Claudius Ptolemy (Latin: Claudius Ptolemaeus; c. 100 – c. 170 AD) was a mathematician, astronomer, natural philosopher, geographer, and astrologer who wrote several scientific treatises, three of which were of importance to later Byzantine, Islamic, and Western European science. The first is the astronomical treatise now known as the Almagest, although it was originally entitled the Mathematical Treatise and then known as The Great Treatise. The second is the Geography, which is a thorough discussion of the geographic knowledge of the Greco-Roman world. The third is the astrological treatise in which he attempted to adapt horoscopic astrology to the Aristotelian natural philosophy of his day.

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An astronomer is a scientist in the field of astronomy who studies stars, planets, moons, comets, and galaxies, as well as many other celestial objects.

A related but distinct subject, cosmology, is concerned with studying the universe as a whole. An astronomer researches the world beyond Earth.