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Paths of the Planets

If you were looking at the Solar System from a few billion miles "above" the Sun, you would have no difficult understanding the paths of the planets. All the planets would be revolving around the Sun at various distances in counter-clockwise motion. Not only do all the planets revolve counter-clockwise around the Sun, but I general they and even their satellites rotate counter-clockwise around their respective axes. The path a planet traces around the Sun is its orbit. The planets are not moving or orbiting around the Sun at a constant speed. The closer a planetís orbit is to the Sun, the more rapid is the planetís motion. Since the outer planets not only travel more slowly, but also have a great distance to travel in order to complete a revolution around the Sun, each planet further from the Sun takes longer to circle the Sun than those that are closer to the Sun, and so has a longer year.

For the moment at least, we'll have to do all our planet watching from within the Solar System, usually on Earth. Not only are all the planets revolving, but to make matters more confusing, so is our viewing platform, the planet Earth. Let us start by taking a look at an "inferior" plane, one whose orbit lies within the Earthís orbit. The two inferior planets are Mercury and Venus. The categorization of the planets into inferior and superior is not a moral judgement but one based on whether their orbit is within the Earthís orbit or not. Once again, the stars are serving as a background to the motions of the nearby planets. One any one given night a planet will move from the eastern horizon towards the west just as the stars do. The apparent motion of the planets is seen only from night to night, as it appears to move past the fixed stars.

In general, an inferior planet will move eastward across the backdrop of the celestial sphere. This is most easy to understand when the Sun is positioned between the Earth and the inferior planet. Both planets are moving counter-clockwise as viewed from far "above" the Solar System, but we on the Earth are seeing things from the side. The Earth on one side of the Sun is moving westward, while the inferior planet on the opposite side from the Sun moves eastward. One westward motion combined with the inferior planetís eastward motion gives it the appearance of moving eastward among the stars. This eastward movement holds true concerning the inferior planets except when they are between the Sun and the Earth, at a moment called the inferior conjunction. During the inferior conjunction of an inferior planet, both it and the Earth are moving westward because they are both moving around the Sun counter-clockwise and furthermore are on the same side of the Sun. But not as if they were attached to a common spoke of a wheel, because the inferior planet is moving faster than the Earth. The inferior planet is now overtaking the Earth. Since the westward movement of the inferior planet is more rapid than our own, it appears at this time to move westward as it overtakes the Earth. The atypical westward motion of a planet is called retrograde motion. The period from the middle of one retrograde motion to the next is called the synodic period. The synodic period may be thought of as the amount time needed for an inferior planet to catch up with the Earth, or the time needed for the Earth to catch up to a superior planet. The importance of some of the planetís synodic period will become apparent as soon as we start discussing each one in detail.

To summarize the motion of either of the two inferior planets Mercury or Venus, during each synodic period they continually move eastward, except at inferior conjunction when they loop around in a westward movement.

Now let us turn to the remaining superior planets. They also generally move eastward. The moment when a superior planet changes direction and shows retrograde motion occurs at the time of opposition; this is when it, the Earth and the Sun are in a direct line with the Earth in the middle. During the time of opposition, the more rapidly moving Earth overtakes the slower moving superior planet and it appears to move westward through the sky. The retrograde movement of the superior planets takes place when they are at their closes point to Earth and are easiest to observe. The synodic period of the various superior planets varies from little over a single year to over two full years. The period of time that superior planet exhibits retrograde motion also widely varies.

The synodic period of a planet and its retrograde motion is determined by its distance from the Sun, which also determines its speed around the Sun, but that alone does not fully determine a planetís path as seen from the Earth. A planet's movement is affected by eccentricity of its orbit, in other words, how squashed down its elliptical orbit is from a perfect circle. Furthermore, although I have described all the planets and the Sun lying in a common plane, that is not entirely true. Most of the planetary orbits have a slight inclination of a few degrees from the rest of the Solar System. As a result of this inclination, we see the planets a little above or below the ecliptic.


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