The 7 Planets of the Ancients

In ancient times there were thought to be 7 wanderers or planets (only 5 of which are really planets): Mercury, Venus, Mars, Jupiter, Saturn, the Moon, and the Sun
The Sun and the Moon were classified as planets because they wandered on the celestial sphere

Retrograde Motion

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

The usual motion of planets as they "wandered" on the celestial sphere was eastward against the background stars. = "Direct Motion"
It was observed that at times the planets moved westward for some period on the celestial sphere = "Retrograde Motion"
The planets were observed to be brighter at certain times than others.

Aristotle (350 B.C.)

A major reason for the difficulty of explaining these features was the dominance of the Greek philosopher Aristotle on medieval thought.
The heavens were more perfect than the Earth
Objects in the heavens were unchanging
Believed that the only motion permitted objects in the heavens was uniform circular motion
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, with the Earth at the center
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

A solution to Aristotle's problem: Planets were attached to circles called "Epicycles", which rotated in uniform circular motion at a constant velocity
These epicycles were attached to the concentric sphere called "Deferents", which also rotated in uniform circular motion at the same velocity

The net effect is indicated by a line drawn from the Earth through the planet and projected onto the celestial sphere

The resulting apparent path against the background stars accounts for retrograde motion and varying brightness

Even the center of the epicycle had to be displaced by some distance from the center of the deferent to account for some movements

These ideas concerning uniform circular motion and epicycles were catalogued by Ptolemy, in 150 A.D, in his "Almagest" (literally, "The Greatest")
Ptolemy was from then considered "the father of the geocentric theory" (the Earth is the center of the Universe)

Medieval Aristotelian Astronomy

Three ideas, that we now know to be completely wrong, were so ingrained in the astronomers of an earlier age that they were essentially never questioned:

All motion in the heavens is uniform circular motion.
The objects in the heavens are made from perfect material, and cannot change their intrinsic properties (e.g., their brightness).
The Earth is at the center of the Universe.

By the Middle Ages, these ideas 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

Arab Astronomy (8th & 9th Centuries)

Translated and kept alive the astronomy of the ancient Greeks (translated Ptolemy's Almagest)
Built observatories and timekeeping devices
Kept careful records of planetary positions; 400,000 manuscripts in their libraries

Spanish Astronomy (13th Century)

King Alfonso X of Spain facilitated the production of the Alphonsine Tables (computed astronomical tables)

Eastern European, Regiomontanus (15th Century)