Hipparchus (c.190-c.120 BC), Ancient Greek astronomer, with the armillary sphere he invented (lower right) and representations of the signs of the zodiac in the night sky. Hipparchus is considered one of the greatest astronomers of antiquity. He calculated the length of the year and discovered the precession of the equinoxes. It is thought that the later work of Ptolemy was based on the work done by Hipparchus. Artwork from the 19th century book Vies des Savants Illustres.