Hipparchus's determination of the length of the tropical year and the rate of precession

Abstract

Of the numerous astronomical works of HlPPARCHUS only one, the Commentary on Aratus, has survived beyond late antiquity. While the ARATUS commentary has proved of great value in distinguishing the star catalogues of Ptolemy and Hipparchus, and provides the instructive example of a far less organized state of establishing celestial coordinates than the systematic use of longitude and latitude in the Almagest, it tells us nothing of Hipparchus's more important work in solar and lunar theory, and of his attempt to construct a useful planetary theory. For these matters we must rely upon scattered remarks in ancient authors, first and foremost Ptolemy who, when he is providing either specific confirmation or correction, sometimes gives a good deal of information. However, since Ptolemy's intention was not to write an account of the motivation and achievement of his predecessor, whose books were still available to the interested reader, his remarks, even the most extended such as the account of Hipparchus's uncertainty about the uniformity of the tropical year, are not necessarily easy to follow. The sketchiness of some of Ptolemy's comments, along with the retention of some Hipparchan parameters in the Almagest, have led to two unfortunate interpretations. The first of these is that throughout the Almagest Ptolemy, more a compiler than an original investigator, was in great part reproducing the work of Hipparchus with but small modifications. This opinion is now so thoroughly discredited by modern studies that we shall not concern ourselves