The motion of the planets brings about many interesting observations. If observed over a series of even a few nights, we can notice a change in position of the planets. This is especially true of Venus and Mars, since they move quickly in their orbits. This is why they are called "planets", which means wanderer. They wander through the sky.

What does that mean to wander through the sky. If you observe a planet tonight, you'll see the planet move from east to west along with the stars. However, if you observe the position of the planet among the stars over a period of several nights, you'll notice that it changes position. It wanders among the stars. Its usual motion through the stars is toward the east. This is referred to as prograde motion (or direct motion).

However, the planets will sometimes move toward the west over a period of days. This apparently backward motion is called retrograde motion. Retrograde motion is one of the reasons for the downfall of the geocentric solar system. None of the geocentric models could correctly predict the positions of the planets and account for retrograde motion. The system that came the closest to working out the bugs was created by Hipparchus and refined by Ptolemy. Because Ptolemy refined the system to its often referred to as the Ptolemaic system.

This system uses two circles called an epicycle and a deferent. The planets move around a small circle called an epicycle. The center of the epicycle will then move in a circle (the deferent) around the earth. The combination of these two circles will account for retrograde motion. However, Ptolemy was never able to refine the system to the point of predicting the position of the planets exactly.

Recall the stipulations of the scientific method. In order for a model to be accepted, it must correctly predict future outcomes. Since the Ptolemaic system was unable to do this, it needed to be discarded or modified. It would be over a thousand years before a system to correctly predict the position of the planets would be brought forth.

Below are two diagrams of retrograde motion. The first shows a view of Mars and its positions starting on Dec 18, 1998 (follow the dots from right to left). On about March 19, 1999 retrograde motion begins (follow the dots from left to right). On about July 1, 1999 retrograde motion ends (follow the dots from right to left). In the second diagram you can see the position of Mars and the Earth as they orbit the sun on these same dates. The beginning and end of retrograde motion are shown. Notice that the Earth (the inner orbit) is passing Mars (the outer orbit) during these times. But this only works with the planets orbiting the sun. It would be Copernicus who first explained the orbits in this fashion.

This page was last updated on 06/06/01.