Hipparchus of Nicaea (second century BCE): Greek astronomer, discoverer of the precession.
The stellar catalogue made by Hipparchus of Nicaea contains observations that can be dated to 162-128 BCE, and this gives us an indication about the period in which he lived. Unfortunately, this is about everything we know about this astronomer, who was the first to discover the precession, i.e. the slow reorientation of the earth's axis. He was able to do this because he possessed the sky map of Eratosthenes and ancient Babylonian observations.
Unfortunately, Hipparchus ignored Babylonian calculations of the length of the solar year. The Greek astronomer's estimate had an error of 6½ minutes, whereas the Babylonian calculation by Kidinnu was only 4½ short of the real length of the year. Nevertheless, Hipparchus was able to improve the Greek calendar. Another triumph was the accurate calculation of the distance to the moon, and the improvement of the world map of Eratosthenes.