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A montage of all the planets, except Pluto.
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NASA/JPL.

Motions of the Planets

For many years, people believed that the Earth was the center of the universe, that the Earth didn't move and that the planets, Sun, moon, and the stars moved on spheres around the Earth. Astronomers such as Copernicus and Galileo suggested that the Sun was the center of the solar system. But people weren't ready to accept that the Earth wasn't the center of the universe.

Johannes Kepler studied the planets and the work of his teacher, Tycho Brahe. He proved that this theory could explain the motions of planets. His work completely changed the way we view the universe.

Kepler formulated three laws which describe how the planets move on their orbits around the Sun. Kepler derived these laws, but he didn't understand why planets are forced to move in this way. Gravity wasn't "discovered" until Sir Isaac Newton, who could then show that Kepler's laws are simply a consequence of the force of gravity between the Sun and the planets.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA