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Astronomy, study of the planets, stars, and galaxies. To most early astronomers, the earth appeared to be surrounded by a sphere that contained the stars. The high point of early astronomy was the work of the Greeks. Pythagoras (6th century B.C.), who imagined the sun, moon, and planets positioned on transparent spheres that moved along with the sphere of stars, introduced the notion of “the music of the spheres.” Hipparchus (2nd century B.C.) compiled—without a telescope—an accurate catalog of 850 stars; his system of magnitude ratings is the basis for the one used today. Ptolemy (2nd century A.D.) laid out a geocentric scheme of the universe, with the earth at the center, that was regarded as gospel for 1,500 years. In 1543, the Polish mathematician Nicholas Copernicus challenged Ptolemy's theory with the idea that the sun is stationary, with everything else circling around it. In 1609, the Italian Galileo Galilei made his own telescope and confirmed Copernicus's heliocentric theory. At the same time, the German mathematician Johannes Kepler finally discredited the geocentric theory. In the late 17th century English mathematician Isaac Newton formulated laws of motion to explain why objects move as they do, how the planets stay in orbit, and why their orbits are elliptical.

Nineteenth-century astronomers analyzed the composition of stars and wondered what causes them to burn. In the early 20th century, Albert Einstein announced his theory that mass and energy are equivalent, and the idea of nuclear power was introduced. It is now known that the sun produces energy by nuclear fusion.

The branch of astronomy called cosmology seeks to find out how the universe originated. One clue discovered in the 1930s by Edwin Hubble resulted in the idea of an expanding universe and the possibility that it started off with a giant explosion—the big bang theory. An opposing idea, promoted by Fred Hoyle, the steady state theory, holds that the universe remains stable because new matter is created to fill the gaps created as galaxies expand. Space exploration and new techniques (advances in radio astronomy, and methods based on gamma, ultraviolet, and X rays) continue to expand our knowledge of the universe.

See also: Cosmology.

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