**Next:** From the Middle Ages **Up:** Aristotle and Ptolemy **Previous:** The motion according to

The Aristotelian system was modified by Hipparchus whose ideas were popularized
and perfected by Ptolemy. In his treatise the *Almagest* (``The Great System'') Ptolemy provided a mathematical theory of the motions
of the Sun, Moon, and planets. Ptolemy vision (based on previous work by
Hipparchus) was to envision the Earth surrounded by circles, on these circles
he imagined other (smaller) circles moving, and the planets, Sun, etc. moving
on these smaller circles. This model remained unchallenged for 14 centuries.

The system of circles upon circles was called a system of *epicycles* (see Fig. 2.14). It was extremely complicated (requiring several correction factors) but
it did account for all the observations of the time, including the peculiar
behavior of the planets as illustrated in Fig. 2.15. The Almagest was not superseded until a century after Copernicus presented
his heliocentric theory in Copernicus' *De Revolutionibus* of 1543.

This model was devised in order to explain the motion of certain planets. Imagine that the stars are a fixed background in which the planets move, then you can imagine tracing a curve which joins the positions of a given planet everyday at midnight (a ``join the dots'' game); see, for example Fig 2.13. Most of the planets move in one direction, but Mars does not, its motion over several months is seen sometimes to backtrack (the same behavior would have been observed for other celestial objects had Ptolemy had the necessary precision instruments).

**Next:** From the Middle Ages **Up:** Aristotle and Ptolemy **Previous:** The motion according to