The Ptolemaic System

Explaining the nonuniform motions of the five planets was one

of the main challenges facing the astronomers of antiquity. The

Greeks developed many theories to account for retrograde motion

and the loops that the planets trace out against the background

stars. One of the most successful and enduring models was originated

by Apollonius of Perga and by Hipparchus in the second

century B.C. and expanded upon by Ptolemy, the last of the great

Greek astronomers, during the second century A.D. 

sketches the basic concept, usually called the Ptolemaic system.

Each planet is assumed to move in a small circle called an epicycle,

whose center in turn moves in a larger circle, called a deferent,

which is centered approximately on the Earth. Both the

epicycle and deferent rotate in the same direction

As viewed from Earth, the epicycle moves eastward along the

deferent. Most of the time the eastward motion of the planet on

its epicycle adds to the eastward motion of the epicycle on the

deferent . Then the planet is seen to be in direct

(eastward) motion against the background stars. However, when

the planet is on the part of its epicycle nearest Earth, the motion

of the planet along the epicycle is opposite to the motion of the

epicycle along the deferent. The planet therefore appears to slow

down and halt its usual eastward movement among the constellations,

and actually goes backward in retrograde (westward) motion

for a few weeks or months. Thus, the concept

of epicycles and deferents enabled Greek astronomers to explain

the retrograde loops of the planets.

Using the wealth of astronomical data in the library at Alexandria,

including records of planetary positions for hundreds of

years, Ptolemy deduced the sizes and rotation rates of the epicycles

and deferents needed to reproduce the recorded paths of the

planets. After years of tedious work, Ptolemy assembled his calculations

into 13 volumes, collectively called the Almagest. His

work was used to predict the positions and paths of the Sun,

Moon, and planets with unprecedented accuracy. In fact, the Almagest

was so successful that it became the astronomer’s bible,

and for more than 1000 years, the Ptolemaic system endured as

a useful description of the workings of the heavens.