Ptolemy (about A.D. 100 to about A.D. 170) was an Egyptian astronomer, mathematician, and geographer of Greek descent who flourished in Alexandria during the second century A.D.

In several fields his writings represent the culminating achievement of Greco-Roman science, particularly his geocentric (earth-centered) model of the universe now known as the Ptolemaic system.

Nothing is known about Ptolemy's life except what can be inferred from his writings. His first major astronomical work, the Almagest (from a hybrid of Arabic and Greek, "the greatest"), was completed about A.D 150 and contains reports of astronomical observations which Ptolemy made over the preceding quarter of a century.

The size and content of his subsequent literary production suggests that he lived until about A.D. 170.