He put Greek astronomy on a more scientific footing, introducing arithmetic and early trigonometric methods. His many accurate astronomical observations resulted in a catalogue of 850 stars, in which he gave their coordinates and divided them into six magnitude classes. Ptolemy incorporated the catalogue and other findings by Hipparchus in the Almagest. Hipparchus made surprisingly accurate measurements of the precession of the equinoxes, the length of the year, and (from observations of eclipses) the Moon’s distance. He may have been the inventor of the astrolabe.