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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