Four Lost Episodes in Ancient Solar Theory

The most widely known solar theory from antiquity is that of Hipparchus. All ancient sources are agreed that Hipparchus ’ model was a simple eccentric or an equivalent epicycle. The eccentric version of the model is shown in Figure 1. The Earth E is displaced a distance e from the center Z of the deferent. The purpose of the displacement is to allow the Sun at S to appear to move alternately slower and faster when seen from the Earth, even though it is at all times moving uniformly around the deferent as seen from the deferent center Z. Assuming a year length of 365 days and adjusting e and the direction of EZ with respect to the stars, one can account for the lengths of the seasons, which Hipparchus took as 94 days for spring and 92 days for summer, and it also follows that there are 88 days in autumn and 90 days in winter