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 Greek philosophers thought that the planets and stars were part of the gods'
realm and followed a "natural motion." They did not realize that Gravity is
involved. The Greeks' ideas stuck around until the 16th century.
 Beginning in the 1500s, though, astronomers like Galileo and Brahe discovered
that the earth and other planets revolved around the sun. Kepler showed that
they moved in an elliptical orbit, not a circle. The question was why.

Sir Isaac Newton -- The Discoverer of Gravity!

 Sir Isaac Newton was an English mathematician and mathematician and
physicist who lived from 1642-1727.
 The legend is that Newton discovered Gravity when he saw a falling apple while
thinking about the forces of nature.
 Whatever really happened, Newton realized that some force must be acting on
falling objects like apples because otherwise they would not start moving from
rest.
 Newton also realized that the moon would fly off away from Earth in a straight
line tangent to its orbit if some force was not causing it to fall toward the Earth.
The moon is only a projectile circling around the Earth under the attraction of
Gravity.
 Newton called this force "gravity" and determined that gravitational forces exist
between all objects.
 Using the idea of Gravity, Newton was able to explain the astronomical
observations of Kepler.
 The work of Galileo, Brahe, Kepler, and Newton proved once and for all that the
Earth wasn't the center of the solar system. The Earth, along with all other
planets, orbits around the sun.
 Two astronomers, J.C. Adams and U.J.J. LeVerrier, later used the concept of
Gravity to predict that the planet Neptune would be discovered. They realized
that there must be another planet exerting a gravitational force on Uranus
because Uranus had odd perturbations in its orbit. (Perturbations are deviations
in orbits.)

Albert Einstein -- The General Theory of Relativity

 Einstein developed a whole new idea about Gravity.
 According to Einstein, Gravity arises from the "warping" of space and time.
 Einstein's new theory of Gravity explains a number of phenomena that would
violate Newton's theory. For example, light bends when passing near massive
objects like the Sun. And a clock rose above the Earth speeds up relative to a
clock on the surface.

Basic Facts About Gravity