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26.2 Observing the Solar System: A History KEY IDEA When you look up and notice that the sun has moved from east to west over the course of a few hours, doesn’t it seem as though Earth is standing still and the sun is moving across the sky? This perception—that Earth is stationary—was the basis of ancient Greek theories that prevailed for over two thousand years. The Movements of Planets and Stars For thousands of years, the predominant model of the universe stated that Earth stood still at the center of the universe. Such a model of the universe is called a geocentric (JEE-oh-SEN-trihk), or Earth-centered, model. As long as 6000 years ago, astronomers were recording the movements of the stars. They noted that the stars appeared to move across the sky, but they did not move in relation to each other. To explain the apparent motions of the stars, early astronomers envisioned the stars as holes in a solid celestial sphere that surrounded Earth. Beyond the sphere, they imagined, was a source of intense light that shone through the holes. They concluded that the stars moved around Earth as the sphere rotated. Early astronomers also noticed that the same constellations, or groups of stars, became visible at the same time every year. People in many different cultures noticed this phenomenon and used the changing constellations as a basis for a calendar. Not all points of light in the sky are fixed in constellations. A few seem to wander across the sky, changing position over the course of days, weeks, and months. These wandering points of light are planets, and early astronomers inferred correctly that planets are closer to Earth than the stars are. Early astronomers noticed that most of the time the planets moved eastward in front of the background of constellations. Periodically, however, the planets stopped moving eastward and moved westward for a few weeks, then resumed their eastward paths. This pattern of apparent backward motion is called retrograde motion. Mar. April Sept. Aug. Jan. Feb. May Throughout history scientists have developed models to account for their observations of the stars and planets. The work of past scientists contributes to today’s sun-centered model of the universe. KEY VOCABULARY • geocentric • heliocentric • gravitation DEVELOP MODELS Design a way to demonstrate retrograde motion. Dec. June July West East RETROGRADE MOTION Viewed from Earth, the planet Mars seems to go backward during its orbit. The effect is due to Earth catching up with and passing Mars. Chapter 26 The Sun and the Solar System 577