Characterizing Ptolemy

Judging by the simplicity of his observations we could ask if Ptolemy was a naive impressionist? Did he assume that what appears to be true is actually true? The counter to that is that all measurements have a context associated with them and this is the simplest system to use. There are advantages for uniformity over space and time which produces a consistent set of data to work with. Perhaps it would be best to judge him solely as an observer.

He tells us that the Sun's motion is not entirely uniform and presents an anomaly based on the accepted times for the lengths of the seasons. His epicycle theory implies an eccentricity for the Earth's orbit of about 1/50 which can be compared to the modern value of about 1/60. He gives values for the times of the equinoxes and solstices which tells us about the length of the year and an estimate of the time when the Earth and the Sun are closest together. Although the epicycle theory seems a little absurb today it is now common practice to use Fourier series to represent periodic changes in time. He presents a fairly good picture of the Earth's relative motion to the Sun for his time.

Ptolemy systematize the current understanding of astronomy and prepared the way for future advances which the later increase in the precision of measurement brought about. We now know of secular changes, changes from one century to the next, in the orbits of the planets. Ancient observations are useful today in that they provide a check for modern theories of the motion of the planets throughout the ages.