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Why did Hipparchus not create a complete theory of planetary motion?

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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 create a complete theory of planetary motion because he recognized that the movements of the planets were significantly more complex than those of the Sun and Moon. While he successfully developed numerically accurate geometrical theories for the Sun and Moon using eccentric circles and epicycles, he found that the planetary models circulating in his time did not agree with observed phenomena.12

Rather than proposing an incomplete or inaccurate model, Hipparchus contented himself with demonstrating the failures of existing theories. His approach reflected a high standard for scientific integrity; he insisted that a geometrical theory, if true, must work in detail. Consequently, he left the task of fashioning a fully predictive planetary system to later astronomers, such as Ptolemy, who eventually compiled a systematic account in the Almagest.23