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Claudius Ptolemaeus (Greek: Klaudios Ptolemaios; A.D. circa 85 - circa 165), known in English as Ptolemy, was a Greek geographer, astronomer, and astrologer who probably lived and worked in Alexandria in Egypt.

Ptolemy was the author of two important scientific treatises. One is the astronomical treatise that is now known as the Almagest (in Greek Hè Megalè Syntaxis, "The Great Treatise"). It was preserved, like most of Classical Greek science, in Arabic manuscripts (hence its familiar name) and only made available in Latin translation (by Gerard of Cremona) in the 12th Century.

In this work, one of the most influential books of Antiquity, Ptolemy compiled the astronomical knowledge of the ancient Greek and Babylonian world; he relied mainly on the work of Hipparchus of three centuries earlier. Ptolemy formulated a geocentric model (see: Ptolemaic system) of the solar system which remained the generally accepted model in the Western and Arab worlds until it was superseded by the heliocentric solar system of Copernicus. Likewise his computational methods (supplemented in the 12th Century with the Arabic computational ' Tables of ToledoGerard of Cremona edited for Latin readers the Tables of Toledo the most accurate compilation of astronomical data ever seen in Europe at the time. The Tables were partly the work of Al-Zarqali, known to the West as Arzachel, a mathematician and astronome') were of sufficient accuracy to satisfy the needs of astronomers, astrologers and navigators, until the time of the great explorations. They were also adopted in the Arab world and in India. The Almagest also contains a star catalogueIn astronomy, many stars are referred to simply by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, and this article covers only some of the more frequently quoted ones., which is probably an updated version of a catalogue created by Hipparchus. Its list of 48 constellationsOrion is a remarkable constellation, visible from most places on the globe (but not always the whole year long). A constellation is a group of stars visibly related to each other in a particular configuration. In three-dimensional space, most of the stars is ancestral to the modern system of constellations, but unlike the modern system they did not cover the whole sky.

Ptolemy's other main work is his Geography. This too is a compilation, of what was known about the world's geographyGeography is the scientific study of the locational and spatial variation in both physical and human phenomena on Earth. The word derives from the Greek words g ("the Earth") and graphein ("to write," as in "to describe"). Geography is also the title of v in the Roman empire at his time. He relied mainly on the work of an earlier geographer, Marinos of Tyre, and on gazetteers of the Roman and ancient Persian empire, but most of his sources beyond the perimeter of the Empire were unreliable.

The first part of the Geography is a discussion of the data and of the methods he used. Like with the model of the solar system in the Almagest, Ptolemy put all this information into a grand scheme. He assigned coordinates to all the places and geographic features he knew, in a grid that spanned the globe. Latitude was measured from the equator, as it is today, but Ptolemy preferred to express it in the length of the longest day rather than degrees of arc (the length of the midsummer day increases from 12h to 24h as you go from the equator to the polar circle). He put the meridian of 0 longitude at the most western land he knew, the Canary Islands.

Ptolemy also devised and provided instructions on how to create maps both of the whole inhabited world (oikoumenè) and of the Roman provinces. In the second part of the Geography he provided the necessary topographic lists, and captions for the maps. His oikoumenè spanned 180 degrees of longitude from the Canary islands in the Atlantic Ocean to China, and about 80 degrees of latitude from the Arctic to the East-indies and deep into Africa; Ptolemy was well aware that he knew about only a quarter of the globe.

The maps in surviving manuscripts of Ptolemy's Geography however, date only from about 1300, after the text was rediscovered by Maximus PlanudesMaximus Planudes (c. 1260 1330), Byzantine grammarian and theologian, flourished during the reigns of Michael VIII and Andronicus II. He was born at Nicomedia in Bithynia, but the greater part of his life was spent in Constantinople, where as a monk he de.

In his Optics, a work which survives only in a poor Arabic translation, he writes about properties of light, including reflection, refraction and colour. His other works include Planetary Hypothesis, Planisphaerium and Analemma.