The Theories of the Moon
Having finished his theory of the Sun, Ptolemy proceeds with the motion of the Moon. This disposition of the subject matter is not obvious although Ptolemy declares that he has adopted the logical order [IV, 1; Hei 1, 265]. From the pedagogical point of view it would have been more considerate towards the reader to go on with the relatively simple theories of the three superior planets Mars, Jupiter and Saturn in-stead of developing the much more intricate lunar theory. Actually, the reason for dealing with the Moon at this place becomes clear only later: the theories of the other planets are built on observations among which a certain number measure the distance between the planet and a fixed star. But the longitudes of the stars are determined by means of the position of the Moon (see page 240 ff.). Thus the ecliptic position of the planet cannot be found unless we know where the Moon is, i.e. unless we have a theory of its motion. This is another testimony to the fact that the Almagest is no student's text-book of astronomy, but a monograph on the construction of planetary theories.