Ptolemy Claudius Ptolemaeus

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Ptolemy, Claudius Ptolemaeus (83-168) Greek Mathematician, Astronomer, Astrologer, Geographer Claudius Ptolemaeus, known in English as Ptolemy, was an important ancient Greek mathematician, astronomer, and geographer whose works contributed to building the basis for European and Islamic sciences. Ethnically Ptolemy was Greek, though his name Claudius shows he had Roman citizenship. Egyptians knew him as "The Upper Egyptian," suggesting he was from southern Egypt, but this is not certain. His works show that he had access to the older Babylonian astronomical observations and data. He is most famous for Almagest (The Great Treatise, or The Mathematical Treatise). His second main work was Geographia, which discussed the geography of the Greco-Roman world. The third major treatise that bears Ptolemy's name is the Tet-rabiblos (Four Books), a discourse on astrology and natural philosophy.

Ptolemy's Almagest is the oldest complete discussion of astronomy surviving from the ancient world, though older discussions are known from the Babylonian astronomers. Ptolemy claimed to have used notes from older astronomers going back more than 800 years before him in Almagest, yet these sources have largely disappeared with time. Almagest contains star tables, a star catalog, and a description of the 48 ancient constellations. Ptolemy used a geocentric model for the solar system and universe, which would be later challenged by Polish astronomer Nicolaus Copernicus, who advocated a heliocentric model. Ptolemy used a system of nested spheres to describe his geocentric model of the universe and calculated that the Sun was located 1,210 Earth radii from the Earth, while so-called fixed stars were located at 20,000 radii from the Earth, which was located at the center of the universe. Almagest contained a group of tables that contained the data to predict the locations of the Sun, Moon, planets, and eclipses, and these became popular. Ptolemy used them to produce a star calendar and almanac that was favored among astrologers. The popularity of Almagest meant that it was translated into Arabic and Latin from Greek, hence was preserved well and survives to this day.

The second main scientific work of Ptolemy was his Geographia (geography), a compilation of the known geographical features of the Roman Empire at the time. This included Europe and the Mediterranean area, North Africa, Arabia, Persia, and southern Asia. Many of the details of the maps shown by Ptolemy came from the earlier Greek geographer Marinos of Tyre, as well as maps from the Persians. Geographia contains several sections. The first includes data and methods and defines a coordinate system that included latitude measured from the equator. Ptolemy measured latitude in terms of the length of the longest day of the year at that location, which increases from 12 hours to 24 hours between the equator and the Arctic (and Antarctic) circles. For longitude Ptolemy used degrees, plotted from a meridian he placed at the westernmost known lands (Cape Verde Islands), which he called the Fortunata Islands. Ptolemy's maps of the east show China extending southward well into the Pacific Ocean, suggesting the Greek geographers of the time knew less about Asia than about the Mediterranean. Ptolemy worked on map projections, which are ways to project the three-dimensional globe onto a two-dimensional flat map. Ptolemy's maps appear quite distorted compared to modern maps, partly because of the primitive nature of the knowledge of the land, and partly because Ptolemy used an estimate for the size of the Earth that is much smaller than the real value.

The Tetrabiblos, Ptolemy's treatise on astrology, was very popular for predicting horoscopes through celestial positions and was widely translated into Arabic and Latin. The Tetrabiblos was written in general terms that common people could understand. In it, Ptolemy cautioned readers to use astrology as a compilation of astronomical data and not to be overly interpretive of the numerological significance of the astronomical data as was popular at the time. Most historians suggest that Ptolemy compiled the

Tetrabiblos from earlier sources and that his main contribution was to organize it in a rational and systematic way.

Ptolemy also wrote on music, publishing Harmonics about the mathematics of music, suggesting that music should be based on mathematic ratios, and he demonstrated how music could be translated into mathematics. He called this technique Pythagorean tuning, after Pythagoras, who first described the relation between music and math. Ptolemy wrote about light and optics, although his works in these fields are not well preserved.

See also astronomy; constellation; Copernicus, Nicolaus; Hipparchus; solar system.

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