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What observations led Hipparchus to discover precession of the equinoxes?

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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 discovered the precession of the equinoxes by comparing his own astronomical observations of fixed stars with those recorded by his predecessors.1

Specifically, he utilized observations of several fixed stars taken in relation to the eclipsed Moon that had been documented by earlier astronomers. By comparing these historical records with his own eclipse observations, he deduced that the fixed stars were shifting eastward by at least 1° every 100 years. This slow motion is now understood to be caused by a wobble in the orientation of Earth’s axis of rotation over a cycle of approximately 25,772 years.1

While some historians suggest that earlier Babylonian astronomers may have had an awareness of this phenomenon through their shifting definitions of the equinox and solstices, Hipparchus is credited with the formal deduction of this motion based on his systematic comparison of celestial data.1