Ptolemy

Ptolemy (about 90-168 ad), Greek astronomer and geographer, b. in Egypt.

Claudius Ptolemaeus, known as Ptolemy, made his astronomical observations from Alexandria in about 150 ad. He produced a record of these, together with his ideas about the universe, in thirteen volumes entitled Mathematical Composition. The books are always referred to as the Almagest, an Arab term meaning 'the greatest'.

Ptolemy said that the world stood still in the center of the universe. The Sun, the Moon and the planets moved around it at various speeds. This idea was accepted until 1543 when Nicolas Copernicus said it was wrong.

Ptolemy worked out a value for pi (written ?), the Greek letter. This letter stands for the number by which the diameter of a circle must be multiplied to find its circumference. In decimals this value is 3.1416, and that is the value of ? used today.

See Ptolemy's Almagest