Editing Hipparchus (section)

===Orbit of the Moon===
It had been known for a long time that the motion of the Moon is not uniform: its speed varies. This is called its ''anomaly'' and it repeats with its own period; the [[anomalistic month]]. The Chaldeans took account of this arithmetically, and used a table giving the daily motion of the Moon according to the date within a long period. However, the Greeks preferred to think in geometrical models of the sky. At the end of the third century BC, [[Apollonius of Perga]] had proposed two models for lunar and planetary motion:
# In the first, the Moon would move uniformly along a circle, but the Earth would be eccentric, i.e., at some distance of the center of the circle. So the apparent angular speed of the Moon (and its distance) would vary.
# The Moon would move uniformly (with some mean motion in anomaly) on a secondary circular orbit, called an ''epicycle'' that would move uniformly (with some mean motion in longitude) over the main circular orbit around the Earth, called ''deferent''; see [[deferent and epicycle]].

Apollonius demonstrated that these two models were in fact mathematically equivalent. However, all this was theory and had not been put to practice. Hipparchus is the first astronomer known to attempt to determine the relative proportions and actual sizes of these orbits. Hipparchus devised a geometrical method to find the parameters from three positions of the Moon at particular phases of its anomaly. In fact, he did this separately for the eccentric and the epicycle model. Ptolemy describes the details in the ''Almagest'' IV.11. Hipparchus used two sets of three lunar eclipse observations that he carefully selected to satisfy the requirements. The eccentric model he fitted to these eclipses from his Babylonian eclipse list: 22/23 December 383&nbsp;BC, 18/19 June 382&nbsp;BC, and 12/13 December 382&nbsp;BC. The epicycle model he fitted to lunar eclipse observations made in Alexandria at 22 September 201&nbsp;BC, 19 March 200&nbsp;BC, and 11 September 200&nbsp;BC.
* For the eccentric model, Hipparchus found for the ratio between the radius of the [[eccenter]] and the distance between the center of the eccenter and the center of the ecliptic (i.e., the observer on Earth): 3144 : {{frac|327|2|3}};
* and for the epicycle model, the ratio between the radius of the deferent and the epicycle: {{frac|3122|1|2}} : {{frac|247|1|2}} .

These figures are due to the cumbersome unit he used in his chord table and may partly be due to some sloppy rounding and calculation errors by Hipparchus, for which Ptolemy criticised him while also making rounding errors. A simpler alternate reconstruction{{sfn|Thurston|2002}} agrees with all four numbers. Hipparchus found inconsistent results; he later used the ratio of the epicycle model ({{frac|3122|1|2}} : {{frac|247|1|2}}), which is too small (60 : 4;45 sexagesimal). Ptolemy established a ratio of 60 : {{frac|5|1|4}}.{{r|toomer1968}} (The maximum angular deviation producible by this geometry is the arcsin of {{frac|5|1|4}} divided by 60, or approximately 5° 1', a figure that is sometimes therefore quoted as the equivalent of the Moon's [[equation of the center]] in the Hipparchan model.)