How did Hipparchus criticize the writings of Aratus and Eudoxus?

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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 criticized the writings of Aratus and Eudoxus of Cnidus in his only surviving book, titled Commentary on the Phaenomena of Aratus and Eudoxus.1

In this work, Hipparchus ruthlessly exposed errors found in Aratus’s popular poem, Phaenomena, which was based on a now-lost treatise by Eudoxus that described and named the constellations. His critique was part of a broader commitment to accuracy; he was characterized by Ptolemy as a "lover of truth," a trait that drove him to correct his predecessors and even revise his own beliefs when presented with new evidence.1