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 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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