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Ptolemaic system(tŏl'əmā`ĭk), historically the most influential of the geocentric cosmological theories, i.e., theories that placed the earth motionless at the center of the universe with all celestial bodies revolving around it (see cosmologycosmology,
area of science that aims at a comprehensive theory of the structure and evolution of the entire physical universe. Modern Cosmological Theories
..... Click the link for more information. ). The system is named for the Greco-Egyptian astronomer Ptolemy (fl. 2d cent. A.D.); it dominated astronomy until the advent of the heliocentric Copernican systemCopernican system,
first modern European theory of planetary motion that was heliocentric, i.e., that placed the sun motionless at the center of the solar system with all the planets, including the earth, revolving around it.
..... Click the link for more information. in the 16th cent.
The Roots of the Ptolemaic System
The ancient philosophers imagined the universe to resemble a complex clockwork consisting of concentric crystalline spheres, nested inside one another, which carried the sun, moon, and planets in their motions and made the "music of the spheres" as they revolved. Professional astronomers did not claim that such a mechanism physically existed; rather, they treated it as the hypothetical basis for constructing geometrical schemes that would allow them to make accurate predictions of the motions and future positions of celestial bodies.
However, the motions of the planets against the stars are not uniform and circular but exhibit a host of irregularities. For a superior planet (Mars and those farther from the sun), the most important of these is the planet's retrograde motionretrograde motion,
in astronomy, real or apparent movement of a planet, dwarf planet, moon, asteroid, or comet from east to west relative to the fixed stars. The most common direction of motion in the solar system, both for orbital revolution and axial rotation, is from west to
..... Click the link for more information. at the time of opposition. The planet seems to halt and then reverse its motion for a few months, so that its complete circuit of the eclipticecliptic
, the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic). Because of the earth's yearly revolution around the sun, the sun appears to move in an annual journey through the heavens with the ecliptic as its
..... Click the link for more information. is attended by a series of yearly loops or switchbacks.
The Fundamentals of the Ptolemaic System
Partly on aesthetic grounds and partly because no other hypothesis suggested itself, Ptolemy generally retained the semimystical Pythagorean belief that nothing but motion at constant speed in a perfect circle is worthy of a celestial body. He combined simple circular motions to explain the complicated wanderings of the planets against the background of the fixed stars. Ptolemy explained retrograde motion by assuming that each planet moved in a circle called an epicycle, whose center was in turn carried around the earth in a circular orbit called a deferent. Thus the motion of all the planets around the earth in the Ptolemaic system was somewhat similar to the motion that modern astronomy ascribes to the moon as it revolves around the earth while the earth itself is revolving around the sun. The fact that the inferior planets (Venus and Mercury) never stray far from the sun was explained by the provision that the centers of their epicycles always had to lie on the line connecting the earth and sun.
In the final version of his system Ptolemy modified the postulate of uniform motion in order to explain the variations in the apparent speeds of the planets. He found that these variations could be reproduced most conveniently by displacing the earth from the center of the deferent to a point called the eccentric. He then assumed that the motion of the center of the epicycle along the deferent appeared uniform, not from the center of the deferent or from the eccentric, but from a third point symmetrically displaced from the eccentric, called the equant. This modification was tantamount to abandoning the postulate of uniform motion. Ptolemy considered it more important to achieve a closer agreement with the observed astronomical data than to adhere to any preconceived first principles. His work thus anticipates the positivist spirit of modern empirical science, which makes no ontological claim for its constructs but merely asserts that nature behaves "as if" these constructs lay behind appearances.
Ptolemaic system(tol-ĕ-may -ik) A model that attempted to explain the observed motions of the Sun, Moon, and planets and predict their future positions. Originally proposed by Apollonius of Perga in the third century bc and developed by Hipparchus, it was completed by Claudius Ptolemaeus of Alexandria in the second century ad. Ptolemy assumed a geocentric system, possibly for ease of calculation, in which the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn moved around the Earth in circular orbits of increasing radius. The position of the Earth, E, was somewhat offset from the center of the orbit, C (see illustration). The orbiting body moved at a uniform rate about a point A rather than about C. A, C, and E were colinear and AC = CE. This improved the accuracy of earlier theories in which Ptolemy had made the Moon or planet move in a small circle (an epicycle) the center of which moved around the circumference of a larger circle (the deferent), which was the body's orbit. By a suitable choice of the relative size of epicycle to deferent, the relative rates of motion in these circles, and the distance CE, Ptolemy was able to predict planetary positions to within about 1°. Comparison between prediction and observation were much simpler with a geocentric rather than a heliocentric system. It is not known however whether Ptolemy believed in an actual geocentric Solar System, as did his later followers. The Ptolemaic system, with minor modifications, was in use until the Copernican system was finally accepted.