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As far back as 1100 bc, Mesopotamian astronomers drew constellations, or formations of stars perceived to form shapes. Some of today’s constellation names date back to that time. Mesopotamian and Babylonian cultures mapped the motion of the planets across the sky by observing how they moved against the background of stars.

Until the 16th century, most people (including early astronomers) considered Earth to be at the center of the universe. Greek philosopher Aristotle proposed a cosmology in about 350 bc that held for thousands of years. Aristotle theorized that the Sun, the Moon, and the planets all revolved around Earth on a set of celestial spheres. These celestial spheres were made of the quintessence—a perfect, unchanging, transparent element. According to Aristotle, the outermost sphere was made of the stars, which appear to be fixed in position. Early astronomers called the stars “fixed stars” to differentiate between stars and planets. The spheres inside the sphere of the fixed stars held the planets, which astronomers called the “wandering stars.” The Sun and Moon occupied the two innermost spheres. Four elements (earth, air, fire, and water) less pure than the quintessence made up everything below the innermost sphere of the Moon. In about 250 bc, Greek astronomer Aristarchus of Sámos became the first known person to assert that Earth moved around the Sun, but Aristotle’s model of the universe prevailed for almost 1,800 years after that assertion.

Early astronomers called the planets wandering stars because they move against the background of the stars. Astronomers noted that the planets sometimes moved ahead with respect to the stars but sometimes reversed themselves, making retrograde loops. In about ad 140, Greek scientist Ptolemy explained the retrograde motion as the result of a set of small circles, called epicycles, on which the planets moved. Ptolemy hypothesized that the epicycles moved on larger circles called deferents and that the combination of these motions caused the dominant forward motion and the occasional retrograde loops.


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