The Sun is about 400 times greater then the Moon but its distance from us is about 400 times greater.
It is the reason that both the Moon and the Sun appear to be approximately of the same angular
size in the sky. Neither the Earth's orbit around the Sun nor Moon's orbit around the Earth
are circles. It means that the distances Earth - Sun and Moon - Earth are changing.
Booth the Moon and the Sun can be of greater angular size then the other. It means that three types of
solar eclipse can be observed on a particular observing place at particular observing time - partial,
total, annular.
The lunar shadow consists of three parts - umbra, penumbra and anti-umbra.
The darkest part of the shadow is called umbra. If we find ourselves in umbra
we can observe total solar eclipse. Anti-umbra and penumbra are brighter parts of the lunar shadow
with some amount of sunshine. If we find ourselves in anti-umbra we can
observe annular eclipse and only partial eclipse can be seen in penumbra.
The Moon is illuminated by the Earth during the total eclipse. This illumination (reflected
sunshine) is strong enough to enable observing of lunar surface details.
The extreme contrast between dark lunar surface and bright inner solar corona makes the
observation of lunar surface during total eclipse very difficult.
The image on the page top is computer made montage of
corona
composite image and Full Moon image. We are not able to see the Moon so clearly during
total solar eclipse, still the image itself is not very far from reality.
Types of Solar Eclipses
If we consider the solar eclipse from global point of view we can distinguish four types of solar
eclipse.
1) The eclipse is called partial if only the penumbra strikes the Earth during the eclipse.
Partial eclipse can be seen in some part of Earth's surface. Neither total nor annular
eclipse can be seen in that case on any particular place all over the world during the whole eclipse.
2) The eclipse is called total if only umbra and penumbra strike the Earth's surface during the eclipse.
Not only partial eclipse can be seen in some parts of Earth's surface but
there exist places where total eclipse can be seen.
3) The eclipse is called annular if only anti-umbra and penumbra strike the Earth's surface during the eclipse.
In this case the eclipse seen from a particular place on Earth's surface can be partial or annular.
This type of eclipse is very rear.
4) The eclipse is called hybrid if umbra, anti-umbra and penumbra strike the Earth's surface during the eclipse.
This type of eclipse is very rear again. This type of eclipse occurs if there is a point on the surface
of Earth where the angular size of the Moon a the Sun are exactly the same at the moment of total
eclipse. Because the Earth is round the distance from observing place to Moon is different for different
observing places. It causes that the annular size of the Moon changes slightly during the shadow
passing the Earth's surface. Therefore, the eclipse can be total on some place and annular on an other place.
Central and Noncentral eclipses
The eclipse is called central if the line defined by the solar center and lunar center
(axis of the lunar shadow) intersects the Earth. It is obvious that a central eclipse can
be total, annular or hybrid because umbra or antiumbra must strike the Earth's surface.
If the axis of the lunar shadow has no intersection with the Earth the eclipse is called
noncentral and usually it is a partial eclipse. Noncentral eclipse can be total or annular
too but it is very a rear case and the area from which total or annular eclipse are visible is
very small.
Path of Totality
During a total eclipse, umbra moves across the Earth's surface from the West to the East
and its trace is called the path of totality. The total eclipse can be seen
only within this zone. The path of totality is 270 km wide maximum (during 1999 Eclipse only
112 km). The zone within the partial eclipse can be seen is much wider (about 7,000 km).
Duration of Total Solar Eclipse
Total solar eclipse is unfortunately a very short-lasting phenomenon. The longest possible duration
of it is 7 min 31 sec. The total eclipse of August 11, 1999 was long 2 min 22.9 sec (maximum
in Romania). If we take into consideration that there is one single total eclipse during 1 to 2
years and on one particular place there is a total eclipse only once during 360 years on the average,
it is obvious that total solar eclipse is extremely rare.
Saros Cycle
Solar eclipses (also
Lunar Eclipses) recur periodically.
This periodicity was already known to ancient civilizations but it was not easy to understand
the periodicity in its full complexity. The recurrence of eclipses is governed by so-called
Saros cycle which is the key to understanding the eclipse periodicity. The word "saros" was used
first of all by Edmund Halley and it means the key in the language of ancient Babylonians.
The Saros cycle is of period S = 6585 days 7 hours 42 minutes. The period converted to years
is 18 years and 10.3 days or 11.3 days (it depends on number of leap years). The saros period
can be derived from following three periods.
Synodic month M = 29.53059 days = 29d 12h 44m
Draconic month D = 27.21222 days = 27d 05h 06m
Anomalistic month A = 27.55455 days = 27d 13h 19m
Synodic month is New Moon to New Moon period, draconic month is node to node period
(nodes are intersections of ecliptics and lunar orbit) and
anomalistic month is perigee to perigee period. The Saros period is the least entire product
of these three periods
M,
D,
A.
S = 223 M S = 242 D S = 239 A (*)
These equations are valid with a very small error of several hours. It means that two eclipses
separated by one Saros period are very similar from geometrical point of view. In addition,
these eclipses are nearly at the same time of a year (10 or 11 days shift). The Saros period
S = 6585 days 7 hours 42 minutes is not equal to a whole number of days. The Earth
makes approximately 1/3 of its revolution during these 7 hours 42 minutes. This results
the longitude shifting of each successive eclipse in Saros cycle by approximately 120░ westward.
However, successive eclipses in Saros cycle are shifted not only in longitude. More important
is the latitude shift caused by small inaccuracy in equation
S = 242 D. This inaccuracy
causes that the Saros cycle does not last for ever because after 70 to 80 Saros periods the Moon
is so far from the node that the shadow passes above or below our planet without any eclipse.
It means that each Saros cycle (series of eclipses) lasts about 12 - 13 centuries. It was not
easy to understand this organization to Saros "families" of similar eclipses because observation
for very long time and various places are necessary to obtain suitable data.
Two types of Saros cycle
Ecliptics and lunar orbit represent two circles on celestial sphere. Points of intersection of these
two circles are called nodes. The node in which the Moon is moving from position under ecliptics
to position above ecliptics is called ascending node, the opposite one is called descending node.
The Moon is moving on celestial sphere slower than stars, i.e. each successive day it rises
about one hour later than the day before. It means that the Moon is moving relative to stars
and nodes eastward. The solar eclipse occurs if the New Moon is near (less than about 18░) to
ascending or descending node. Successive eclipse in Saros series has not exactly the same geometry
as the previous one because of Moon's relative shift to the node. If we compute precisely we realize that
the equation
223 M = 242 D is in reality inequality
223 M > 242 D.
The Moon is slightly late on its orbit when one Saros period has passed. Its time lag is
223 M - 242 D = 0.03567 day.
This time lag causes the shift of about 0.5░ relative to node. Therefore the Moon occurs
within "eclipse zone" only for about 70 - 80 Saros periods. During that time there are about
50 central eclipses, the other eclipses are noncentral, usually only partial ones. The scenario
of Saros series differs if the Moon is near ascending node or descending
node during series.
Saros cycle at descending node
The series begins when the New Moon occurs about 18 degrees eastward from the descending node.
The first eclipse is noncentral and umbra of lunar shadow passes about 3,500 km
below the Earth (for an observer standing on the northern hemisphere). Only small partial
eclipse is visible from the South polar region in that case. On the following eclipse of series,
the Moon shifts 0.5░ closer to node and umbra passes about 300 km closer to the Earth.
It takes about 11 Saros periods before the first central eclipse occurs near
the South pole. This first central eclipse starts the series of about 50 central
eclipses. Each successive eclipse has the path of totality displaced northward
by approximately 300 km. Eclipses of the longest duration will occur near
the equator. Later on the path of totality shifts more and more northward and
finally the Saros series ends at the
north pole about 13 centuries after its beginning.
Saros cycle at ascending node
The series begins when the New Moon occurs about 18 degrees eastward from the ascending node.
The first eclipse is noncentral and umbra of lunar shadow passes about 3,500 km
above the Earth, i.e. small partial eclipse is visible from the north polar region in that case.
Each successive eclipse in Saros series the lunar shadow shifts southward
of about 300 km. The series consists of about 11 noncentral eclipses,
about 20 central ones later on and finally about 11 noncentral eclipses.
The Saros series ends at the south pole about 13 centuries after its beginning.
Number of Saros series simultaneously in progress
Approximately forty different Saros series are simultaneously in progress
producing 2 - 5 solar eclipses every year. Thirteen centuries old series terminate
and are replaced by new ones. For instance, during the second half of the twentieth century,
there were 41 different Saros series in progress and 26 of them are producing central eclipses.
One of them was
total solar eclipse of August 11, 1999.
