Ptolemy (ca. 100-170)

Greek astronomer

Very little is known about Ptolemy's early life. Born in Alexandria, Egypt, as Ptolemais Hermii, his name was later latinized as Claudius Ptolemaeus, and later Ptolemy.

Ptolemy's chief contribution to science is a series of books in which he compiled the knowledge of the ancient Greeks, his primary source being Hipparchus (fl. second centuryb.c.). Because most of Hipparchus' writings have not survived from antiquity, many of the ideas he espoused about the universe have become known as the Ptolemaic system.

Ptolemy's system placed Earth directly at the center of the universe. The Sun, Moon and planets all orbited Earth. However, since such a scheme did not match the observed motions of the planets, Ptolemy added small orbits to the planets called epicycles, and introduced other mathematical devices to make a better fit.

Despite its errors and complications, the Ptolemaic system was adequate enough to make predictions of planetary positions, and it influenced thinking for 1,400 years. It was not until 1543 that Polish astronomer Nicolaus Copernicus (1473–1543) published his book refuting the Ptolemaic system. After Danish astronomer Tycho Brahe's (1546–1601) exceptionally accurate measurements of the positions of the planets showed Ptolemy's system was inaccurate, it fell upon German astronomer and mathematician Johannes Kepler (1571–1630) to devise a better explanation of planetary orbits.

Hipparchus had made a catalogue of stars, which were grouped into 48 constellations. Ptolemy placed them in his book and gave these patterns the names that are still in use today. He also included Hipparchus'work on trigonometry, his estimate of the distance between Earth and the Moon, which was fairly accurate, as well as Aristarchus' (third centuryb.c.) incorrect estimate of Earth's distance from the Sun.

Ptolemy's book was entitled Mega (mathematike) syntaxis ("Great [mathematical] compilation") although Mega was sometimes replaced by Megiste ("Greatest"). When the Arabs adopted the work, they called it Al-majisti ("The Greatest"), which it is known as today. It was translated into Latin in 1175 (as "Almagesti" or "Almagestum") and dominated European thinking for four centuries.

In the field of optics, Ptolemy wrote about the reflection and refraction of light. He lists tables for the refraction of light as it passes into water at different angles. Another book, Tetrabiblos, is a serious treatment of astrology.

Ptolemy also wrote a treatise that dealt with geography and included maps as well as tables of latitude and longitude . It explained how those lines could be mathematically determined, but only a few latitudes were calculated. He had accepted Poseidonius' (ca. 135–51 b.c.) erroneously small estimate of the size of Earth, instead of Eratosthenes' (ca. 276–194 b.c.) more accurate figure, and Ptolemy unwittingly may have altered the history of the world. After his geography had been translated into Latin, it eventually came to the attention of Christopher Columbus (1451–1506), who accepted the incorrect size and concluded that his search for a short-cut to Asia was possible.

See also History of exploration I (Ancient and classical); History of exploration II (Age of exploration)

Ptolemy (fl. AD 139–61). Egyptian geographer and astronomer sometimes called Claudius Ptolemaeus to distinguish him from the several Egyptian kings also named Ptolemy. Better known as an astronomer, he was the most important theorist before Copernicus (1543). His Geographia retains a better reputation today, as it corrects the observations of his predecessors and includes some of the most reliable data to be found in the ancient world. Divided into eight books and illustrated with twenty-six maps, including a map of the then known world, the Geographia is primarily a catalogue of places, with their latitudes and longitudes, and a brief description of each continent, country, and people.

Bibliography

See Geography, ed. P. J. Fischer (London, 1932);
Geographia (3 vols., Hildesheim, 1966);
T. G. Rylands, The Geography of Ptolemy (London, 1893);
Geography of Claudius Ptolemy, ed. Edward L. Stevenson (New York, 1932);
Walter M. Ellis , Ptolemy of Egypt (London and New York, 1994).

Ptolemies The Macedonian dynasty that ruled Egypt from 323 to 30 BC. Ptolemy I was an officer of Alexander the Great who, after the king's death, was appointed satrap of Egypt. He proclaimed himself king in 304, and by the time of his death in 283–82 he had established control over Cyprus, Palestine, and many cities in the Aegean and Asia Minor. The reigns of the Ptolemies who succeeded him were characterized externally by struggles with the SELEUCIDS for control of Syria, Asia Minor, and the Aegean; and internally by dissatisfaction and rebellion among the native Egyptians. Contact with the rising power of Rome came to a head during the reign of CLEOPATRA VII, whose liaison with MARK ANTONY led ultimately to defeat at Actium, suicide, and the annexation of Egypt by Octavian.

Ptolemy( Claudius Ptolemaeus) (2nd century AD)Egyptianastronomer and geographer. He produced the Almagest, a compendium of contemporary astronomical knowledge, drawing on writers, such as Plato and Hipparchus, whose works were kept in the great library at Alexandria. His Ptolemaic system was a geocentric model of the Universe. Highly contrived as it now appears, it accounted for the observed apparent motions of the planets reasonably well, and remained largely unquestioned until the 16th century, when it was challenged by N.Copernicus. Ptolemy's Geography enjoyed a similar period of dominance (it convinced Columbus that he could sail westwards to India); his Tetrabiblos was an astrological treatise.

Ptolemy The kings who ruled Egypt from 323 BCE after the death of Alexander the Great took this name; their aim was to introduce Greek culture and learning at Alexandria (where there lived a large community of Jews). In the 2nd cent. BCE there was considerable strife within the dynasty, with the Seleucids of Syria, and finally with the Romans. The last of the line was Cleopatra VII, married to Mark Antony, who gave her his territory in Palestine; but when Augustus defeated their combined navies at Actium in Sept. 31 BCE Cleopatra committed suicide.

Ptolemy2 (2nd century), Greek astronomer and geographer. His teachings had enormous influence on medieval thought, the geocentric view of the cosmos (the Ptolemaic system outlined in his major work Almagest) being adopted as Christian doctrine until the late Renaissance. Ptolemy's Geography, giving lists of places with their longitudes and latitudes, was also a standard work for centuries, despite its inaccuracies.

Ptolemy (ad 90–168) ( Claudius Ptolemaeus) Greek astronomer and geographer. He worked at the library of Alexandria, Egypt. Ptolemy's chief astronomical work, the Almagest, drew heavily on the work of Hipparchus. The Ptolemaic system is based on the geocentric world system of the ancient Greeks. His Geography, which provided the basis for a world map, was a definitive text until the Renaissance.

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