Used with
permission of Maiken Naylor, University at Buffalo, Buffalo, NY, USA,
http://ublib.buffalo.edu/libraries/units/sel/exhibits/stamps
Hipparchus (2nd century BC), the greatest astronomer of the ancient world, is thought to have invented the astrolabe. He continued the work of Aristarchus in calculating the distance of the moon by measuring its parallax against the sphere of fixed stars. His result of 30 earth diameters is correct. From his observations he produced the first star catalogue, grading the stars by brightness. He also discovered the precession of the equinoxes.