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Motions of the Planets - Windows to the Universe

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A montage of all the planets, except Pluto.
Click on image for full size
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 used the very precise observations of the positions of planets made by Tycho Brahe to prove 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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The Summer 2010 issue of The Earth Scientist, available in our online store, includes articles on rivers and snow, classroom planetariums, satellites and oceanography, hands-on astronomy, and global warming.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF