Claudius Ptolemy was a astronomer, mathematician, and geographer who probably lived around 87-150ce. He was probably descended from a Greek family living in Egypt, and probably a citizen of Rome. Theon of Smyrna was probably his teacher.

Apparently, nobody knows much about him for certain.

He wrote He mathematike syntaxis ("The Mathematical Collection"), later titled Ho megas astronomos ("The Great Astronomer") and eventually known as Almagest, which were thirteen books describing his geocentric theory (the Ptolemaic system) and included a catalog of 1,022 stars (~850 may have been taken from Hipparchus's catalog).

Later, Tycho Brahe accused Ptolemy of messing up, followed by more accusation by others, including Isaac Newton stating that Ptolemy "developed certain astronomical theories and discovered that they were not consistent with observation. Instead of abandoning the theories, he deliberately fabricated observations from the theories so that he could claim that the observations prove the validity of his theories. In every scientific or scholarly setting known, this practice is called fraud, and it is a crime against science and scholarship."

The Ptolemaic system which Ptolemy described in Almagest lasted until Nicolaus Copernicus replaced it with his heliocentric theory in 1543.

Ptolemy's major work in geography, consisted of eight books listed the coordinates for about 8,000 places.
He showed Asia extending much too far east, which may have been a factor in Christopher Columbus' decision to sail west for the Indies.

In Optics Ptolemy studies colour, reflection, refraction, and mirrors of different shapes.

Among other things, Ptolemy attempted to prove a postulate on parallel lines created by Euclid.