ProfitPress Mega CDROM2 …eeware (MSDOS)(1992)(Eng)/ProfitPress-MegaCDROM2.B6I/MISC/EDUCATIO/EPHEM421.ZIP/MAN.TXT

Text File | 1990-09-13 | 143.0 KB | 3,565 lines

                           Ephem V4.20 - August 21, 1990

                    Copyright (c) 1990 by Elwood Charles Downey
                               Chaska, Minnesota, USA

                                  [e-mail]


                                 Table of Contents
      1.  Introduction ...................................................  3
      2.  Running Ephem ..................................................  3
      2.1.  Command Line Format ..........................................  3
      2.2.  Program Operation ............................................  4
      3.  Screen Fields ..................................................  5
      3.1.  Top Screen Fields ............................................  6
      3.2.  Data format columns ..........................................  7
      3.3.  RiseSet format columns .......................................  8
      3.4.  Separation format fields .....................................  8
      4.  Date and Time Formats ..........................................  8
      5.  Configuration File .............................................  9
      5.1.  Configuration File fields ....................................  9
      5.2.  Example ephem.cfg ............................................ 11
      6.  Menu options ................................................... 11
      6.1.  Adaptive vs. Standard hzn .................................... 11
      6.2.  Geocentric vs. Topocentric ................................... 12
      7.  User Defined Objects: X and Y .................................. 12
      7.1.  Controlling Object-X or Y Operation .......................... 12
      7.1.1.  Fixed ...................................................... 13
      7.1.2.  Elliptical ................................................. 13
      7.1.3.  Parabolic .................................................. 13
      7.1.4.  Lookup ..................................................... 14
      7.1.5.  On or Off .................................................. 14
      7.2.  Magnitude models ............................................. 14
      7.2.1.  g/k model .................................................. 14
      7.2.2.  H/G model .................................................. 15
      7.3.  Database File ................................................ 15
      8.  Plotting ....................................................... 15
      8.1.  Defining plot fields ......................................... 16
      8.2.  Displaying a plot file ....................................... 16
      8.3.  Cartesian or Polar coords .................................... 16
      8.4.  Begin Plotting ............................................... 17
      8.5.  Stopping Plotting ............................................ 17
      9.  Listing ........................................................ 17
      10.  Watching ...................................................... 17
      10.1.  Trails ...................................................... 17
      10.2.  Sky dome .................................................... 18
      10.3.  Alt/az sky .................................................. 18
      10.4.  Solar System ................................................ 18
      11.  Searching ..................................................... 18
      11.1.  Find extreme ................................................ 19
      11.2.  Find 0 ...................................................... 19
      11.3.  Binary ...................................................... 19
      11.4.  Define a New function ....................................... 19
      11.4.1.  Intrinsic functions ....................................... 20









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      11.4.2.  Field Specifiers .......................................... 20
      11.4.3.  Constants ................................................. 21
      11.4.4.  Operators ................................................. 21
      11.5.  Specifying Search Accuracy .................................. 21
      11.6.  Stop ........................................................ 21
      11.7.  Example Searches ............................................ 22
      11.8.  Another Example ............................................. 22
      11.9.  Caution ..................................................... 22
      12.  Implementation Notes .......................................... 23
      12.1.  Program limits .............................................. 24
      13.  DOS Installation Procedure .................................... 24
      13.1.  Setting TZ .................................................. 24
      14.  Known Bugs and Wish List ...................................... 25
      15.  Sample Screens ................................................ 26

















































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     1.  Introduction

     Ephem is a program that displays ephemerides for all the planets plus any
     two additional objects.  The additional objects may be fixed or specified
     via heliocentric elliptical or parabolic orbital elements to accommodate
     solar system objects such as comets or asteroids.

     Information displayed about each object includes RA and Dec precessed to
     any epoch, heliocentric coordinates, local azimuth and altitude, distance
     from sun and earth, solar elongation, angular size, visual magnitude,
     illumination percentage, local rise, transit and set times, length of time
     up, and topocentric or geocentric angular separations between all
     combinations of objects.

     Observing circumstance information includes UTC and local date and time,
     local sidereal time, times of astronomical twilight, length of day and
     night, local temperature, pressure and height above sea level for the
     refraction model and a monthly calendar.

     RA/Dec calculations are geocentric and include the effects of light travel
     time, nutation, aberration and precession.  Alt/az and rise/set/transit
     and, optionally, angular separation calculations are topocentric and
     include the additional effects of parallax and refraction.

     Plot and listing files of selected field values may be generated as the
     program runs.  The plot files are full precision floating point values in
     ASCII intended for input to other plotting programs.  The listing files
     are tables formatted for general reading.  Ephem includes simple quick-
     look facilities to view these files.

     One may watch the sky or the solar system with a simple screen-oriented
     display.

     Ephem may be asked to search for interesting conditions automatically,
     using several algorithms. Most fields displayed on the screen may be used
     as terms in an arbitrary arithmetic expression that can be solved for zero
     or minimized or maximized, or the time of state change of any boolean
     expression can be found.

     The program is written in C for unix or DOS. It uses only a very simple
     set of io routines and should be easily ported to any ASCII display.

     The planetary data and correction algorithms are taken, with permission,
     from "Astronomy With Your Personal Computer", by Peter Duffett-Smith,
     Cambridge University Press, 1985.

     2.  Running Ephem


     2.1.  Command Line Format

     To run ephem, just type "ephem".  You may also specify an alternate
     configuration file, an alternate database file, and specify initial values
     for several screen fields.  The command line syntax can be summarized as









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     follows:

         ephem [-c <config_file>] [-d <database_file>] [field=value  ...]


     The default configuration file is named ephem.cfg in the current
     directory.  Ephem also looks for one named by the EPHEMCFG environment
     variable, if it is set. You may specify an arbitrary name with the -c
     option.

     The default database file is named ephem.db in the current directory.
     Ephem also looks for one named by the EPHEMDB environment variable, if it
     is set. You may specify an arbitrary name with the -d option.

     The exact format of these files is described below.

     Any additional command line arguments are treated exactly as if they too
     came from the configuration file.

     2.2.  Program Operation

     When ephem starts, it first displays a disclaimer banner.  Then, after any
     key is pressed, it reads the configuration file and processes the command
     line arguments to set the initial values of several fields, accessing the
     database file if OBJX or OBJY is set, It then draws all fields on the
     screen with their initial values.  The program then loops advancing time
     each step, by some amount you may control, and updating all fields each
     loop.

     There are three fields that control this looping behavior.  NStep controls
     the number of steps, StpSz the amount of time to add each step, and Pause
     is the amount of real seconds to pause between steps.  Ephem does not
     pause between steps when plotting or searching is on.  When the number of
     steps, NStep, goes to 0 or any key is pressed, the looping stops and you
     enter a command mode.

     Command mode allows you to modify most of the fields.  The idea is that
     you move to each field on the screen you wish to change and change it.
     When you have changed everything you want to, type "q" to resume screen
     updates.

     To change a field:

         1) move the cursor to the field (see below);
         2) type RETURN;
         3) type in the new value along the command line at the top according
            to the format indicated in the prompt. To accept the new value
            type RETURN, or to leave it unchanged after all type "q".


     A few fields don't require you to type anything; just typing RETURN does
     all the work.  If you can't move to it, you can't change it.

     The arrow keys on most systems move the cursor around.  If these do not









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     function or function incorrectly, the h/j/k/l keys also move the cursor
     left/down/up/right, respectively.  Motions off any edge of the screen will
     wrap around.

     Several "hot-keys" move the cursor immediately to frequently used fields.
     You may move the cursor immediately to a planet row by typing one of the
     characters SMevmjsunpxy.  To avoid conflict with j, jupiter's row must
     actually be typed as control-j.  "x" and "y" are for the user-defined
     objects X and y on the bottom rows.  Also, the characters c, d, o, w, L
     and z move you to the Menu, UT Date, Epoch, Watch, Listing and StpSz
     fields immediately.

     When you have changed a field that would invalidate any of the other
     fields the message NEW CIRCUMSTANCES appears in the top center of the
     screen.  This will remain until you type "q" to allow at least one screen
     update loop to occur.  If you change any field that causes new
     circumstances, the StpSz value is not added to the first loop.  Note also
     that after a series of loops, NStep is automatically reset to 1 so "q"
     will do exactly one loop and return you to command mode.

     On some systems, you may temporarily escape from ephem while in command
     mode to run your operating system's command interpretor. This is done by
     typing an exclamation point (!) followed by your command.  When the
     command completes, you will return back to ephem where you left off.

     To quit the program, type control-d from command mode.  For a little more
     help, type ?.  The entire screen may be erased and redrawn with control-l.

     3.  Screen Fields

     The screen is divided into two halves, top and bottom.  The top fields are
     always present. They define the general observing circumstances and
     control features.

     The planets and two additional objects are displayed in a table in the
     bottom portion of the screen.  There is one object per row, and several
     columns.  There are three forms of this portion selected by picking the
     Menu selection.

     Some things may be turned off to reduce compute times.  Calculations for
     each planet may be turned on and off by selecting the planet name field.
     Calculations for Dawn/Dusk/NiteLn may be turned off by selecting any of
     these fields.  Planet positions are only updated as often as necessary to
     match the display precision of the screen unless plotting or searching is
     on.  In these cases full precision is desired at all times and so
     positions are always fully recalculated at each iteration.

     Follows is a list and description of each of the fields in each section.
     Following each name a parenthetical "p" indicates the field may be
     selected for plotting (see later). All fields may be selected for
     changing.












                                       - 6 -


     3.1.  Top Screen Fields

     LTZ           the local timezone name.  The name field may be changed to
                   any three-character mnemonic.
     LT(p)
     LD(p)         The local time and date are not labeled as such but are to
                   the right of the local timezone name.  They are individually
                   selectable.  Time and date fields may be changed as
                   described in a later section.  Set to "n" to set to "now"
                   from computer clock.
     UT(p)
     UD(p)         The universally coordinated time and date are not labeled as
                   such but are to the right of the UTC label.  They are
                   individually selectable.  Time and date fields may be
                   changed as described in a later section.  Set to "n" to set
                   to "now" from computer clock.
     JulianDat(p)  the current Julian date, to about 1-second accuracy.
     Listing       controls listing; see complete discussion below.
     Watch         selects the sky dome, altitude/azimuth sky or solar system
                   displays; see complete discussion below.
     Search        controls the automatic search feature of ephem. See the
                   complete discussion below.
     Plot          controls plotting; see complete discussion below.
     Menu          controls which menu is in the bottom half of the screen.
                   See their complete discussion below.
     LST(p)        the current local sidereal time.  set to "n" to set from
                   computer clock.
     Dawn(p)       local time when the sun center is 18 degrees below the
                   horizon before sunrise today.
     Dusk(p)       local time when the sun center is 18 degrees below the
                   horizon after sunset today.
     NiteLn(p)     length of astronomical night, ie, Dawn - Dusk.  If this line
                   is shown as "-----", it means the sun is either always below
                   or always above approximately -18 degrees altitude on this
                   particular day.  This and the Dawn and Dusk lines are blank
                   when their computation has been turned off.
     NStep         The number of times the display with be updated (time
                   advanced by StpSz each step) before entering command mode.
     StpSz         the amount of time UTC (and its derivatives) is incremented
                   each loop.  set this to "r" to use real-time based on the
                   computer clock.  you may also set it in terms of days by
                   appending a "d" after the number when you set it.
     Lat(p)        location latitude, positive degrees north of equator.
     Long(p)       location longitude, positive degrees west of Greenwich
                   meridian.  set to "N" to set from computer clock.
     Elev(p)       local elevation of the ground above sea level, in feet. (see
                   implementation notes).
     Temp(p)       local surface air temperature, in degrees F.
     AtmPr(p)      local surface air pressure, in inches of mercury.
     TZ(p)         hours local time is behind utc, ie, positive west or
                   negative east of Greenwich.
     Epoch         the epoch, to the nearest 0.1 years, to which the ra/dec
                   fields are precessed.  This says (OfDate) when coordinates
                   are not precessed, ie, are in the epoch of date. Set to "e"









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                   to set to epoch of date.
     Pause         number of seconds to pause between screen updates. This is
                   used mainly to set up for free-running unattended operation.
                   This pause also applies to free-running "watch" screen
                   updates.  Pausing is not done when plotting or searching is
                   on.  If pausing is used with StpSz set to RT CLOCK and the
                   time was set with Now then ephem attempts to synchronize the
                   time to an integral multiple of pause seconds after the
                   minute, for aesthetic reasons.
     Also in the upper right of the screen is a calendar for the current local
     month.  Dates of new and full moons are marked NM and FM, respectively.

     3.2.  Data format columns
     Ob            name of object.  Select this to toggle the display and
                   calculations on and off.
     R.A.(p)       apparent geocentric right ascension of object, precessed to
                   given epoch, in hours, minutes and decimal minutes.
     Dec(p)        apparent geocentric declination of object, precessed to
                   given epoch, in degrees and minutes.
     Az(p)         degrees eastward of true north for object.
     Alt(p)        degrees up from a horizontal plane Elev feet above sea
                   level.
     H Long(p)     true heliocentric longitude, in degrees.  Earth's is
                   displayed on the sun's line.  For the moon this is the
                   geocentric longitude.
     H Lat(p)      true heliocentric latitude, in degrees.  For the moon this
                   is the geocentric latitude.
     Ea Dst(p)     true distance from Earth center to object center, in AU,
                   except distance to the moon is in miles.
     Sn Dst(p)     true distance from sun center to object center, in AU.
     Elong(p)      spherical angular separation between sun and given object,
                   calculated from the their geocentric ecliptic coordinates.
                   Note this is not just difference in ecliptic longitude. The
                   sign, however, is simply sign(obj's longitude - sun's
                   longitude), ie, degrees east.  thus, a positive elongation
                   means the object rises after the sun.
     Size(p)       angular size of object, in arc seconds.
     VMag(p)       visual magnitude of object.
     Phs(p)        percent of visible surface in sunlight. Note the moon phase
                   is calculated simplistically as just abs(elongation)/180*100
                   which can be a few degrees off... this means that because of
                   how elongation is defined it doesn't say 0 during new moon
                   (or 100 during full) except during close eclipses (maybe
                   that's a "feature"?).
     Also, some terminals scroll when a character is written to the lower right
     character position. To avoid this, Object Y's phase is left shifted by one
     column. This can look particularly ugly when the phase is 100% because the
     "100" is right next to visual magnitude number.
     If desired, the angle between Earth and Sun from the object, p, can be
     computed from the illumination percentage, i, with the following relation:

         cos (p) = i/50 - 1











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     3.3.  RiseSet format columns
     Rise Time
     Rise Az       The local time and azimuth when the upper limb of the object
                   rises today.
     Transit Time
     Transit Alt   The local time and altitude when the object crosses the
                   meridian today, ie, when its azimuth is true south or, if no
                   precession, when the local sidereal time equals the object's
                   right ascension.
     Set Time
     Set Az        The local time and azimuth when the upper limb of the object
                   sets today.
     Hours Up      The number of hours the object is up on the local date.

     Horizon displacement may be calculated in either of two ways; see the
     horizon discussion in the Menu selection section.

     Various oddball conditions are accounted for, including an object that is
     up sometime during the day but that doesn't rise, transit or set as such
     on that day, an object that is circumpolar or that is never up or one that
     rises twice on the same day.  These are marked as "Never rises", "Never
     transits", "Never sets", "Circumpolar", "Never up" or appended with a plus
     "+" sign, respectively.

     3.4.  Separation format fields
     This format is a table of angular separations between each pair of
     objects.  These angles are based on the local altitude/azimuth, and so in
     general differ somewhat from the elongations reported for the sun in the
     Data menu.
     Unfortunately, with the format "ddd:mm", there is not enough room for a
     space between columns when the angle is at least 100 degrees.  To avoid
     this, I drop the minutes portion if the (rounded) angle is at least 100
     degrees.

     4.  Date and Time Formats
     Times are displayed and entered in h:m:s format.  If you pick a time field
     to change it any of the h, m, and s components that are not specified are
     left unchanged from their current value.  For example, 0:5:0 set hours to
     0, minutes to 5, seconds to 0, whereas :5 sets minutes to 5 but leaves
     hours and seconds unchanged.  A negative time is indicated by a minus sign
     (-) anywhere before the first digit.

     Dates are displayed and entered in American month:day:year format.  As
     with time, components omitted when entering a new value retain the current
     value.  For example, if the current date is 10/20/1988 and you type 20/20
     the new date will become 20/20/1988. Note you must type the full year
     since the program is accurate over several centuries either side of 1900.
     If you change the date, the time (ie, partial day) will not change.

     Two other ways to set the date are supported for compatibility with some
     published comet ephemerides.  You may enter the day portion as a real
     number.  When you set the day this way, the time will also change to
     correspond to the fractional portion of the day.










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     You may also enter a date as a decimal year, as in 1990.12345.  This is
     also useful in interpreting plot files that include a date field, since
     date fields are stored in plot files as decimal years.  If no decimal
     point is included, the number is assumed to be a year unless it is in the
     range 1-12, in which case it will be taken to mean that you are just
     changing the month of the current date.  To actually specify the years 1 -
     12, you must append a decimal point to distinguish them from months.

     As a matter of typing convenience, the program accepts most any character
     as the separator; you don't have to type a perfect ":" or "/".

     5.  Configuration File
     The ephem.cfg configuration file allows you to set the initial values of
     many of the screen fields. You can still change any field while the
     program is running too; this file just sets the initial conditions.  Note
     that the order of entries in this file is important because they each take
     effect immediately.  You should put them in the same order you wish them
     to be processed, just as though you were changing the fields interactively
     within ephem.

     The default name of the file is ephem.cfg.  Ephem also looks for one named
     by the EPHEMCFG environment variable (if defined) or you may specify any
     name using the -c command line option.

     The format of the file uses the form KEYWORD=VALUE, where the possible
     KEYWORDS and the types of VALUES for each are described below. Any
     KEYWORDS not in the file will take on some sort of default. The separator
     need not be an actual equals sign; any char will do because the VALUE is
     assumed to start one character after the KEYWORD, regardless.

     Blank lines and lines that begin with an asterisk (*) or whitespace (space
     or tab) are ignored and may be used for comments.

     Note:  because of the way unspecified time and date components are left
     unchanged (see section on Date and Time Formats) always specify the
     complete time and date for all entries in the configuration file. For
     example, to initialize the longitude to zero degrees, say 0:0:0, not just
     0.

     5.1.  Configuration File fields
     UD        initial UTC date, such as 10/20/1988, or "NOW" to use the
               computer clock.
     UT        initial UTC time, such as 12:0:0, or "NOW" to use the computer
               clock.
     TZONE     hours the local time is behind utc, such as 5:0:0.  you need not
               set this if you use "NOW" for UT or UD.
     TZNAME    name of the local time zone, such as CDT. 3 chars max.  you need
               not set this if you use "NOW" for UT or UD.
     LONG      longitude, in degrees west of Greenwich, in the form d:m:s.
     LAT       latitude, in degrees north of the equator, in the form d:m:s.
     HEIGHT    height above sea level, in feet, such as 800
     TEMP      air temperature, in degrees F, such as 50











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     PRES      air pressure, in inches of Mercury, such as 29
     STPSZ     the time increment between screen updates, such as "1" to give
               one hour updates. this can be a specific amount or RTC to use
               the system clock as a real-time source. You may also specify a
               time in days, by appending a D (or d) after the number.
     PROPTS    this selects what you want included initially in the display.
               since IBM-PC math is not very fast, you can reduce the time to
               update the screen by only printing those fields of interest. the
               VALUE is a collection of letters to turn on each item from the
               following set:

                       T       twilight (dawn-dusk)
                       S       circumstances for the sun
                       M       circumstances for the moon
                       e       circumstances for mercury
                       v       circumstances for venus
                       m       circumstances for mars
                       j       circumstances for jupiter
                       s       circumstances for saturn
                       u       circumstances for uranus
                       n       circumstances for neptune
                       p       circumstances for pluto
                       x       circumstances for object X
                       y       circumstances for object Y

               For example, to just track the sun and saturn, say PROPTS=Ss

               If the delimiter between PROPTS and the selection is a plus (+)
               sign then the given planets are included IN ADDITION TO ones
               already specified. Any other delimiter sets the selection to
               exactly the set specified. This feature was added so that the
               command line version of using PROPTS could add to the set of
               planets giving in the configuration file.
     NSTEP     number of times program will loop before entering command mode.
               see the discussion under Program Operation.
     EPOCH     this sets the desired ra/dec precession epoch. you can put any
               date here or EOD to use the current instant ("Epoch of Date").
     OBJX
     OBJY      These fields specify the optional objects "x" and "y" by naming
               any item in the database file. The form is OBJX=xyz, where xyz
               must be in the database file, case sensitive.  You may define
               one object of each type for each of OBJX and OBJY; the last one
               defined will be the "current" one when ephem gets going.
     PAUSE     The number of seconds to pause between calculation steps. See
               definition of the Pause field in the "Top Screen Fields"
               section.
     MENU      establishes the initial bottom screen menu type.  This should be
               one of the keywords DATA, RISET or SEP.  There is no way to set
               horizon or center suboptions at this time.














                                       - 11 -


     5.2.  Example ephem.cfg

     This is the ephem.cfg file that was in effect when the sample screens (in
     another section) were generated. You might run ephem with this
     configuration file and compare with the samples as a check.

     UT=0;0;0
     UD=5/1/1990
     TZNAME=CDT
     TZONE=5
     LONG=93:42:8
     LAT=44:50:37
     HEIGHT=800
     TEMP=40
     PRES=29.5
     STPSZ=RTC
     PROPTS=TSMevmjsunpxy
     EPOCH=2000
     NSTEP=1

     OBJX=Austin
     OBJY=Juno

     As another common example, this ephem.cfg creates an essentially free-
     running real-time screen based on the computer clock:

     UT=Now
     LONG=90:10:8
     LAT=40:50:20
     HEIGHT=800
     TEMP=50
     PRES=29
     STPSZ=RTC
     PROPTS=TSMevmjsunp
     NSTEP=9999999
     EPOCH=Eod
     PAUSE=30


     6.  Menu options

     When you select "Menu" you can change among the three styles of bottom
     screens. There are also two options that can be set from the Menu quick-
     choice menu. These options toggle when picked and retain their values so
     they need only be changed when desired.

     6.1.  Adaptive vs. Standard hzn

     This selects the horizon refraction displacement algorithm used by the
     Rise/Set menu.  "Adaptive" uses the local atmospheric conditions known to
     ephem and matches the Planet Info times nicely. "Standard" uses the
     "accepted nominal" horizon refraction value of 32 arc minutes and usually
     agrees, to a minute or so, with published tables.










                                       - 12 -


     6.2.  Geocentric vs. Topocentric

     This selects the vantage point for the Separation menu. "Geocentric"
     ignores local conditions and gives the separation as seen from Earth
     center. "Topocentric" uses the local conditions known to ephem.  They are
     particularly critical for lunar occultations, but the effect can be
     significant for the planets.

     Note that searching over a period that will include the rise or set times
     of either object is generally better performed from the geocentric
     viewpoint.  The refraction effect of the topocentric viewpoint causes many
     arcminutes of rapid whiplash displacement as the objects rise and set that
     overlays the smooth celestial motion of the objects.  This rapid position
     variation can confuse the solver algorithms that expect fairly smooth
     functions.

     7.  User Defined Objects: X and Y

     You may specify one or two extra objects for ephem to use. The objects may
     be defined in three different ways:  fixed celestial sphere coordinates,
     or heliocentric elliptical or parabolic orbital elements.  Elliptical
     elements are typically useful for periodic comets or asteroids, and
     parabolic elements are for nonrecurring solar system interlopers such as
     aperiodic comets.

     The parameters for each type of object are stored separately, so you may
     switch between types of objects without losing parameters.

     7.1.  Controlling Object-X or Y Operation

     To control the type and the corresponding details for object X or Y,
     select the corresponding row near the bottom.  (Remember that typing the
     character "x" or "y" is a shorthand way to move to the bottom rows.) It
     will bring up a quick-choice menu as follows:

     Select: Fixed, Elliptical, Parabolic, Lookup, On


     When you first enter the quick-choice menu the cursor will start out
     positioned at the field for the current type of object.  The first three
     selections allow you to enter or review the various parameters required to
     define an object's position of such type, one parameter at a time.

     You set the current object type and begin to view its parameters by
     positioning the cursor over the type and pressing RETURN.  The prompt for
     each item includes a short description, the units to use, and its current
     setting is shown in parentheses. To leave the item unchanged and go to the
     next item, type RETURN.  If you do not wish to change or see any more
     items about the object then type "q" and you will return immediately to
     the object-X quick-choice menu.

     You exit the quick-choice menu by typing "q" while over any field or
     RETURN while over On or Off, as described in a later section.










                                       - 13 -


     As with all dates throughout ephem, the dates for the epochs of perihelion
     and reference epochs may be entered in month/day/year or decimal year
     formats, and the day may be entered as a real number (see the section on
     Date and Time Formats).  All dates given for comet parameters are always
     in UT.

     7.1.1.  Fixed

     This selection will present a series of five prompts, the name and one
     each for the RA, Dec, magnitude and the reference epoch for the
     coordinates of a fixed object.

     7.1.2.  Elliptical

     This will begin a series of twelve prompts asking for a name and the
     parameters that define a heliocentric elliptic orbit and the coefficients
     for either of two magnitude models.  These elements are the same ones
     often listed in the Astronomical Almanac.  The elements are, in order:

             i = inclination, degrees
             O = longitude of ascending node, degrees
             o = argument of perihelion, degrees
             a = mean distance (aka semi-major axis), AU
             n = daily motion, degrees per day
             e = eccentricity
             M = mean anomaly (ie, degrees from perihelion)
             E = epoch date (ie, time of M)
             D = the equinox year (ie, time of i/O/o)
             g/k or H/G  = either of two magnitude models; see below


     You might have other parameters available that can be converted into
     these.  For example, we have the following relationships:

             P = sqrt(a*a*a)
             p = O + o
             n = 360/days_per_year/P ~ 0.98563/P
             T = E - M/n
             q = a*(1-e)
         where
             P = the orbital period, years;
             p = longitude of perihelion, degrees
             n = daily motion, degrees per day;
             T = epoch of perihelion (add multiples of P for desired range)
             q = perihelion distance, AU

     Note that if you know T you can then set E = T and M = 0.

     7.1.3.  Parabolic

     This will begin a series of nine prompts asking for a name and the
     parameters that define a heliocentric elliptic orbit and the magnitude
     model coefficients.  These orbital parameters are, in order:










                                       - 14 -


     epoch of perihelion,
     inclination,
     argument of perihelion,
     perihelion distance,
     longitude of the ascending node,
     and the reference epoch of the parameters.
     absolute magnitude, g
     luminosity index coefficients, k


     7.1.4.  Lookup

     This option lets you define an object from any of those listed in the
     database file, described in a subsequent section.

     If successful, the cursor will move to the type of the new object and it
     becomes the current type.

     7.1.5.  On or Off

     The last selection on the right toggles the calculations for the object On
     and Off.  It toggles when selected with RETURN and then immediately exits
     the quick-choice menu back to the main menu.  If calculations become On,
     then they will be performed for the current type of object; if they become
     Off the object-X or Y row of information will be erased.

     7.2.  Magnitude models

     Ephem supports two different magnitude models. One, denoted here as g/k,
     is generally used for comets and may be used for parabolic and elliptical
     objects. The other, denoted H/G, is generally used for asteroids and is
     the one used in the Astronomical Almanac.

     7.2.1.  g/k model

     This model requires two parameters to be specified.  One, the absolute
     magnitude, g, is the visual magnitude of the object if it were one AU from
     both the sun and the earth.  The other, the luminosity index, k,
     characterizes the brightness change of the object as a function of its
     distance from the sun. This is generally zero, or very small, for inactive
     objects like asteroids.  The model may be expressed as:

         m = g + 5*log10(D) + 2.5*k*log10(r)
       where:
         m = resulting visual magnitude;
         g = absolute visual magnitude;
         D = comet-earth distance, in AU;
         k = luminosity index; and
         r = comet-sun distance.

     Note that this model does not take into account the phase angle of
     sunlight.

     When using this model for elliptical objects, the first of the two









                                       - 15 -


     magnitude fields must be preceded by a letter "g" in both the ephem.db
     database file and the corresponding quick-choice elliptical object
     definition prompt; otherwise the default is the H/G model.

     7.2.2.  H/G model

     This model also requires two parameters. The first, H, is the magnitude of
     the object when one AU from the sun and the earth. The other, G, attempts
     to model the reflection characteristics of a passive surface, such as an
     asteroid. The model may be expressed with the following code fragment:

         beta = acos((rp*rp + rho*rho - rsn*rsn)/ (2*rp*rho));
         psi_t = exp(log(tan(beta/2.0))*0.63);
         Psi_1 = exp(-3.33*psi_t);
         psi_t = exp(log(tan(beta/2.0))*1.22);
         Psi_2 = exp(-1.87*psi_t);
         m = H + 5.0*log10(rp*rho) - 2.5*log10((1-G)*Psi_1 + G*Psi_2);
       where:
         m   = resulting visual magnitude
         rp  = distance from sun to object
         rho = distance from earth to object
         rsn = distance from sun to earth

     Note that this model does not take into account the phase angle of
     sunlight.

     7.3.  Database File

     You may save a list of objects in a file to be used for setting OBJX and
     OBJY. The default name of this file is ephem.db. You may also set it from
     the command line with the -d option, or set it with the EPHEMDB
     environment variable.

     The file consists of one object per line. Lines that begin with an
     asterisk (*) are ignored.  Each line contains several fields, each
     separated by a comma.  The first field is the name of the object.  The
     second field is the type of the object, that is, one of the strings
     "fixed", "elliptical", or "parabolic"; actually, "f", "e" and "p" are
     sufficient.  The remaining fields depend on the type of object.  They are
     exactly the same parameters, and in the same order, as ephem asks for when
     defining the object from the menu.

     8.  Plotting

     Each time a field is drawn on the screen during a full screen update cycle
     (that as, during automatic looping or a manual "q" command character from
     the main menu but not from a screen redraw from control-l or when an
     individual planet is turned on or a single time field is changed) its
     full-precision value may be written to a file.  This implies you may not
     plot a field from other than the current menu at the time plotting is on.
     You can append several plot runs together, however, if necessary.

     Each line in the file consists of a tag character followed by two or three
     floating point variables, all separated by commas. If there are two









                                       - 16 -


     values, they should be interpreted to be x and y (or perhaps r and theta).
     If there is a third, it is a z or trace value.

     For efficiency on systems that can compute a screen full faster than they
     can display it, screen updates are suppressed while plotting is on and
     NStep is greater than 1. This can greatly reduce the time to generate a
     long plot file. Fields are still logged for plotting; they just are not
     drawn on the screen.

     The Plot field controls plotting.  Whether plotting is currently active is
     indicated by "on" or "off" immediately to its right.

     Picking "Plot" brings up a quick-choice menu, as follows:

     Select: Select fields, Display a plot file, Cartesian coords, Begin plotting


     8.1.  Defining plot fields

     Select the "Select fields" option.  You will be asked to move the cursor
     to the field you want to use as the x coordinate (abscissa), then asked to
     choose the y coordinate (ordinate), then asked to choose an optional z
     trace variable and finally a tag character.  (Of course, you may have in
     mind fields that are more appropriately displayed in other than Cartesian
     coordinates, in which case think of x, y and z as dimensions.) If you type
     q for either x or y then no more fields will be defined.  If you type q
     for the z field there will be no z field.  You can not label a plot line
     with the letter "q" at this time.

     This then repeats so you may choose up to ten of these sets for any given
     plot run.  Each set defines what will become a line on the final plot.
     Note that you may select the "Search" field to indicate use of the current
     search function; that function must be defined by the time plotting is
     turned on.

     If you turn plotting off and back on the fields selected for plotting are
     reactivated the same as they were last time. You may change them if
     desired, of course, but there is no need to redefine them if you do not
     wish to change them.

     8.2.  Displaying a plot file

     Select the "Display a plot file" option to generate a crude plot on the
     screen of an existing plot file previously created by ephem.  The entries
     in the file will be drawn on the screen using their tag characters; the
     plot remains on the screen until you type any character.

     The plot may be made in polar or Cartesian coordinates, depending on the
     setting of the plotting mode in the quick-choice (see next section).

     8.3.  Cartesian or Polar coords

     This toggles the plotting mode coordinate system.  The mode remains until
     changed.  Polar coordinates assume the first numeric field in the plot









                                       - 17 -


     file is the radius, and the second is the angle counterclockwise from
     right, in degrees.

     8.4.  Begin Plotting

     If plot field lines are defined then the third option, "Begin plotting"
     will be available.  You will be asked for the name of the file to use.  If
     it already exists you will be asked whether to overwrite it or append to
     it.  You will also be asked for a title line that will become the next
     line of the file, automatically prefixed with a "*" to make it a valid
     comment line.  Once you have chosen a file, plotting is on and the top
     menu plotting status field changes to "on".  The default plot file name is
     ephem.plt.  The values are written to the plot file each time they are
     updated on the screen until you select "Plot" again and select the "Stop"
     option to turn plotting back off.

     8.5.  Stopping Plotting

     If plotting is on, then selecting the Plot field in the top section will
     turn plotting off. You may pick Plot again and resume with the same fields
     by selecting "Begin plotting" again.

     Note that due to internal buffering the plot file will not be completely
     written to disk until plotting is turned off.

     9.  Listing

     This feature works very much like Plotting. However, the fields you select
     define columns of a table generated as ephem runs. These columns are look
     exactly like their corresponding fields on the ephem screen and so are far
     more readable than plot files.

     See the section on Plotting for an explanation of the quick-choice menu.
     The general operation is very much the same.

     10.  Watching

     You may generate a simple drawing on the screen of the local sky in two
     forms or the solar system by selecting "Watch".  It will bring up a
     quick-choice menu as follows:

     Select: Sky dome, Alt/az sky, Solar system, No trails


     10.1.  Trails

     You may either erase after each iteration or leave the tags up, referred
     to as "trails". Picking the right-most choice will toggle between "No
     trails" and "Leave trails"; you should set it as desired before you select
     the style of sky plot you wish.  Ephem will remember your selection.













                                       - 18 -


     10.2.  Sky dome

     This draws the currently active planets within a circle that represents
     the locally visible hemisphere.  This is similar to the formats commonly
     used in the popular astronomy magazines.  East is left, south is down,
     west is right and north is up.

     10.3.  Alt/az sky

     This is a cylindrical projection showing the currently active planets as
     they would appear in the sky looking southwards at the current time and
     date.  The coordinate system is such that 0 degrees azimuth (north)
     through 360 degrees (north, once around) is mapped to the horizontal
     screen dimension, and 0 degrees altitude (level) through 90 degrees (the
     zenith) is mapped to the vertical dimension. Thus, the bottom row is the
     horizon and all across the top is the zenith.

     10.4.  Solar System

     This selection draws the currently active planets as they would appear
     looking "down from the top" of the ecliptic, with the sun at the center
     and zero hours right ascension towards the right.  The scale is adjusted
     to roughly fill the screen according to the outer-most active planet.
     Note that the scaling does not take into account the distances of the user
     defined objects so, if they don't appear, select a planet that is at least
     as far out as they are.  The screen transformation assumes a screen aspect
     width/height ratio of 4/3.  Down the left column of the screen is the
     heliocentric altitude of the planet above or below the ecliptic, drawn to
     the same scale as the circular display.  Values so close as to land on the
     same line are sorted left to right; the S and E symbols always denote
     heliocentric altitude 0.

     In each style of display, pressing RETURN advances the time by whatever
     amount StpSz is set to.  Pressing "h" advances the time by one hour, "d"
     advances by one day, and "w" advances by one week (seven days).  Pressing
     "q" returns to the watch quick-choice menu from which you may select
     another display style or return to the main tabular display by typing
     another "q".  Pressing any other key starts an automatic loop with each
     step advancing by StpSz; pressing any key stops the looping.

     As symbols are placed, collisions (overstrikes) are avoided by moving
     characters in such a way as to maintain increasing sorted order towards
     the right.

     When you return to the main menu, the last watched time will be maintained
     as the current time.  The StpSz is not changed.

     11.  Searching

     Ephem can search for arbitrary conditions to exist among most displayed
     fields.  You first enter a function, then select from among three forms of
     equation solvers to iteratively solve for the next time when the function
     meets the requirements of the solver. The solver selects the next time for
     which it wants the function evaluated and sets StpSz so that the next









                                       - 19 -


     iteration will occur at that time. The solvers continue to iterate until
     either they achieve their goal or NStep reaches 0.

     You may set NStep to be quite large and let ephem search unattended or set
     it to 1 and watch it converge one step at a time. You may also plot at the
     same time as search to record the exact steps ephem took to converge.
     (But recall that screen updates are suppressed if plotting is also on).

     The "Search" selection in the top half of the screen controls all
     searching.  Picking it brings up a quick-choice menu as follows:

     Select: Find extreme, Find 0, Binary, New function, Accuracy


     11.1.  Find extreme

     This search algorithm searches for a local maximum or a minimum in the
     search function, whichever it finds first. It begins by evaluating the
     search function at the current time then for two more times each separated
     by StpSz. It then fits these three points to a parabola and solves it for
     the time of its maximum (or minimum). StpSz is set so that the next
     iteration will evaluate at this point.  This parabolic fit solution keeps
     repeating until StpSz changes by less than the desired accuracy or until
     the curve becomes so flat that an extrema appears too broad to find.

     11.2.  Find 0

     This search algorithm uses the secant method to solve for the time at
     which the search function is zero. The function is evaluated at the
     current time and then again StpSz later to establish a slope for which the
     x-intercept is found as the next zero guess. This is used to set StpSz for
     the next desired time value and the slope hunting process repeats until
     StpSz changes by less than the desired accuracy.

     11.3.  Binary

     This search algorithm must be used with a search function that yields a
     boolean result, ie, a true or false value. The idea is that the function
     is assumed to be one truth value when evaluated at the present time, and
     the opposite truth value when it is evaluated StpSz later. The algorithm
     will then do a binary search for the time when the truth value changes.

     The binary algorithm does not begin until the state change is bounded in
     time.  Initially, as long as the truth value at StpSz is the same as the
     previous value the algorithm will just keep moving in time by StpSz
     looking for when the state changes.  That is, a linear search is initiated
     to bound the state change, then the binary search proceeds.

     11.4.  Define a New function

     Select "New function" to display the current search function.  If you type
     "q" it will be left unchanged.  If you type RETURN it will be erased.  If
     you type anything else it will be compiled and, if there are no errors, it
     will become the new search function.  Once a valid function has been









                                       - 20 -


     stored, it will remain unless changed.  If a search function is selected
     and there is as yet no valid search function defined, you will
     automatically be asked to enter one as though you had selected "New
     function."

     A search function consists of intrinsic functions, field-specifiers,
     constants and operators, and precedence may be overridden with
     parentheses.


     11.4.1.  Intrinsic functions

     In this release, the only intrinsic function available is abs(), which
     returns the absolute value of its argument.

     11.4.2.  Field Specifiers

     A field in the bottom half of the menu is specified in the form of
     "object_name.column_name". The object_name is enough of the planet name to
     be unique; use "x" or "y" for the user-specified object X or Y.  The
     column_name is from the following table, depending on which menu is up. In
     all cases additional characters may be entered but are ignored.

     Planet Data Menu        Rise/Set Menu               Separation Menu
     ------------------      --------------------        ---------------
     al      Alt             hr      Hrs Up, or          j       Jup
     az      Az              hu      Hrs Up              ma      Mars
     d       Dec             raz     Rise Az             me      Merc
     ed      Ea Dst          rt      Rise Time           mo      Moon
     el      Elong           saz     Set Az              n       Nep
     hla     Helio Lat       st      Set Time            pl      Pluto
     hlo     Helio Long      ta      Transit Alt         sa      Saturn
     ph      Phs             tt      Transit Time        su      Sun
     ra      R.A.                                        u       Uranus
     sd      Sn Dst                                      ve      Venus
     si      Size
     vm      VMag


     In addition, the following top-half fields may be used:

     da      Dawn
     du      Dusk
     n       NiteLn



     Remember, searching may only involve fields being calculated for display
     at the time the solver is active.  While you can syntactically include any
     field in a search function it is useless to define a search that uses
     fields from other than the menu that is selected at the time the search is
     running.











                                       - 21 -


     11.4.3.  Constants

     Constants may be integers or floating point numbers. The latter may be
     expressed in scientific notation if desired. Examples include 100, .9,
     1.234, 1e10 and 1.2e-4.  Any number may be preceded by - to make it
     negative.

     11.4.4.  Operators

     The collection of arithmetic, relational and boolean operators provided
     mimics those of C language as listed in the following table, in decreasing
     order of precedence.  Operators grouped together have the same precedence
     and all have left-to-right associativity. Parentheses may be used as
     desired.

     Symbol  Meaning                 Resulting type
     ------  --------------------    --------------
     *       multiply                arithmetic
     /       divide                  arithmetic

     +       add                     arithmetic
     -       subtract                arithmetic

     >       greater than            boolean
     >=      greater than or equal   boolean
     <       less than               boolean
     <=      less than or equal      boolean

     ==      equality                boolean
     !=      inequality              boolean

     &&      logical and             boolean

     ||      logical or              boolean


     11.5.  Specifying Search Accuracy

     Selecting "Accuracy" allows you to specify when the search will stop.  The
     search algorithms will stop when StpSz becomes equal to or less than this
     value. The default is one minute.  If ephem has not yet converged to the
     specified accuracy but NStep has decremented to 1, the searching will stop
     but the search status field will still indicate which search procedure is
     in effect. To try more iterations you may increase NStep and resume
     searching. If the accuracy was achieved, the search status field will
     switch to "off" with the number of "unused" steps remaining in NStep and
     the last step size in the StpSz fields.

     11.6.  Stop

     If searching is on, this option will also appear on the quick-choice menu
     and may be selected to turn off the search.











                                       - 22 -


     11.7.  Example Searches

     As an example, let's find when Pluto again becomes the furthest planet
     from Sol.  You may find when the difference in their sun distance is zero,
     or you might use a binary search on the condition that Pluto's sun
     distance is larger then Neptune's.

     To try the former approach select Search, select "Find 0", specify the
     search function to be:

     pl.sd - nep.sd

     set StpSz to something large like 10d, NStep to allow several iterations
     like 20, and then type "q" to start the search and watch ephem do the
     hunt.  Ephem will settle on about 21:02 1/10/1999 UT.

     To try a binary search, you first need to have some idea of when the event
     will occur so you can eliminate the initial linear search for the state
     change. We can start at, say, 1/1/1999, set StpSz to 30d, select Binary
     search, specify the search function to be:

     pl.sd > nep.sd

     and go. Once it brackets the state change note how StpSz keeps being cut
     in half but can go in either direction (sign) as it divides each interval
     in half.  Ephem will converge on the same answer.

     11.8.  Another Example

     To find the time of last quarter moon during December, 1989, use the "Find
     0" search algorithm to solve "moon.el + 90".  (At last quarter, the moon
     is 90 degrees west of the sun, or -90 east in ephem's elongation display.)
     Set the initial time to mid-month, 12/15/1989, StpSz to 1 day and NStep to
     10. Ephem takes only a few iterations to settle on 23:57 12/19 UT.

     11.9.  Caution

     Beware that most celestial phenomena are generally pseudo-periodic in
     nature.  In early search steps ephem can easily skip over a local maxima
     and find a later one, which, while correct, may not be what was desired.
     In general, the closer you can be when you start the search the better
     ephem can refine it; it is not as good with very broad searches that can
     go "wild". Set StpSz large enough to offer significant change in the
     function value, but small enough not to skip too far.

     For example, Saturn and Neptune had three close approaches during 1989.
     If you did not know this then just asking ephem to find a minimum would
     have produced different results depending on the starting conditions.
     When starting a search for a certain class of event it is a good idea to
     first use the plotting or watching facility of ephem to get a broad
     picture of the general circumstances then use ephem's search facility to
     refine a given region (or create and inspect a plot file and do your own
     interpolation directly from it separately).










                                       - 23 -


     Similarly, ephem's searching techniques are not good for eclipses because
     the moon and sun are close every month; the trick is sorting through the
     frequent conjunctions for ones that are particularly close. One needs a
     way of establishing an envelope fit to the local extrema of a cyclic
     function in order to find a more global extreme.

     12.  Implementation Notes

     Remember that everything is for the current local time and day. So, for
     example, the calendar marks moon events in local time; commercial
     calendars usually mark the UT date.  Similarly, the rise/set times are for
     the current local day.

     The program uses a horizontal plane tangent to the earth as the horizon
     for all altitude calculations, rise/set events, etc.  This is not the same
     as the angle up from the local horizon unless the observer is directly on
     the ground due to earth's curvature.  The effect can be found from:

             sin(a)**2 = (h**2 + 2Rh) / (R+h)**2
         where:
             R = radius of earth
             h = height above ground (same units as R)
             a = increase in altitude

     For example, the effect is more than two arc minutes at a height of 5
     feet.

     Visual magnitudes are not very accurate ... I haven't bothered to fix.

     The accuracy of ephem can not be specifically stated since the Duffett-
     Smith book does not warrant its planet position polynomials to any given
     degree. I know for sure that better accuracy could be achieved if ephem
     used TDT but I have not yet decided on a suitable algorithm.  Allowing for
     this manually, (see the Wish List section) comparisons with the
     Astronomical Almanac are often within a few arcseconds.

     The program uses double precision throughout. While this precision might
     seem a little ridiculous, it is actually more efficient for most
     traditional K&R C compilers and the search functions seem to be are far
     more stable.

     Searching and plotting always use full precision but if neither of these
     are turned on pure display and watching only recompute a given planets new
     location if the time has changed enough to effect the required display
     precision, based on the planets mean apparent orbital motion.

     The sun-moon distance is the solution for the third side of a planar
     triangle whose two other sides are the earth-moon distance and earth-sun
     distance separated by the angle of elongation.

     Beware of specifying a year of 0, or of computing with the user-defined
     objects before they are properly defined. These conditions can cause ephem
     to blow.










                                       - 24 -


     12.1.  Program limits

     The search function is limited to a maximum of 32 instructions (each
     constant, field spec, and operation is one instruction), with no more than
     a total of 16 constants and field specs. At run time, the function can not
     require more than 16 stacked values (due to operator precedence or
     explicit parenthetical expressions) to evaluate.

     No more than 32 different fields can be tracked simultaneously for
     plotting and/or searching.

     No more than 10 lines may be plotted at once.

     The maximum file name length is 14 characters.

     13.  DOS Installation Procedure

     You must be running DOS V2.0 or later, though somewhere between V2.0 and
     V3.21 the behavior of control-c to terminate the program was fixed.  An
     8087 floating point chip will be used if present.

     The distribution floppy contains five files:
     README describes last minute items and details of this release.
     MAN.TXT is this manual, hopefully formatted and printable on most any
     printer.
     EPHEM.EXE is the executable program.
     EPHEM.CFG is a sample configuration file.
     EPHEM.DB is a sample database.
     To run the program, make working copies of these files in a directory and
     run "ephem" from that directory.

     13.1.  Setting TZ

     Before running ephem, you should set a DOS environment variable, TZ.  It
     is is used to establish the timezone name and hours offset whenever the
     "Now" shorthand is used from ephem, either from the configuration startup
     file or whenever any time field is changed manually.  Set it in the
     following form:

          set TZ=SSSnDDD

     where

     SSS  is the 3-letter abbreviation for the local standard timezone;

     n    is a number between -23 to 24 indicating the number of hours that are
          subtracted from GMT to obtain local standard time;

     DDD  is an optional 3-letter abbreviation for the local daylight savings
          time zone name. Leave it off if you do not have savings time in your
          area or it is not currently in effect. If the changeover dates differ
          from the internal algorithm, just use SSS and n directly.

     For example, in the midwestern United States with savings times set









                                       - 25 -


     TZ=CST6CDT

     If for some reason your system does not change to savings time at the
     right time, then omit the DDD parameter and just set the SSS and n to
     exactly what you want.

     You can put this in your AUTOEXEC.BAT file so it gets set each time you
     boot DOS.

     14.  Known Bugs and Wish List

     incorporate Terrestrial Dynamical Time (known as Ephemeris Time prior to
     1984).  TDT is about 57 seconds ahead of UT1 in 1990.

     it's too easy to turn on objx/y before it's defined and bomb out.

     add explicit searching for eclipses and occultations.

     in watch mode, RTC reverts back to being based off the time when watch was
     first entered.

     add a facility (or tool) to find g/k from a set of observed magnitudes.

     find a better precession algorithm; current one exhibits some hysteresis
     and isn't all that accurate in general.

     add search criteria for database objects.

     add plot options to reverse x and/or y direction.

     handle year 0 properly.

     add moons.






























                                       - 26 -


     15.  Sample Screens

     Here are sample ephem screens. They are generated using the first sample
     ephem.cfg file (listed in the section describing the configuration file).
     There is one for each of the three possible screen formats.  The rise/set
     screen was done using the Adaptive option.  The separations screen was
     done using the Topocentric option.

Move to another field, RETURN to change this field, ? for help, or q to run

CDT 19:00:00  4/30/1990 | LST    8:19:50 | Lat   44:50:37 |      April 1990
UTC  0:00:00  5/01/1990 |                | Long  93:42:08 | Su Mo Tu We Th Fr Sa
JulianDat 2448012.50000 | Dawn      4:10 | Elev    800 ft |  1  2  3  4  5  6  7
Watch                   | Dusk     22:15 | Temp      40 F |  8 FM 10 11 12 13 14
Listing             off | NiteLn    5:55 | AtmPr 29.50 in | 15 16 17 18 19 20 21
Search              off |                | TZ     5:00:00 | 22 23 NM 25 26 27 28
Plot                off | NStep        1 | Epoch   2000.0 | 29 30
Menu        Planet Data | StpSz RT CLOCK | Pause        0 |
--------------------------------------------------------------------------------
Ob   R.A.    Dec    Az     Alt  H Long H Lat  Ea Dst Sn Dst Elong  Size VMag Phs
Su  2:32.3  14:58 278:40  12:38 220:22        1.0075               1905  -27
Mo  8:09.9  21:11 186:06  65:53 119:55   1:04 234821 1.0071   79.5 1897  -12  44
Me  2:49.4  17:39 277:48  17:26 214:08   1:43 0.5764 0.4360    4.9 11.7  1.6   1
Ve 23:49.4  -2:25 296:53 -27:39 282:39  -1:30 0.9288 0.7276  -43.9 18.2 -4.8  64
Ma 22:39.8 -10:09 308:17 -44:14 297:56  -1:43 1.5438 1.4067  -62.9  6.1  0.3  89
Ju  6:30.9  23:23 235:13  59:04 106:16   0:08 5.6806 5.1941   56.6 34.6 -2.0  99
Sa 19:49.6 -20:53  17:24 -65:14 289:45   0:10 9.7077 10.017 -105.0 17.1  1.1 100
Ur 18:41.9 -23:24  51:18 -60:39 276:55  -0:18 18.864 19.401 -120.9  3.5  5.6 100
Ne 19:03.2 -21:46  40:51 -62:01 282:48   0:51 29.754 30.207 -115.8  2.1  7.9 100
Pl 15:14.8  -1:26  81:18 -10:37 226:18  15:28 28.693 29.658 -162.9  0.3 13.6 100
X   0:08.3  36:01 316:59   5:58 238:20  26:29 0.5622 0.6657  -38.4       1.5  33
Y  15:22.9  -2:40  80:43 -12:54 226:17  10:49 2.3635 3.3381 -162.5      10.1100































                                       - 27 -


Move to another field, RETURN to change this field, ? for help, or q to run

CDT 19:00:00  4/30/1990 | LST    8:19:50 | Lat   44:50:37 |      April 1990
UTC  0:00:00  5/01/1990 |                | Long  93:42:08 | Su Mo Tu We Th Fr Sa
JulianDat 2448012.50000 | Dawn      4:10 | Elev    800 ft |  1  2  3  4  5  6  7
Watch                   | Dusk     22:15 | Temp      40 F |  8 FM 10 11 12 13 14
Listing             off | NiteLn    5:55 | AtmPr 29.50 in | 15 16 17 18 19 20 21
Search              off |                | TZ     5:00:00 | 22 23 NM 25 26 27 28
Plot                off | NStep        1 | Epoch   2000.0 | 29 30
Menu      Rise/Set Info | StpSz RT CLOCK | Pause        0 |
--------------------------------------------------------------------------------
Ob  Rise Time    Rise Az  Trans Time  Trans Alt  Set Time    Set Az    Hours Up
Su     6:05       67:48     13:12       60:01     20:20      292:28     14:15
Mo    10:54       57:14     18:49       66:01      1:56      304:46     15:02
Me     6:13       63:34     13:30       62:51     20:47      296:04     14:34
Ve     4:35       93:02     10:29       42:33     16:23      267:14     11:48
Ma     3:57      103:53      9:20       34:52     14:43      256:17     10:45
Ju     9:24       54:59     17:11       68:33      1:00      305:01     15:36
Sa     1:56      119:18      6:31       24:17     11:06      240:42      9:10
Ur     1:01      123:08      5:24       21:47      9:46      236:52      8:45
Ne     1:14      120:39      5:45       23:24     10:16      239:21      9:02
Pl    19:55       91:11      1:57       43:46      7:55      268:48     12:00
X      1:41       32:26     10:51       81:10     20:00      327:20     18:19
Y     20:08       92:55      2:06       42:28      7:59      267:01     11:50


Move to another field, RETURN to change this field, ? for help, or q to run

CDT 19:00:00  4/30/1990 | LST    8:19:50 | Lat   44:50:37 |      April 1990
UTC  0:00:00  5/01/1990 |                | Long  93:42:08 | Su Mo Tu We Th Fr Sa
JulianDat 2448012.50000 | Dawn      4:10 | Elev    800 ft |  1  2  3  4  5  6  7
Watch                   | Dusk     22:15 | Temp      40 F |  8 FM 10 11 12 13 14
Listing             off | NiteLn    5:55 | AtmPr 29.50 in | 15 16 17 18 19 20 21
Search              off |                | TZ     5:00:00 | 22 23 NM 25 26 27 28
Plot                off | NStep        1 | Epoch   2000.0 | 29 30
Menu        Separations | StpSz RT CLOCK | Pause        0 |
--------------------------------------------------------------------------------
Ob  Sun  Moon   Merc  Venus  Mars   Jup    Sat  Uranus  Nep  Pluto    X     Y
Su        79:32   4:52 43:59  62:55 56:31   105:  121:   116:  163:  38:21  162:
Mo  79:32        74:49  124:   142: 23:01   175:  160:   164:  106:  99:52  108:
Me   4:52 74:49        48:43  67:39 51:50   110:  126:   121:  163:  39:57  163:
Ve  43:59  124:  48:43        18:56  100:  61:09 77:00  71:58  129:  38:49  126:
Ma  62:55  142:  67:39 18:56         119:  42:14 58:04  53:04  111:  50:48  108:
Ju  56:31 23:01  51:50  100:   119:         162:  177:   172:  128:  80:40  130:
Sa   105:  175:   110: 61:09  42:14  162:        15:52  10:50 69:37  83:36 67:17
Ur   121:  160:   126: 77:00  58:04  177:  15:52         5:11 54:43  97:18 52:21
Ne   116:  164:   121: 71:58  53:04  172:  10:50  5:11        59:06  92:24 56:46
Pl   163:  106:   163:  129:   111:  128:  69:37 54:43  59:06         125:  2:22
X   38:21 99:52  39:57 38:49  50:48 80:40  83:36 97:18  92:24  125:         124:
Y    162:  108:   163:  126:   108:  130:  67:17 52:21  56:46  2:22   124:
















                           Ephem V4.20 - August 21, 1990

                    Copyright (c) 1990 by Elwood Charles Downey
                               Chaska, Minnesota, USA

                                  [e-mail]


                                 Table of Contents
      1.  Introduction ...................................................  3
      2.  Running Ephem ..................................................  3
      2.1.  Command Line Format ..........................................  3
      2.2.  Program Operation ............................................  4
      3.  Screen Fields ..................................................  5
      3.1.  Top Screen Fields ............................................  6
      3.2.  Data format columns ..........................................  7
      3.3.  RiseSet format columns .......................................  8
      3.4.  Separation format fields .....................................  8
      4.  Date and Time Formats ..........................................  8
      5.  Configuration File .............................................  9
      5.1.  Configuration File fields ....................................  9
      5.2.  Example ephem.cfg ............................................ 11
      6.  Menu options ................................................... 11
      6.1.  Adaptive vs. Standard hzn .................................... 11
      6.2.  Geocentric vs. Topocentric ................................... 12
      7.  User Defined Objects: X and Y .................................. 12
      7.1.  Controlling Object-X or Y Operation .......................... 12
      7.1.1.  Fixed ...................................................... 13
      7.1.2.  Elliptical ................................................. 13
      7.1.3.  Parabolic .................................................. 13
      7.1.4.  Lookup ..................................................... 14
      7.1.5.  On or Off .................................................. 14
      7.2.  Magnitude models ............................................. 14
      7.2.1.  g/k model .................................................. 14
      7.2.2.  H/G model .................................................. 15
      7.3.  Database File ................................................ 15
      8.  Plotting ....................................................... 15
      8.1.  Defining plot fields ......................................... 16
      8.2.  Displaying a plot file ....................................... 16
      8.3.  Cartesian or Polar coords .................................... 16
      8.4.  Begin Plotting ............................................... 17
      8.5.  Stopping Plotting ............................................ 17
      9.  Listing ........................................................ 17
      10.  Watching ...................................................... 17
      10.1.  Trails ...................................................... 17
      10.2.  Sky dome .................................................... 18
      10.3.  Alt/az sky .................................................. 18
      10.4.  Solar System ................................................ 18
      11.  Searching ..................................................... 18
      11.1.  Find extreme ................................................ 19
      11.2.  Find 0 ...................................................... 19
      11.3.  Binary ...................................................... 19
      11.4.  Define a New function ....................................... 19
      11.4.1.  Intrinsic functions ....................................... 20









                                       - 2 -


      11.4.2.  Field Specifiers .......................................... 20
      11.4.3.  Constants ................................................. 21
      11.4.4.  Operators ................................................. 21
      11.5.  Specifying Search Accuracy .................................. 21
      11.6.  Stop ........................................................ 21
      11.7.  Example Searches ............................................ 22
      11.8.  Another Example ............................................. 22
      11.9.  Caution ..................................................... 22
      12.  Implementation Notes .......................................... 23
      12.1.  Program limits .............................................. 24
      13.  DOS Installation Procedure .................................... 24
      13.1.  Setting TZ .................................................. 24
      14.  Known Bugs and Wish List ...................................... 25
      15.  Sample Screens ................................................ 26

















































                                       - 3 -


     1.  Introduction

     Ephem is a program that displays ephemerides for all the planets plus any
     two additional objects.  The additional objects may be fixed or specified
     via heliocentric elliptical or parabolic orbital elements to accommodate
     solar system objects such as comets or asteroids.

     Information displayed about each object includes RA and Dec precessed to
     any epoch, heliocentric coordinates, local azimuth and altitude, distance
     from sun and earth, solar elongation, angular size, visual magnitude,
     illumination percentage, local rise, transit and set times, length of time
     up, and topocentric or geocentric angular separations between all
     combinations of objects.

     Observing circumstance information includes UTC and local date and time,
     local sidereal time, times of astronomical twilight, length of day and
     night, local temperature, pressure and height above sea level for the
     refraction model and a monthly calendar.

     RA/Dec calculations are geocentric and include the effects of light travel
     time, nutation, aberration and precession.  Alt/az and rise/set/transit
     and, optionally, angular separation calculations are topocentric and
     include the additional effects of parallax and refraction.

     Plot and listing files of selected field values may be generated as the
     program runs.  The plot files are full precision floating point values in
     ASCII intended for input to other plotting programs.  The listing files
     are tables formatted for general reading.  Ephem includes simple quick-
     look facilities to view these files.

     One may watch the sky or the solar system with a simple screen-oriented
     display.

     Ephem may be asked to search for interesting conditions automatically,
     using several algorithms. Most fields displayed on the screen may be used
     as terms in an arbitrary arithmetic expression that can be solved for zero
     or minimized or maximized, or the time of state change of any boolean
     expression can be found.

     The program is written in C for unix or DOS. It uses only a very simple
     set of io routines and should be easily ported to any ASCII display.

     The planetary data and correction algorithms are taken, with permission,
     from "Astronomy With Your Personal Computer", by Peter Duffett-Smith,
     Cambridge University Press, 1985.

     2.  Running Ephem


     2.1.  Command Line Format

     To run ephem, just type "ephem".  You may also specify an alternate
     configuration file, an alternate database file, and specify initial values
     for several screen fields.  The command line syntax can be summarized as









                                       - 4 -


     follows:

         ephem [-c <config_file>] [-d <database_file>] [field=value  ...]


     The default configuration file is named ephem.cfg in the current
     directory.  Ephem also looks for one named by the EPHEMCFG environment
     variable, if it is set. You may specify an arbitrary name with the -c
     option.

     The default database file is named ephem.db in the current directory.
     Ephem also looks for one named by the EPHEMDB environment variable, if it
     is set. You may specify an arbitrary name with the -d option.

     The exact format of these files is described below.

     Any additional command line arguments are treated exactly as if they too
     came from the configuration file.

     2.2.  Program Operation

     When ephem starts, it first displays a disclaimer banner.  Then, after any
     key is pressed, it reads the configuration file and processes the command
     line arguments to set the initial values of several fields, accessing the
     database file if OBJX or OBJY is set, It then draws all fields on the
     screen with their initial values.  The program then loops advancing time
     each step, by some amount you may control, and updating all fields each
     loop.

     There are three fields that control this looping behavior.  NStep controls
     the number of steps, StpSz the amount of time to add each step, and Pause
     is the amount of real seconds to pause between steps.  Ephem does not
     pause between steps when plotting or searching is on.  When the number of
     steps, NStep, goes to 0 or any key is pressed, the looping stops and you
     enter a command mode.

     Command mode allows you to modify most of the fields.  The idea is that
     you move to each field on the screen you wish to change and change it.
     When you have changed everything you want to, type "q" to resume screen
     updates.

     To change a field:

         1) move the cursor to the field (see below);
         2) type RETURN;
         3) type in the new value along the command line at the top according
            to the format indicated in the prompt. To accept the new value
            type RETURN, or to leave it unchanged after all type "q".


     A few fields don't require you to type anything; just typing RETURN does
     all the work.  If you can't move to it, you can't change it.

     The arrow keys on most systems move the cursor around.  If these do not









                                       - 5 -


     function or function incorrectly, the h/j/k/l keys also move the cursor
     left/down/up/right, respectively.  Motions off any edge of the screen will
     wrap around.

     Several "hot-keys" move the cursor immediately to frequently used fields.
     You may move the cursor immediately to a planet row by typing one of the
     characters SMevmjsunpxy.  To avoid conflict with j, jupiter's row must
     actually be typed as control-j.  "x" and "y" are for the user-defined
     objects X and y on the bottom rows.  Also, the characters c, d, o, w, L
     and z move you to the Menu, UT Date, Epoch, Watch, Listing and StpSz
     fields immediately.

     When you have changed a field that would invalidate any of the other
     fields the message NEW CIRCUMSTANCES appears in the top center of the
     screen.  This will remain until you type "q" to allow at least one screen
     update loop to occur.  If you change any field that causes new
     circumstances, the StpSz value is not added to the first loop.  Note also
     that after a series of loops, NStep is automatically reset to 1 so "q"
     will do exactly one loop and return you to command mode.

     On some systems, you may temporarily escape from ephem while in command
     mode to run your operating system's command interpretor. This is done by
     typing an exclamation point (!) followed by your command.  When the
     command completes, you will return back to ephem where you left off.

     To quit the program, type control-d from command mode.  For a little more
     help, type ?.  The entire screen may be erased and redrawn with control-l.

     3.  Screen Fields

     The screen is divided into two halves, top and bottom.  The top fields are
     always present. They define the general observing circumstances and
     control features.

     The planets and two additional objects are displayed in a table in the
     bottom portion of the screen.  There is one object per row, and several
     columns.  There are three forms of this portion selected by picking the
     Menu selection.

     Some things may be turned off to reduce compute times.  Calculations for
     each planet may be turned on and off by selecting the planet name field.
     Calculations for Dawn/Dusk/NiteLn may be turned off by selecting any of
     these fields.  Planet positions are only updated as often as necessary to
     match the display precision of the screen unless plotting or searching is
     on.  In these cases full precision is desired at all times and so
     positions are always fully recalculated at each iteration.

     Follows is a list and description of each of the fields in each section.
     Following each name a parenthetical "p" indicates the field may be
     selected for plotting (see later). All fields may be selected for
     changing.












                                       - 6 -


     3.1.  Top Screen Fields

     LTZ           the local timezone name.  The name field may be changed to
                   any three-character mnemonic.
     LT(p)
     LD(p)         The local time and date are not labeled as such but are to
                   the right of the local timezone name.  They are individually
                   selectable.  Time and date fields may be changed as
                   described in a later section.  Set to "n" to set to "now"
                   from computer clock.
     UT(p)
     UD(p)         The universally coordinated time and date are not labeled as
                   such but are to the right of the UTC label.  They are
                   individually selectable.  Time and date fields may be
                   changed as described in a later section.  Set to "n" to set
                   to "now" from computer clock.
     JulianDat(p)  the current Julian date, to about 1-second accuracy.
     Listing       controls listing; see complete discussion below.
     Watch         selects the sky dome, altitude/azimuth sky or solar system
                   displays; see complete discussion below.
     Search        controls the automatic search feature of ephem. See the
                   complete discussion below.
     Plot          controls plotting; see complete discussion below.
     Menu          controls which menu is in the bottom half of the screen.
                   See their complete discussion below.
     LST(p)        the current local sidereal time.  set to "n" to set from
                   computer clock.
     Dawn(p)       local time when the sun center is 18 degrees below the
                   horizon before sunrise today.
     Dusk(p)       local time when the sun center is 18 degrees below the
                   horizon after sunset today.
     NiteLn(p)     length of astronomical night, ie, Dawn - Dusk.  If this line
                   is shown as "-----", it means the sun is either always below
                   or always above approximately -18 degrees altitude on this
                   particular day.  This and the Dawn and Dusk lines are blank
                   when their computation has been turned off.
     NStep         The number of times the display with be updated (time
                   advanced by StpSz each step) before entering command mode.
     StpSz         the amount of time UTC (and its derivatives) is incremented
                   each loop.  set this to "r" to use real-time based on the
                   computer clock.  you may also set it in terms of days by
                   appending a "d" after the number when you set it.
     Lat(p)        location latitude, positive degrees north of equator.
     Long(p)       location longitude, positive degrees west of Greenwich
                   meridian.  set to "N" to set from computer clock.
     Elev(p)       local elevation of the ground above sea level, in feet. (see
                   implementation notes).
     Temp(p)       local surface air temperature, in degrees F.
     AtmPr(p)      local surface air pressure, in inches of mercury.
     TZ(p)         hours local time is behind utc, ie, positive west or
                   negative east of Greenwich.
     Epoch         the epoch, to the nearest 0.1 years, to which the ra/dec
                   fields are precessed.  This says (OfDate) when coordinates
                   are not precessed, ie, are in the epoch of date. Set to "e"









                                       - 7 -


                   to set to epoch of date.
     Pause         number of seconds to pause between screen updates. This is
                   used mainly to set up for free-running unattended operation.
                   This pause also applies to free-running "watch" screen
                   updates.  Pausing is not done when plotting or searching is
                   on.  If pausing is used with StpSz set to RT CLOCK and the
                   time was set with Now then ephem attempts to synchronize the
                   time to an integral multiple of pause seconds after the
                   minute, for aesthetic reasons.
     Also in the upper right of the screen is a calendar for the current local
     month.  Dates of new and full moons are marked NM and FM, respectively.

     3.2.  Data format columns
     Ob            name of object.  Select this to toggle the display and
                   calculations on and off.
     R.A.(p)       apparent geocentric right ascension of object, precessed to
                   given epoch, in hours, minutes and decimal minutes.
     Dec(p)        apparent geocentric declination of object, precessed to
                   given epoch, in degrees and minutes.
     Az(p)         degrees eastward of true north for object.
     Alt(p)        degrees up from a horizontal plane Elev feet above sea
                   level.
     H Long(p)     true heliocentric longitude, in degrees.  Earth's is
                   displayed on the sun's line.  For the moon this is the
                   geocentric longitude.
     H Lat(p)      true heliocentric latitude, in degrees.  For the moon this
                   is the geocentric latitude.
     Ea Dst(p)     true distance from Earth center to object center, in AU,
                   except distance to the moon is in miles.
     Sn Dst(p)     true distance from sun center to object center, in AU.
     Elong(p)      spherical angular separation between sun and given object,
                   calculated from the their geocentric ecliptic coordinates.
                   Note this is not just difference in ecliptic longitude. The
                   sign, however, is simply sign(obj's longitude - sun's
                   longitude), ie, degrees east.  thus, a positive elongation
                   means the object rises after the sun.
     Size(p)       angular size of object, in arc seconds.
     VMag(p)       visual magnitude of object.
     Phs(p)        percent of visible surface in sunlight. Note the moon phase
                   is calculated simplistically as just abs(elongation)/180*100
                   which can be a few degrees off... this means that because of
                   how elongation is defined it doesn't say 0 during new moon
                   (or 100 during full) except during close eclipses (maybe
                   that's a "feature"?).
     Also, some terminals scroll when a character is written to the lower right
     character position. To avoid this, Object Y's phase is left shifted by one
     column. This can look particularly ugly when the phase is 100% because the
     "100" is right next to visual magnitude number.
     If desired, the angle between Earth and Sun from the object, p, can be
     computed from the illumination percentage, i, with the following relation:

         cos (p) = i/50 - 1











                                       - 8 -


     3.3.  RiseSet format columns
     Rise Time
     Rise Az       The local time and azimuth when the upper limb of the object
                   rises today.
     Transit Time
     Transit Alt   The local time and altitude when the object crosses the
                   meridian today, ie, when its azimuth is true south or, if no
                   precession, when the local sidereal time equals the object's
                   right ascension.
     Set Time
     Set Az        The local time and azimuth when the upper limb of the object
                   sets today.
     Hours Up      The number of hours the object is up on the local date.

     Horizon displacement may be calculated in either of two ways; see the
     horizon discussion in the Menu selection section.

     Various oddball conditions are accounted for, including an object that is
     up sometime during the day but that doesn't rise, transit or set as such
     on that day, an object that is circumpolar or that is never up or one that
     rises twice on the same day.  These are marked as "Never rises", "Never
     transits", "Never sets", "Circumpolar", "Never up" or appended with a plus
     "+" sign, respectively.

     3.4.  Separation format fields
     This format is a table of angular separations between each pair of
     objects.  These angles are based on the local altitude/azimuth, and so in
     general differ somewhat from the elongations reported for the sun in the
     Data menu.
     Unfortunately, with the format "ddd:mm", there is not enough room for a
     space between columns when the angle is at least 100 degrees.  To avoid
     this, I drop the minutes portion if the (rounded) angle is at least 100
     degrees.

     4.  Date and Time Formats
     Times are displayed and entered in h:m:s format.  If you pick a time field
     to change it any of the h, m, and s components that are not specified are
     left unchanged from their current value.  For example, 0:5:0 set hours to
     0, minutes to 5, seconds to 0, whereas :5 sets minutes to 5 but leaves
     hours and seconds unchanged.  A negative time is indicated by a minus sign
     (-) anywhere before the first digit.

     Dates are displayed and entered in American month:day:year format.  As
     with time, components omitted when entering a new value retain the current
     value.  For example, if the current date is 10/20/1988 and you type 20/20
     the new date will become 20/20/1988. Note you must type the full year
     since the program is accurate over several centuries either side of 1900.
     If you change the date, the time (ie, partial day) will not change.

     Two other ways to set the date are supported for compatibility with some
     published comet ephemerides.  You may enter the day portion as a real
     number.  When you set the day this way, the time will also change to
     correspond to the fractional portion of the day.










                                       - 9 -


     You may also enter a date as a decimal year, as in 1990.12345.  This is
     also useful in interpreting plot files that include a date field, since
     date fields are stored in plot files as decimal years.  If no decimal
     point is included, the number is assumed to be a year unless it is in the
     range 1-12, in which case it will be taken to mean that you are just
     changing the month of the current date.  To actually specify the years 1 -
     12, you must append a decimal point to distinguish them from months.

     As a matter of typing convenience, the program accepts most any character
     as the separator; you don't have to type a perfect ":" or "/".

     5.  Configuration File
     The ephem.cfg configuration file allows you to set the initial values of
     many of the screen fields. You can still change any field while the
     program is running too; this file just sets the initial conditions.  Note
     that the order of entries in this file is important because they each take
     effect immediately.  You should put them in the same order you wish them
     to be processed, just as though you were changing the fields interactively
     within ephem.

     The default name of the file is ephem.cfg.  Ephem also looks for one named
     by the EPHEMCFG environment variable (if defined) or you may specify any
     name using the -c command line option.

     The format of the file uses the form KEYWORD=VALUE, where the possible
     KEYWORDS and the types of VALUES for each are described below. Any
     KEYWORDS not in the file will take on some sort of default. The separator
     need not be an actual equals sign; any char will do because the VALUE is
     assumed to start one character after the KEYWORD, regardless.

     Blank lines and lines that begin with an asterisk (*) or whitespace (space
     or tab) are ignored and may be used for comments.

     Note:  because of the way unspecified time and date components are left
     unchanged (see section on Date and Time Formats) always specify the
     complete time and date for all entries in the configuration file. For
     example, to initialize the longitude to zero degrees, say 0:0:0, not just
     0.

     5.1.  Configuration File fields
     UD        initial UTC date, such as 10/20/1988, or "NOW" to use the
               computer clock.
     UT        initial UTC time, such as 12:0:0, or "NOW" to use the computer
               clock.
     TZONE     hours the local time is behind utc, such as 5:0:0.  you need not
               set this if you use "NOW" for UT or UD.
     TZNAME    name of the local time zone, such as CDT. 3 chars max.  you need
               not set this if you use "NOW" for UT or UD.
     LONG      longitude, in degrees west of Greenwich, in the form d:m:s.
     LAT       latitude, in degrees north of the equator, in the form d:m:s.
     HEIGHT    height above sea level, in feet, such as 800
     TEMP      air temperature, in degrees F, such as 50











                                       - 10 -


     PRES      air pressure, in inches of Mercury, such as 29
     STPSZ     the time increment between screen updates, such as "1" to give
               one hour updates. this can be a specific amount or RTC to use
               the system clock as a real-time source. You may also specify a
               time in days, by appending a D (or d) after the number.
     PROPTS    this selects what you want included initially in the display.
               since IBM-PC math is not very fast, you can reduce the time to
               update the screen by only printing those fields of interest. the
               VALUE is a collection of letters to turn on each item from the
               following set:

                       T       twilight (dawn-dusk)
                       S       circumstances for the sun
                       M       circumstances for the moon
                       e       circumstances for mercury
                       v       circumstances for venus
                       m       circumstances for mars
                       j       circumstances for jupiter
                       s       circumstances for saturn
                       u       circumstances for uranus
                       n       circumstances for neptune
                       p       circumstances for pluto
                       x       circumstances for object X
                       y       circumstances for object Y

               For example, to just track the sun and saturn, say PROPTS=Ss

               If the delimiter between PROPTS and the selection is a plus (+)
               sign then the given planets are included IN ADDITION TO ones
               already specified. Any other delimiter sets the selection to
               exactly the set specified. This feature was added so that the
               command line version of using PROPTS could add to the set of
               planets giving in the configuration file.
     NSTEP     number of times program will loop before entering command mode.
               see the discussion under Program Operation.
     EPOCH     this sets the desired ra/dec precession epoch. you can put any
               date here or EOD to use the current instant ("Epoch of Date").
     OBJX
     OBJY      These fields specify the optional objects "x" and "y" by naming
               any item in the database file. The form is OBJX=xyz, where xyz
               must be in the database file, case sensitive.  You may define
               one object of each type for each of OBJX and OBJY; the last one
               defined will be the "current" one when ephem gets going.
     PAUSE     The number of seconds to pause between calculation steps. See
               definition of the Pause field in the "Top Screen Fields"
               section.
     MENU      establishes the initial bottom screen menu type.  This should be
               one of the keywords DATA, RISET or SEP.  There is no way to set
               horizon or center suboptions at this time.














                                       - 11 -


     5.2.  Example ephem.cfg

     This is the ephem.cfg file that was in effect when the sample screens (in
     another section) were generated. You might run ephem with this
     configuration file and compare with the samples as a check.

     UT=0;0;0
     UD=5/1/1990
     TZNAME=CDT
     TZONE=5
     LONG=93:42:8
     LAT=44:50:37
     HEIGHT=800
     TEMP=40
     PRES=29.5
     STPSZ=RTC
     PROPTS=TSMevmjsunpxy
     EPOCH=2000
     NSTEP=1

     OBJX=Austin
     OBJY=Juno

     As another common example, this ephem.cfg creates an essentially free-
     running real-time screen based on the computer clock:

     UT=Now
     LONG=90:10:8
     LAT=40:50:20
     HEIGHT=800
     TEMP=50
     PRES=29
     STPSZ=RTC
     PROPTS=TSMevmjsunp
     NSTEP=9999999
     EPOCH=Eod
     PAUSE=30


     6.  Menu options

     When you select "Menu" you can change among the three styles of bottom
     screens. There are also two options that can be set from the Menu quick-
     choice menu. These options toggle when picked and retain their values so
     they need only be changed when desired.

     6.1.  Adaptive vs. Standard hzn

     This selects the horizon refraction displacement algorithm used by the
     Rise/Set menu.  "Adaptive" uses the local atmospheric conditions known to
     ephem and matches the Planet Info times nicely. "Standard" uses the
     "accepted nominal" horizon refraction value of 32 arc minutes and usually
     agrees, to a minute or so, with published tables.










                                       - 12 -


     6.2.  Geocentric vs. Topocentric

     This selects the vantage point for the Separation menu. "Geocentric"
     ignores local conditions and gives the separation as seen from Earth
     center. "Topocentric" uses the local conditions known to ephem.  They are
     particularly critical for lunar occultations, but the effect can be
     significant for the planets.

     Note that searching over a period that will include the rise or set times
     of either object is generally better performed from the geocentric
     viewpoint.  The refraction effect of the topocentric viewpoint causes many
     arcminutes of rapid whiplash displacement as the objects rise and set that
     overlays the smooth celestial motion of the objects.  This rapid position
     variation can confuse the solver algorithms that expect fairly smooth
     functions.

     7.  User Defined Objects: X and Y

     You may specify one or two extra objects for ephem to use. The objects may
     be defined in three different ways:  fixed celestial sphere coordinates,
     or heliocentric elliptical or parabolic orbital elements.  Elliptical
     elements are typically useful for periodic comets or asteroids, and
     parabolic elements are for nonrecurring solar system interlopers such as
     aperiodic comets.

     The parameters for each type of object are stored separately, so you may
     switch between types of objects without losing parameters.

     7.1.  Controlling Object-X or Y Operation

     To control the type and the corresponding details for object X or Y,
     select the corresponding row near the bottom.  (Remember that typing the
     character "x" or "y" is a shorthand way to move to the bottom rows.) It
     will bring up a quick-choice menu as follows:

     Select: Fixed, Elliptical, Parabolic, Lookup, On


     When you first enter the quick-choice menu the cursor will start out
     positioned at the field for the current type of object.  The first three
     selections allow you to enter or review the various parameters required to
     define an object's position of such type, one parameter at a time.

     You set the current object type and begin to view its parameters by
     positioning the cursor over the type and pressing RETURN.  The prompt for
     each item includes a short description, the units to use, and its current
     setting is shown in parentheses. To leave the item unchanged and go to the
     next item, type RETURN.  If you do not wish to change or see any more
     items about the object then type "q" and you will return immediately to
     the object-X quick-choice menu.

     You exit the quick-choice menu by typing "q" while over any field or
     RETURN while over On or Off, as described in a later section.










                                       - 13 -


     As with all dates throughout ephem, the dates for the epochs of perihelion
     and reference epochs may be entered in month/day/year or decimal year
     formats, and the day may be entered as a real number (see the section on
     Date and Time Formats).  All dates given for comet parameters are always
     in UT.

     7.1.1.  Fixed

     This selection will present a series of five prompts, the name and one
     each for the RA, Dec, magnitude and the reference epoch for the
     coordinates of a fixed object.

     7.1.2.  Elliptical

     This will begin a series of twelve prompts asking for a name and the
     parameters that define a heliocentric elliptic orbit and the coefficients
     for either of two magnitude models.  These elements are the same ones
     often listed in the Astronomical Almanac.  The elements are, in order:

             i = inclination, degrees
             O = longitude of ascending node, degrees
             o = argument of perihelion, degrees
             a = mean distance (aka semi-major axis), AU
             n = daily motion, degrees per day
             e = eccentricity
             M = mean anomaly (ie, degrees from perihelion)
             E = epoch date (ie, time of M)
             D = the equinox year (ie, time of i/O/o)
             g/k or H/G  = either of two magnitude models; see below


     You might have other parameters available that can be converted into
     these.  For example, we have the following relationships:

             P = sqrt(a*a*a)
             p = O + o
             n = 360/days_per_year/P ~ 0.98563/P
             T = E - M/n
             q = a*(1-e)
         where
             P = the orbital period, years;
             p = longitude of perihelion, degrees
             n = daily motion, degrees per day;
             T = epoch of perihelion (add multiples of P for desired range)
             q = perihelion distance, AU

     Note that if you know T you can then set E = T and M = 0.

     7.1.3.  Parabolic

     This will begin a series of nine prompts asking for a name and the
     parameters that define a heliocentric elliptic orbit and the magnitude
     model coefficients.  These orbital parameters are, in order:










                                       - 14 -


     epoch of perihelion,
     inclination,
     argument of perihelion,
     perihelion distance,
     longitude of the ascending node,
     and the reference epoch of the parameters.
     absolute magnitude, g
     luminosity index coefficients, k


     7.1.4.  Lookup

     This option lets you define an object from any of those listed in the
     database file, described in a subsequent section.

     If successful, the cursor will move to the type of the new object and it
     becomes the current type.

     7.1.5.  On or Off

     The last selection on the right toggles the calculations for the object On
     and Off.  It toggles when selected with RETURN and then immediately exits
     the quick-choice menu back to the main menu.  If calculations become On,
     then they will be performed for the current type of object; if they become
     Off the object-X or Y row of information will be erased.

     7.2.  Magnitude models

     Ephem supports two different magnitude models. One, denoted here as g/k,
     is generally used for comets and may be used for parabolic and elliptical
     objects. The other, denoted H/G, is generally used for asteroids and is
     the one used in the Astronomical Almanac.

     7.2.1.  g/k model

     This model requires two parameters to be specified.  One, the absolute
     magnitude, g, is the visual magnitude of the object if it were one AU from
     both the sun and the earth.  The other, the luminosity index, k,
     characterizes the brightness change of the object as a function of its
     distance from the sun. This is generally zero, or very small, for inactive
     objects like asteroids.  The model may be expressed as:

         m = g + 5*log10(D) + 2.5*k*log10(r)
       where:
         m = resulting visual magnitude;
         g = absolute visual magnitude;
         D = comet-earth distance, in AU;
         k = luminosity index; and
         r = comet-sun distance.

     Note that this model does not take into account the phase angle of
     sunlight.

     When using this model for elliptical objects, the first of the two









                                       - 15 -


     magnitude fields must be preceded by a letter "g" in both the ephem.db
     database file and the corresponding quick-choice elliptical object
     definition prompt; otherwise the default is the H/G model.

     7.2.2.  H/G model

     This model also requires two parameters. The first, H, is the magnitude of
     the object when one AU from the sun and the earth. The other, G, attempts
     to model the reflection characteristics of a passive surface, such as an
     asteroid. The model may be expressed with the following code fragment:

         beta = acos((rp*rp + rho*rho - rsn*rsn)/ (2*rp*rho));
         psi_t = exp(log(tan(beta/2.0))*0.63);
         Psi_1 = exp(-3.33*psi_t);
         psi_t = exp(log(tan(beta/2.0))*1.22);
         Psi_2 = exp(-1.87*psi_t);
         m = H + 5.0*log10(rp*rho) - 2.5*log10((1-G)*Psi_1 + G*Psi_2);
       where:
         m   = resulting visual magnitude
         rp  = distance from sun to object
         rho = distance from earth to object
         rsn = distance from sun to earth

     Note that this model does not take into account the phase angle of
     sunlight.

     7.3.  Database File

     You may save a list of objects in a file to be used for setting OBJX and
     OBJY. The default name of this file is ephem.db. You may also set it from
     the command line with the -d option, or set it with the EPHEMDB
     environment variable.

     The file consists of one object per line. Lines that begin with an
     asterisk (*) are ignored.  Each line contains several fields, each
     separated by a comma.  The first field is the name of the object.  The
     second field is the type of the object, that is, one of the strings
     "fixed", "elliptical", or "parabolic"; actually, "f", "e" and "p" are
     sufficient.  The remaining fields depend on the type of object.  They are
     exactly the same parameters, and in the same order, as ephem asks for when
     defining the object from the menu.

     8.  Plotting

     Each time a field is drawn on the screen during a full screen update cycle
     (that as, during automatic looping or a manual "q" command character from
     the main menu but not from a screen redraw from control-l or when an
     individual planet is turned on or a single time field is changed) its
     full-precision value may be written to a file.  This implies you may not
     plot a field from other than the current menu at the time plotting is on.
     You can append several plot runs together, however, if necessary.

     Each line in the file consists of a tag character followed by two or three
     floating point variables, all separated by commas. If there are two









                                       - 16 -


     values, they should be interpreted to be x and y (or perhaps r and theta).
     If there is a third, it is a z or trace value.

     For efficiency on systems that can compute a screen full faster than they
     can display it, screen updates are suppressed while plotting is on and
     NStep is greater than 1. This can greatly reduce the time to generate a
     long plot file. Fields are still logged for plotting; they just are not
     drawn on the screen.

     The Plot field controls plotting.  Whether plotting is currently active is
     indicated by "on" or "off" immediately to its right.

     Picking "Plot" brings up a quick-choice menu, as follows:

     Select: Select fields, Display a plot file, Cartesian coords, Begin plotting


     8.1.  Defining plot fields

     Select the "Select fields" option.  You will be asked to move the cursor
     to the field you want to use as the x coordinate (abscissa), then asked to
     choose the y coordinate (ordinate), then asked to choose an optional z
     trace variable and finally a tag character.  (Of course, you may have in
     mind fields that are more appropriately displayed in other than Cartesian
     coordinates, in which case think of x, y and z as dimensions.) If you type
     q for either x or y then no more fields will be defined.  If you type q
     for the z field there will be no z field.  You can not label a plot line
     with the letter "q" at this time.

     This then repeats so you may choose up to ten of these sets for any given
     plot run.  Each set defines what will become a line on the final plot.
     Note that you may select the "Search" field to indicate use of the current
     search function; that function must be defined by the time plotting is
     turned on.

     If you turn plotting off and back on the fields selected for plotting are
     reactivated the same as they were last time. You may change them if
     desired, of course, but there is no need to redefine them if you do not
     wish to change them.

     8.2.  Displaying a plot file

     Select the "Display a plot file" option to generate a crude plot on the
     screen of an existing plot file previously created by ephem.  The entries
     in the file will be drawn on the screen using their tag characters; the
     plot remains on the screen until you type any character.

     The plot may be made in polar or Cartesian coordinates, depending on the
     setting of the plotting mode in the quick-choice (see next section).

     8.3.  Cartesian or Polar coords

     This toggles the plotting mode coordinate system.  The mode remains until
     changed.  Polar coordinates assume the first numeric field in the plot









                                       - 17 -


     file is the radius, and the second is the angle counterclockwise from
     right, in degrees.

     8.4.  Begin Plotting

     If plot field lines are defined then the third option, "Begin plotting"
     will be available.  You will be asked for the name of the file to use.  If
     it already exists you will be asked whether to overwrite it or append to
     it.  You will also be asked for a title line that will become the next
     line of the file, automatically prefixed with a "*" to make it a valid
     comment line.  Once you have chosen a file, plotting is on and the top
     menu plotting status field changes to "on".  The default plot file name is
     ephem.plt.  The values are written to the plot file each time they are
     updated on the screen until you select "Plot" again and select the "Stop"
     option to turn plotting back off.

     8.5.  Stopping Plotting

     If plotting is on, then selecting the Plot field in the top section will
     turn plotting off. You may pick Plot again and resume with the same fields
     by selecting "Begin plotting" again.

     Note that due to internal buffering the plot file will not be completely
     written to disk until plotting is turned off.

     9.  Listing

     This feature works very much like Plotting. However, the fields you select
     define columns of a table generated as ephem runs. These columns are look
     exactly like their corresponding fields on the ephem screen and so are far
     more readable than plot files.

     See the section on Plotting for an explanation of the quick-choice menu.
     The general operation is very much the same.

     10.  Watching

     You may generate a simple drawing on the screen of the local sky in two
     forms or the solar system by selecting "Watch".  It will bring up a
     quick-choice menu as follows:

     Select: Sky dome, Alt/az sky, Solar system, No trails


     10.1.  Trails

     You may either erase after each iteration or leave the tags up, referred
     to as "trails". Picking the right-most choice will toggle between "No
     trails" and "Leave trails"; you should set it as desired before you select
     the style of sky plot you wish.  Ephem will remember your selection.













                                       - 18 -


     10.2.  Sky dome

     This draws the currently active planets within a circle that represents
     the locally visible hemisphere.  This is similar to the formats commonly
     used in the popular astronomy magazines.  East is left, south is down,
     west is right and north is up.

     10.3.  Alt/az sky

     This is a cylindrical projection showing the currently active planets as
     they would appear in the sky looking southwards at the current time and
     date.  The coordinate system is such that 0 degrees azimuth (north)
     through 360 degrees (north, once around) is mapped to the horizontal
     screen dimension, and 0 degrees altitude (level) through 90 degrees (the
     zenith) is mapped to the vertical dimension. Thus, the bottom row is the
     horizon and all across the top is the zenith.

     10.4.  Solar System

     This selection draws the currently active planets as they would appear
     looking "down from the top" of the ecliptic, with the sun at the center
     and zero hours right ascension towards the right.  The scale is adjusted
     to roughly fill the screen according to the outer-most active planet.
     Note that the scaling does not take into account the distances of the user
     defined objects so, if they don't appear, select a planet that is at least
     as far out as they are.  The screen transformation assumes a screen aspect
     width/height ratio of 4/3.  Down the left column of the screen is the
     heliocentric altitude of the planet above or below the ecliptic, drawn to
     the same scale as the circular display.  Values so close as to land on the
     same line are sorted left to right; the S and E symbols always denote
     heliocentric altitude 0.

     In each style of display, pressing RETURN advances the time by whatever
     amount StpSz is set to.  Pressing "h" advances the time by one hour, "d"
     advances by one day, and "w" advances by one week (seven days).  Pressing
     "q" returns to the watch quick-choice menu from which you may select
     another display style or return to the main tabular display by typing
     another "q".  Pressing any other key starts an automatic loop with each
     step advancing by StpSz; pressing any key stops the looping.

     As symbols are placed, collisions (overstrikes) are avoided by moving
     characters in such a way as to maintain increasing sorted order towards
     the right.

     When you return to the main menu, the last watched time will be maintained
     as the current time.  The StpSz is not changed.

     11.  Searching

     Ephem can search for arbitrary conditions to exist among most displayed
     fields.  You first enter a function, then select from among three forms of
     equation solvers to iteratively solve for the next time when the function
     meets the requirements of the solver. The solver selects the next time for
     which it wants the function evaluated and sets StpSz so that the next









                                       - 19 -


     iteration will occur at that time. The solvers continue to iterate until
     either they achieve their goal or NStep reaches 0.

     You may set NStep to be quite large and let ephem search unattended or set
     it to 1 and watch it converge one step at a time. You may also plot at the
     same time as search to record the exact steps ephem took to converge.
     (But recall that screen updates are suppressed if plotting is also on).

     The "Search" selection in the top half of the screen controls all
     searching.  Picking it brings up a quick-choice menu as follows:

     Select: Find extreme, Find 0, Binary, New function, Accuracy


     11.1.  Find extreme

     This search algorithm searches for a local maximum or a minimum in the
     search function, whichever it finds first. It begins by evaluating the
     search function at the current time then for two more times each separated
     by StpSz. It then fits these three points to a parabola and solves it for
     the time of its maximum (or minimum). StpSz is set so that the next
     iteration will evaluate at this point.  This parabolic fit solution keeps
     repeating until StpSz changes by less than the desired accuracy or until
     the curve becomes so flat that an extrema appears too broad to find.

     11.2.  Find 0

     This search algorithm uses the secant method to solve for the time at
     which the search function is zero. The function is evaluated at the
     current time and then again StpSz later to establish a slope for which the
     x-intercept is found as the next zero guess. This is used to set StpSz for
     the next desired time value and the slope hunting process repeats until
     StpSz changes by less than the desired accuracy.

     11.3.  Binary

     This search algorithm must be used with a search function that yields a
     boolean result, ie, a true or false value. The idea is that the function
     is assumed to be one truth value when evaluated at the present time, and
     the opposite truth value when it is evaluated StpSz later. The algorithm
     will then do a binary search for the time when the truth value changes.

     The binary algorithm does not begin until the state change is bounded in
     time.  Initially, as long as the truth value at StpSz is the same as the
     previous value the algorithm will just keep moving in time by StpSz
     looking for when the state changes.  That is, a linear search is initiated
     to bound the state change, then the binary search proceeds.

     11.4.  Define a New function

     Select "New function" to display the current search function.  If you type
     "q" it will be left unchanged.  If you type RETURN it will be erased.  If
     you type anything else it will be compiled and, if there are no errors, it
     will become the new search function.  Once a valid function has been









                                       - 20 -


     stored, it will remain unless changed.  If a search function is selected
     and there is as yet no valid search function defined, you will
     automatically be asked to enter one as though you had selected "New
     function."

     A search function consists of intrinsic functions, field-specifiers,
     constants and operators, and precedence may be overridden with
     parentheses.


     11.4.1.  Intrinsic functions

     In this release, the only intrinsic function available is abs(), which
     returns the absolute value of its argument.

     11.4.2.  Field Specifiers

     A field in the bottom half of the menu is specified in the form of
     "object_name.column_name". The object_name is enough of the planet name to
     be unique; use "x" or "y" for the user-specified object X or Y.  The
     column_name is from the following table, depending on which menu is up. In
     all cases additional characters may be entered but are ignored.

     Planet Data Menu        Rise/Set Menu               Separation Menu
     ------------------      --------------------        ---------------
     al      Alt             hr      Hrs Up, or          j       Jup
     az      Az              hu      Hrs Up              ma      Mars
     d       Dec             raz     Rise Az             me      Merc
     ed      Ea Dst          rt      Rise Time           mo      Moon
     el      Elong           saz     Set Az              n       Nep
     hla     Helio Lat       st      Set Time            pl      Pluto
     hlo     Helio Long      ta      Transit Alt         sa      Saturn
     ph      Phs             tt      Transit Time        su      Sun
     ra      R.A.                                        u       Uranus
     sd      Sn Dst                                      ve      Venus
     si      Size
     vm      VMag


     In addition, the following top-half fields may be used:

     da      Dawn
     du      Dusk
     n       NiteLn



     Remember, searching may only involve fields being calculated for display
     at the time the solver is active.  While you can syntactically include any
     field in a search function it is useless to define a search that uses
     fields from other than the menu that is selected at the time the search is
     running.











                                       - 21 -


     11.4.3.  Constants

     Constants may be integers or floating point numbers. The latter may be
     expressed in scientific notation if desired. Examples include 100, .9,
     1.234, 1e10 and 1.2e-4.  Any number may be preceded by - to make it
     negative.

     11.4.4.  Operators

     The collection of arithmetic, relational and boolean operators provided
     mimics those of C language as listed in the following table, in decreasing
     order of precedence.  Operators grouped together have the same precedence
     and all have left-to-right associativity. Parentheses may be used as
     desired.

     Symbol  Meaning                 Resulting type
     ------  --------------------    --------------
     *       multiply                arithmetic
     /       divide                  arithmetic

     +       add                     arithmetic
     -       subtract                arithmetic

     >       greater than            boolean
     >=      greater than or equal   boolean
     <       less than               boolean
     <=      less than or equal      boolean

     ==      equality                boolean
     !=      inequality              boolean

     &&      logical and             boolean

     ||      logical or              boolean


     11.5.  Specifying Search Accuracy

     Selecting "Accuracy" allows you to specify when the search will stop.  The
     search algorithms will stop when StpSz becomes equal to or less than this
     value. The default is one minute.  If ephem has not yet converged to the
     specified accuracy but NStep has decremented to 1, the searching will stop
     but the search status field will still indicate which search procedure is
     in effect. To try more iterations you may increase NStep and resume
     searching. If the accuracy was achieved, the search status field will
     switch to "off" with the number of "unused" steps remaining in NStep and
     the last step size in the StpSz fields.

     11.6.  Stop

     If searching is on, this option will also appear on the quick-choice menu
     and may be selected to turn off the search.











                                       - 22 -


     11.7.  Example Searches

     As an example, let's find when Pluto again becomes the furthest planet
     from Sol.  You may find when the difference in their sun distance is zero,
     or you might use a binary search on the condition that Pluto's sun
     distance is larger then Neptune's.

     To try the former approach select Search, select "Find 0", specify the
     search function to be:

     pl.sd - nep.sd

     set StpSz to something large like 10d, NStep to allow several iterations
     like 20, and then type "q" to start the search and watch ephem do the
     hunt.  Ephem will settle on about 21:02 1/10/1999 UT.

     To try a binary search, you first need to have some idea of when the event
     will occur so you can eliminate the initial linear search for the state
     change. We can start at, say, 1/1/1999, set StpSz to 30d, select Binary
     search, specify the search function to be:

     pl.sd > nep.sd

     and go. Once it brackets the state change note how StpSz keeps being cut
     in half but can go in either direction (sign) as it divides each interval
     in half.  Ephem will converge on the same answer.

     11.8.  Another Example

     To find the time of last quarter moon during December, 1989, use the "Find
     0" search algorithm to solve "moon.el + 90".  (At last quarter, the moon
     is 90 degrees west of the sun, or -90 east in ephem's elongation display.)
     Set the initial time to mid-month, 12/15/1989, StpSz to 1 day and NStep to
     10. Ephem takes only a few iterations to settle on 23:57 12/19 UT.

     11.9.  Caution

     Beware that most celestial phenomena are generally pseudo-periodic in
     nature.  In early search steps ephem can easily skip over a local maxima
     and find a later one, which, while correct, may not be what was desired.
     In general, the closer you can be when you start the search the better
     ephem can refine it; it is not as good with very broad searches that can
     go "wild". Set StpSz large enough to offer significant change in the
     function value, but small enough not to skip too far.

     For example, Saturn and Neptune had three close approaches during 1989.
     If you did not know this then just asking ephem to find a minimum would
     have produced different results depending on the starting conditions.
     When starting a search for a certain class of event it is a good idea to
     first use the plotting or watching facility of ephem to get a broad
     picture of the general circumstances then use ephem's search facility to
     refine a given region (or create and inspect a plot file and do your own
     interpolation directly from it separately).










                                       - 23 -


     Similarly, ephem's searching techniques are not good for eclipses because
     the moon and sun are close every month; the trick is sorting through the
     frequent conjunctions for ones that are particularly close. One needs a
     way of establishing an envelope fit to the local extrema of a cyclic
     function in order to find a more global extreme.

     12.  Implementation Notes

     Remember that everything is for the current local time and day. So, for
     example, the calendar marks moon events in local time; commercial
     calendars usually mark the UT date.  Similarly, the rise/set times are for
     the current local day.

     The program uses a horizontal plane tangent to the earth as the horizon
     for all altitude calculations, rise/set events, etc.  This is not the same
     as the angle up from the local horizon unless the observer is directly on
     the ground due to earth's curvature.  The effect can be found from:

             sin(a)**2 = (h**2 + 2Rh) / (R+h)**2
         where:
             R = radius of earth
             h = height above ground (same units as R)
             a = increase in altitude

     For example, the effect is more than two arc minutes at a height of 5
     feet.

     Visual magnitudes are not very accurate ... I haven't bothered to fix.

     The accuracy of ephem can not be specifically stated since the Duffett-
     Smith book does not warrant its planet position polynomials to any given
     degree. I know for sure that better accuracy could be achieved if ephem
     used TDT but I have not yet decided on a suitable algorithm.  Allowing for
     this manually, (see the Wish List section) comparisons with the
     Astronomical Almanac are often within a few arcseconds.

     The program uses double precision throughout. While this precision might
     seem a little ridiculous, it is actually more efficient for most
     traditional K&R C compilers and the search functions seem to be are far
     more stable.

     Searching and plotting always use full precision but if neither of these
     are turned on pure display and watching only recompute a given planets new
     location if the time has changed enough to effect the required display
     precision, based on the planets mean apparent orbital motion.

     The sun-moon distance is the solution for the third side of a planar
     triangle whose two other sides are the earth-moon distance and earth-sun
     distance separated by the angle of elongation.

     Beware of specifying a year of 0, or of computing with the user-defined
     objects before they are properly defined. These conditions can cause ephem
     to blow.










                                       - 24 -


     12.1.  Program limits

     The search function is limited to a maximum of 32 instructions (each
     constant, field spec, and operation is one instruction), with no more than
     a total of 16 constants and field specs. At run time, the function can not
     require more than 16 stacked values (due to operator precedence or
     explicit parenthetical expressions) to evaluate.

     No more than 32 different fields can be tracked simultaneously for
     plotting and/or searching.

     No more than 10 lines may be plotted at once.

     The maximum file name length is 14 characters.

     13.  DOS Installation Procedure

     You must be running DOS V2.0 or later, though somewhere between V2.0 and
     V3.21 the behavior of control-c to terminate the program was fixed.  An
     8087 floating point chip will be used if present.

     The distribution floppy contains five files:
     README describes last minute items and details of this release.
     MAN.TXT is this manual, hopefully formatted and printable on most any
     printer.
     EPHEM.EXE is the executable program.
     EPHEM.CFG is a sample configuration file.
     EPHEM.DB is a sample database.
     To run the program, make working copies of these files in a directory and
     run "ephem" from that directory.

     13.1.  Setting TZ

     Before running ephem, you should set a DOS environment variable, TZ.  It
     is is used to establish the timezone name and hours offset whenever the
     "Now" shorthand is used from ephem, either from the configuration startup
     file or whenever any time field is changed manually.  Set it in the
     following form:

          set TZ=SSSnDDD

     where

     SSS  is the 3-letter abbreviation for the local standard timezone;

     n    is a number between -23 to 24 indicating the number of hours that are
          subtracted from GMT to obtain local standard time;

     DDD  is an optional 3-letter abbreviation for the local daylight savings
          time zone name. Leave it off if you do not have savings time in your
          area or it is not currently in effect. If the changeover dates differ
          from the internal algorithm, just use SSS and n directly.

     For example, in the midwestern United States with savings times set









                                       - 25 -


     TZ=CST6CDT

     If for some reason your system does not change to savings time at the
     right time, then omit the DDD parameter and just set the SSS and n to
     exactly what you want.

     You can put this in your AUTOEXEC.BAT file so it gets set each time you
     boot DOS.

     14.  Known Bugs and Wish List

     incorporate Terrestrial Dynamical Time (known as Ephemeris Time prior to
     1984).  TDT is about 57 seconds ahead of UT1 in 1990.

     it's too easy to turn on objx/y before it's defined and bomb out.

     add explicit searching for eclipses and occultations.

     in watch mode, RTC reverts back to being based off the time when watch was
     first entered.

     add a facility (or tool) to find g/k from a set of observed magnitudes.

     find a better precession algorithm; current one exhibits some hysteresis
     and isn't all that accurate in general.

     add search criteria for database objects.

     add plot options to reverse x and/or y direction.

     handle year 0 properly.

     add moons.






























                                       - 26 -


     15.  Sample Screens

     Here are sample ephem screens. They are generated using the first sample
     ephem.cfg file (listed in the section describing the configuration file).
     There is one for each of the three possible screen formats.  The rise/set
     screen was done using the Adaptive option.  The separations screen was
     done using the Topocentric option.

Move to another field, RETURN to change this field, ? for help, or q to run

CDT 19:00:00  4/30/1990 | LST    8:19:50 | Lat   44:50:37 |      April 1990
UTC  0:00:00  5/01/1990 |                | Long  93:42:08 | Su Mo Tu We Th Fr Sa
JulianDat 2448012.50000 | Dawn      4:10 | Elev    800 ft |  1  2  3  4  5  6  7
Watch                   | Dusk     22:15 | Temp      40 F |  8 FM 10 11 12 13 14
Listing             off | NiteLn    5:55 | AtmPr 29.50 in | 15 16 17 18 19 20 21
Search              off |                | TZ     5:00:00 | 22 23 NM 25 26 27 28
Plot                off | NStep        1 | Epoch   2000.0 | 29 30
Menu        Planet Data | StpSz RT CLOCK | Pause        0 |
--------------------------------------------------------------------------------
Ob   R.A.    Dec    Az     Alt  H Long H Lat  Ea Dst Sn Dst Elong  Size VMag Phs
Su  2:32.3  14:58 278:40  12:38 220:22        1.0075               1905  -27
Mo  8:09.9  21:11 186:06  65:53 119:55   1:04 234821 1.0071   79.5 1897  -12  44
Me  2:49.4  17:39 277:48  17:26 214:08   1:43 0.5764 0.4360    4.9 11.7  1.6   1
Ve 23:49.4  -2:25 296:53 -27:39 282:39  -1:30 0.9288 0.7276  -43.9 18.2 -4.8  64
Ma 22:39.8 -10:09 308:17 -44:14 297:56  -1:43 1.5438 1.4067  -62.9  6.1  0.3  89
Ju  6:30.9  23:23 235:13  59:04 106:16   0:08 5.6806 5.1941   56.6 34.6 -2.0  99
Sa 19:49.6 -20:53  17:24 -65:14 289:45   0:10 9.7077 10.017 -105.0 17.1  1.1 100
Ur 18:41.9 -23:24  51:18 -60:39 276:55  -0:18 18.864 19.401 -120.9  3.5  5.6 100
Ne 19:03.2 -21:46  40:51 -62:01 282:48   0:51 29.754 30.207 -115.8  2.1  7.9 100
Pl 15:14.8  -1:26  81:18 -10:37 226:18  15:28 28.693 29.658 -162.9  0.3 13.6 100
X   0:08.3  36:01 316:59   5:58 238:20  26:29 0.5622 0.6657  -38.4       1.5  33
Y  15:22.9  -2:40  80:43 -12:54 226:17  10:49 2.3635 3.3381 -162.5      10.1100































                                       - 27 -


Move to another field, RETURN to change this field, ? for help, or q to run

CDT 19:00:00  4/30/1990 | LST    8:19:50 | Lat   44:50:37 |      April 1990
UTC  0:00:00  5/01/1990 |                | Long  93:42:08 | Su Mo Tu We Th Fr Sa
JulianDat 2448012.50000 | Dawn      4:10 | Elev    800 ft |  1  2  3  4  5  6  7
Watch                   | Dusk     22:15 | Temp      40 F |  8 FM 10 11 12 13 14
Listing             off | NiteLn    5:55 | AtmPr 29.50 in | 15 16 17 18 19 20 21
Search              off |                | TZ     5:00:00 | 22 23 NM 25 26 27 28
Plot                off | NStep        1 | Epoch   2000.0 | 29 30
Menu      Rise/Set Info | StpSz RT CLOCK | Pause        0 |
--------------------------------------------------------------------------------
Ob  Rise Time    Rise Az  Trans Time  Trans Alt  Set Time    Set Az    Hours Up
Su     6:05       67:48     13:12       60:01     20:20      292:28     14:15
Mo    10:54       57:14     18:49       66:01      1:56      304:46     15:02
Me     6:13       63:34     13:30       62:51     20:47      296:04     14:34
Ve     4:35       93:02     10:29       42:33     16:23      267:14     11:48
Ma     3:57      103:53      9:20       34:52     14:43      256:17     10:45
Ju     9:24       54:59     17:11       68:33      1:00      305:01     15:36
Sa     1:56      119:18      6:31       24:17     11:06      240:42      9:10
Ur     1:01      123:08      5:24       21:47      9:46      236:52      8:45
Ne     1:14      120:39      5:45       23:24     10:16      239:21      9:02
Pl    19:55       91:11      1:57       43:46      7:55      268:48     12:00
X      1:41       32:26     10:51       81:10     20:00      327:20     18:19
Y     20:08       92:55      2:06       42:28      7:59      267:01     11:50


Move to another field, RETURN to change this field, ? for help, or q to run

CDT 19:00:00  4/30/1990 | LST    8:19:50 | Lat   44:50:37 |      April 1990
UTC  0:00:00  5/01/1990 |                | Long  93:42:08 | Su Mo Tu We Th Fr Sa
JulianDat 2448012.50000 | Dawn      4:10 | Elev    800 ft |  1  2  3  4  5  6  7
Watch                   | Dusk     22:15 | Temp      40 F |  8 FM 10 11 12 13 14
Listing             off | NiteLn    5:55 | AtmPr 29.50 in | 15 16 17 18 19 20 21
Search              off |                | TZ     5:00:00 | 22 23 NM 25 26 27 28
Plot                off | NStep        1 | Epoch   2000.0 | 29 30
Menu        Separations | StpSz RT CLOCK | Pause        0 |
--------------------------------------------------------------------------------
Ob  Sun  Moon   Merc  Venus  Mars   Jup    Sat  Uranus  Nep  Pluto    X     Y
Su        79:32   4:52 43:59  62:55 56:31   105:  121:   116:  163:  38:21  162:
Mo  79:32        74:49  124:   142: 23:01   175:  160:   164:  106:  99:52  108:
Me   4:52 74:49        48:43  67:39 51:50   110:  126:   121:  163:  39:57  163:
Ve  43:59  124:  48:43        18:56  100:  61:09 77:00  71:58  129:  38:49  126:
Ma  62:55  142:  67:39 18:56         119:  42:14 58:04  53:04  111:  50:48  108:
Ju  56:31 23:01  51:50  100:   119:         162:  177:   172:  128:  80:40  130:
Sa   105:  175:   110: 61:09  42:14  162:        15:52  10:50 69:37  83:36 67:17
Ur   121:  160:   126: 77:00  58:04  177:  15:52         5:11 54:43  97:18 52:21
Ne   116:  164:   121: 71:58  53:04  172:  10:50  5:11        59:06  92:24 56:46
Pl   163:  106:   163:  129:   111:  128:  69:37 54:43  59:06         125:  2:22
X   38:21 99:52  39:57 38:49  50:48 80:40  83:36 97:18  92:24  125:         124:
Y    162:  108:   163:  126:   108:  130:  67:17 52:21  56:46  2:22   124: