SOLAR SYSTEM ORBITS

An orbit is the curved path of a planet, satellite (moon) or other body such as a comet or asteroid. Kepler in 1609 discovered that the orbits of the major planets are not circular but elliptical.

Shape

Astronomers use several terms to describe the shape of an orbital path. The dimensions of an ellipse are determined by its long axis called the major axis and by the short axis which is called the minor axis. Many orbits are nearly circular and the distances of the major and minor axes are almost, but not quite, the same. In a scaled down representation the orbit will be indistinguishable from a circle. To make comparisons between such orbits a lot easier, eccentricity or 'departure from circularity' is calculated.

Eccentricity is worked out by dividing the distance between the centre of an ellipse and one of its focal points, by the length of its semimajor axis (half the length of its long, or major axis). A perfect circle has an eccentricity of 0, while a parabola has an eccentricity of 1.

Though the orbits of the Solar System planets are sometimes represented as being in the same plane, they are actually inclined at angles to one another. The inclination of an orbital path is measured as the angle between its plane and the plane of the ecliptic (the plane of the Earth's orbit round the Sun).

Most planets have inclinations of just a few degrees. But some comets, Hale-Bopp for example, have a path which crosses the plane of the ecliptic at a right angle.

Movement

The fundamental principles of the movement of orbiting bodies were worked out by Kepler and are embodied in his three laws.

1. Planets move around the Sun in ellipses. The Sun forms one focus of the ellipse, while the other is empty.

2. A line drawn between the planet and the Sun (radius vector) sweeps out equal areas in an equal amount of time. To do this the planet must move fastest when it is near the Sun.

3. The square of the time taken for a planet to complete an orbit equals the cube of the planet's distance from the Sun. For example, a body 4 AU (astronomical units) from the Sun would take 8 years to move around the Sun. (43 = 64, 64 = 82).

When comparing the direction of movement, the terms retrograde and prograde are used. Prograde means that an object is moving on a path in the same direction as which the body being orbited (the primary) is spinning. A retrograde orbiting object moves in the opposite direction to that of the bodies of the Solar System.

Retrograde and prograde are sometimes used to describe the rotation of a body itself. Venus for example rotates on its axis retrograde but moves around the Sun, like the other planets, prograde.

It is also useful to able to describe the nearest and furthest point that a body moves to and from the primary. The closest point is called the periapsis and the furthest point from the primary is the apoapsis. When we talk about an object moving about the Sun the periapsis and the apoapsis are called the perihelion and the aphelion (helios = Sun).

Because some orbits are inclined, we might want to specify whether the moving body is climbing or descending with respect to the plane of the ecliptic, or the equatorial plane of a planet.

The ascending node is the point where the orbital path of a travelling orbit moves above the plane of the ecliptic. The descending node is the point where the orbital path of a travelling orbit moves below the plane of the ecliptic.

Changes

There are several ways in which the orbit of a body can be altered. A common effect occurs when a body approaches one or more other bodies; their gravitational pull disturbs it's orbit. This is called a perturbation.

Perturbations can be caused by massive objects at a great distance. It is thought that the orbits of comets far beyond the outer planets, in the Oort cloud, are altered by the gravitational affect of stars over interstellar space.

Cometary orbits are altered in a variety of ways. Comets can be perturbed when they pass through the Solar System, by the gravity of the planets, depending on their configuration. Orbital modification can occur when gas jets from their surface as they approach the Sun. The loss of mass they experience, too, will modify their orbit slightly.

Within the asteroid belt it is found that there are concentric gaps, called Kirkwood gaps. It is thought that these regions are swept clear by a 'resonance' or periodic gravitational perturbation, in this case by Jupiter.

Resonant orbits or 'locations', as they are known, occur in ratios, for example at the 4:1 resonance a body will make four orbits of the Sun for every one made by Jupiter. There are also major gaps in the asteroid belt at the 3:1 and 2:1 resonances.

Nearer Jupiter the situation is reversed. Resonances, rather than sweep a region clear, have served to concentrate asteroids in groups.

Resonances coincide with features in Saturn's ring system, and with the location of the Galilean satellites about Jupiter. Io, Europa, and Ganymede have orbital distances from Jupiter which form a ratio- 1:2:4.

Perturbations occurring as a result of resonances in the asteroid belt can have unpredictable effects. An object ejected from a resonant orbit into a more populated region of the asteroid belt may stir up asteroids. It may send these on a different course, and cause some of them to collide.

Though collisions between bodies presumably still happen from time to time, such events probably take place with a far lower frequency than during the early history of the Solar System when there was a lot more fragmentary material in orbit about the Sun.

Rotation

A synchronous orbit is when one body orbits another in the same time as the primary takes to rotate. Synchronous rotation, however, is when a body makes one complete rotation on its axis (spin) for every orbit of the primary. Our Moon has been slowed down by tidal forces into a pattern of synchronous rotation. This is the reason why it always presents the same faces towards us.

The moon Charon, orbiting Pluto, has a synchronous orbit as well as synchronous rotation. To an observer on the surface of Pluto, the same face of Charon is always apparent, but because Charon moves about the planet at the same speed Pluto rotates, Charon appears 'fixed' in the sky.