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of Alexandria. The most celebrated of ancient astronomers, believed to have been a native of Ptolemais in the Thebaid; he lived in the first half of the second century of our era. His great astronomical treatise was entitled "Mathematical Arrangement", and by the Arabian philosophers "Almagest" (al magiste, "the greatest"); it gives an exposition of the system of the universe, the interrelations and revolutions of the heavenly bodies, as understood in Ptolemy's time. He also wrote treatises on "Geography", "Trigonometry", "Chronology", "Optics", and other subjects pertaining to mathematical and physical science. The "Ptolemaic System" was the accepted and ruling astronomical authority down to Copernicus's time, and his work on geography was the chief authority up to the time of the great discoveries of the 15th century. His system of map orientation (north at the top and east at the right) is still the universal one.