The Apparent Motion of Planets
on the Celestial Sphere

Two observations concerning the planets were very difficult to explain for astronomers of the Middle Ages:

  1. The usual motion of planets as they "wandered" on the celestial sphere was eastward against the background stars. This is called "Direct" Motion". However, it was observed that at times the planets moved westward for some period on the celestial sphere; this was termed "Retrograde Motion". The episodes of retrograde motion were difficult to explain.

  2. The planets were observed to be brighter at certain times than others. This varying brightness was also a challenge to explain.
Both retrograde motion and varying brightness are illustrated in the adjacent animation.

A major reason for the difficulty of explaining these features was the dominance of the Greek philosopher Aristotle on medieval thought. This philosophy held that the heavens were more perfect than the Earth, and that objects in the heavens were unchanging. As part of this philosophy, it was believed that the only motion permitted objects in the heavens was uniform circular motion (motion at constant angular speed on a circle), because such motion brought one back cyclically to the starting point and therefore was (in a sense) unchanging.

This philosophy had no problem with the direct motion of the planets, since that could be explained by uniform circular motion of the planets about the Earth (which, as any fool who looked at the sky could see, was clearly the center of the Universe). However, neither the varying brightness of the planets, nor the occasional retrograde motion of the planets on the celestial sphere, were easily reconciled with this idea of unchanging objects executing uniform circular motion in the heavens.

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