The Fixed Stars
In Books VII and VIII of the Almagest Ptolemy interrupts the development of the planetary theories in order to deal with a number of problems connected with the fixed stars. The reason for this is explained in the prologue to this second part of the Almagest [VII, 1; Hei 2, 2], where Ptolemy tells his friend or protector Syrus (see page 26) that the further development of planetary theory presupposes a chapter on the so-called fixed stars. The reason is that while the solar theory was founded upon simple observations of solstitial and equinoctial times, and the lunar theory upon eclipses, the theories of the remaining five planets are, to a great extent, founded upon exact determinations of planetary longitudes derived from the distance of the planet in question from a fixed star, measured with the astrolabon (see page 183). Thus a position of Mercury is referred to Aldebaran (α Tauri) [IX, 7; Hei 2, 262] or to Regulus (a Leonis) [ibid.; Hei 2, 263] just as a longitude of Venus is found relative to that of Antares (a Scorpii) [X, 3; Hei 2, 303].