What did Hipparchus study in his work Against the Geography of Eratosthenes?

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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.

In his work Against the Geography of Eratosthenes, the Greek astronomer Hipparchus focused primarily on the accurate determination of terrestrial locations.1

According to the Greek geographer Strabo, who quoted the text, Hipparchus used this commentary to critique the work of his predecessor, Eratosthenes. Hipparchus was known for being unforgiving of loose or inconsistent reasoning in the scientific works of others, and he applied this critical approach to Eratosthenes' geographical theories.2