Britannica Sources used in this answer

What problem did Hipparchus identify with existing Greek theories of the planets?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Engraving of six astronomers, with Hipparchus seated at center and, from left to right, Galileo, Johannes Hevelius, Tycho Brahe, Nicolaus Copernicus, and Ptolemy standing, each holding scientific instruments. — Hipparchus “Six astronomers,” the frontispiece of *Astronomica Institutio* (1692) by Jan Luyts, engraving after drawing by Gerard Hoet. Hipparchus is seated at center with from left to right: Galileo, Johannes Hevelius, Tycho Brahe, Nicolaus Copernicus, and Ptolemy.

Hipparchus did not identify a specific theoretical error in the sense of proposing a new model to replace existing ones; rather, he recognized that the planetary theories circulating in his time failed to agree with observed phenomena.1

While Hipparchus successfully developed numerically accurate geometrical models for the Sun and the Moon, he found that the complex movements of the planets could not be accounted for by the same simple eccentric circle and epicycle models that worked for the Sun and Moon. Consequently, he did not attempt to devise a new, satisfactory planetary theory himself, contenting himself with the observation that the existing models were insufficient to explain the actual motions of the planets. His work was significant because it established a new standard in Greek astronomy: the insistence that a geometrical theory must work in detail to be considered true.12