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developer of the "Earth-centered universe theory"
Claudius Ptolemy was born some time around 100, and died about 165. Nothing is known about his personal life or early work, only that he began conducting astronomical observations in Alexandria, Egypt, around 150.
Ptolemy's most important known work was the 13-volume Mathematike Syntasis (Mathematical Composition). Now commonly known as the Almagest (a Greek-Arabic term meaning "the greatest"), it was this work that established the theory of how the universe is structured that served as the basis of astronomy for 1,400 years. In it, Ptolemy explained that if the Earth moved, as some earlier philosophers had suggested, then certain phenomena should be observed. In particular, he argued that since all bodies fall to the center of the universe, the Earth must be fixed there at the center, otherwise falling objects would not be seen to drop toward the center of the Earth. And, if the Earth rotated once every 24 hours, a body thrown vertically upward should not fall back to the same place, as it was always seen to do. Therefore, the only logical explanation for the apparent movement of the Moon, planets, Sun, and stars was that the Earth is a stationary object at the center of a series of spheres.
In the Ptolemaic System, each planet is moved by two or more spheres. One sphere is its deferent, a circle centered on a point halfway between the equant (directly opposite the Earth from the center of the deferent) and Earth. Another sphere is the epicycle, which is embedded in the deferent, and within which the planet is embedded. The deferent rotates around the Earth while the epicycle rotates within the deferent, causing the planet to move closer to and farther from Earth at different points in its orbit, and even to slow down, stop, and move backward (retrograde motion). The epicycles of Venus and Mercury are always centered on a line between Earth and the Sun, which explains why they are always near it in the sky. The order of these spheres, from center out, are Earth, Moon, Mercury, Venus, Sun, Mars, Jupiter, Saturn, and stars.
Two volumes of the Almagest described the mathematical arrangement of the stars and charted the celestial latitudes and longitudes of 1,022 stars, grouped into 48 constellations. The vast majority of Ptolemy's constellations are still included among the 88 constellations now commonly listed.
Ptolemy's other major work, the 8-volume Geographica, began with a theory of map projection and included a list of places with longitudes and latitudes, 26 color maps, and a map of the world. Although it was the most comprehensive geographical work of his day, Ptolemy's world map was also seriously flawed, as it greatly exaggerated the size of the Eurasian land mass and greatly underestimated the size of the ocean. It was Ptolemy's map that convinced Christopher Columbus that Asia could be easily reached from Europe by sailing due west.
Ptolemy also devised new geometrical proofs and theorems; and, in a book entitled Analemma, he discussed the details of the projection of points on the celestial sphere onto three planes at right angles to each other -- the horizon, the meridian, and the prime vertical. Another book, the Planisphaerium, dealt with stereographic projection (the delineation of the forms of solid bodies on a plane), and in it he used the south celestial pole as his center of projection.
Ptolemy also prepared a calendar that gave, in addition to weather indications, the risings and settings of the stars in the morning and evening twilight. Other mathematical publications include the 2-volume work Hypotheseis ton planomenon (Planetary Hypothesis) and two separate geometrical works, one of which is concerned with proving that there cannot be more than three dimensions of space, the other containing an attempted proof for a postulate on parallel lines that had been devised by Euclid. In his 5-volume work Optica (Optics), Ptolemy discussed the refraction of light as it passes from one medium into another of different density. The work also included a table of refractions. Ptolemy was also the author of Harmonica, a 3-volume treatise on music.
This page was last updated on 02/21/2017.