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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 unmoving center of the universe and that the planets,Sun, moon, and the stars moved on spheres around the Earth. Astronomers such as Copernicus and Galileo suggested that a Sun centered solar system offered a better way to understand the motions of these objects in the sky. 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, and proved that this theory could explain the motions of planets. His research revolutionized astronomy.

From these observations, Kepler formulated three laws of planetary orbits which describe how the planets move on their orbits around the Sun. By studying the observations, Kepler derived these laws, but he didn't understand why planets are forced to move in this way. The concept of gravity wasn't fully understood until Isaac Newton, who could then show that Kepler's laws are simply a consequence of the gravitational attraction 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