Definition: Ptolemy (ca. 100-ca. 170): A.k.a. Claudius Ptolemaeus. Ptolemy believed the planets and Sun to orbit the Earth in the order Mercury, Venus, Sun, Mars, Jupiter, Saturn. This system became known as the Ptolemaic system and predicted the positions of the planets accurately enough for naked-eye observations (although it made some ridiculous predictions, such as that the distance to the moon should vary by a factor of two over its orbit). He authored a book called Mathematical Syntaxis (widely known as the Almagest). The Almagest included a star catalog containing 48 constellations, using the names we still use today.
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