Hipparchus' many important and lasting contributions to astronomy included practical and well as theoretical innovations. He employed geometrical models, including the deferent-epicycle and eccentric previously used by Apollonius (flourished ca. 200 B.C.). One of his contributions appears to have been the incorporation of numerical data based on observations into the geometrical models developed to account for the astronomical motions; Gerald Toomer has credited Hipparchus with the founding of trigonometry. Hipparchus was very interested in observation; his recorded observations span the years 147 to 127 BC. He used an instrument described by Ptolemy as a dioptra and may have invented the planispheric astrolabe. Hipparchus made extensive observations of star positions, and is credited by some with the production of the first known catalogue of stars. He turned his attention to a wide variety of astronomical questions, including the length of the year, the determination of lunar distance and the computation of lunar and solar eclipses. He developed theories for the Sun and Moon demonstrating (as Ptolemy explained, Almagest, 421) 'that they are represented by uniform circular motions'. Ptolemy noted that as far as he knew, Hipparchus did not establish theories for the five planets, 'not at least in his writings which have come down to us'. But Hipparchus did compile the planetary observations to which he had access into a more useful arrangement, and demonstrated that the phenomena were 'not in agreement with the hyotheses of the astronomers of that time'. Hipparchus' discussion of the motion of the points of solstice and equinox slowly from east to west against the background of the fixed stars is perhaps his most famous achievement; he has been therefore credited with the discovery of the precession of the equinoxes. Otto Neugebauer has suggested that Hipparchus, in fact, invented the theory of trepidation.
Perhaps most intriguing for historians of astronomy is Hipparchus' use of Babylonian astronomical material, including methods as well as observations. Many questions remain regarding the relationship between Babylonian and Greek astronomy, but Hipparchus' work provides a clear link. Toomer has argued that Hipparchus was responsible for the direct transmission of both Babylonian observations and procedures and for the successful synthesis of Babylonian and Greek astronomy.
The historian of astronomy Otto Neugebauer, in his 'Notes on Hipparchus', remarked that:
Even the most casual discussion of ancient astronomy will not fail to call Hipparchus of Nicaea in Bithynia "the greatest astronomer of antiquity." It is obvious enough that classifications of greatness are usually void of any precise meaning, though it is equally evident that they will remain a stock phrase in the histories of science. It is perhaps not useless, however, to underline how little we actually know about Hipparchus' astronomy and its relation to his predecessors and followers.