The celestial sphere that we introduced previously is a convenient fiction to locate objects in the sky. However, the Greek philosopher Aristotle (many of Aristotles works are available at the Internet Classics Archive) proposed that the heavens were literally composed of 55 concentric, crystalline spheres to which the celestial objects were attached and which rotated at different velocities (but the angular velocity was constant for a given sphere), with the Earth at the center. The following figure illustrates the ordering of the spheres to which the Sun, Moon, and visible planets were attached.
(The diagram is not to scale, and the planets are aligned for convenience in illustration; generally they were distributed around the spheres.) There were additional "buffering" spheres that lay between the spheres illustrated. The sphere of the stars lay beyond the ones shown here for the planets; finally, in the Aristotelian conception there was an outermost sphere that was the domain of the "Prime Mover". The Prime Mover caused the outermost sphere to rotate at constant angular velocity, and this motion was imparted from sphere to sphere, thus causing the whole thing to rotate.
By adjusting the velocities of these concentric spheres, many features of
planetary motion could be explained. However, the troubling observations of
varying planetary brightness and retrograde motion could not be accommodated:
the spheres moved with constant angular velocity, and the objects
attached to them were always the same distance from the earth because they
moved on spheres with the earth at the center.
effect was as illustrated in the following animation. As the center of the
epicycle moves around the deferent at constant angular velocity, the planet
moves around the epicycle, also at constant angular velocity. The apparent
position of the planet on the celestial sphere at each time is indicated by the
line drawn from the earth through the planet and projected onto the celestial
sphere. The resulting apparent path against the background stars is indicated
Now, in this tortured model one sees that it is possible to have retrograde
motion and varying brightness, since at times as viewed from the earth the
planet can appear to move "backward" on the celestial sphere. Obviously,
of the planet from the Earth also varies with time, which leads to variations
Thus, the idea of uniform
circular motion is saved (at least in some sense) by this scheme, and it allows
a description of retrograde motion and varying planetary brightness.
Thus, the ideas largely originating with pagan Greek philosophers were baptized into the Catholic church and eventually assumed the power of religious dogma: to challenge this view of the Universe was not merely a scientific issue; it became a theological one as well, and subjected dissenters to the considerable and not always benevolent power of the Church.