Here is what Johannes Kepler discovered:
-- IF we ASSUME that the orbits of the Earth and the other
planets are ellipses with the Sun at one focus,
AND
-- we ASSUME that each planet moves slower in its orbit
when it's farther from the Sun, and moves faster in its orbit
when it's closer to the Sun,
THEN
-- this model is simpler than any model that has ever been
suggested to explain how everything is put together,
AND
-- this model matches the real motions of the real planets that
we really see in the real sky, closer than any other model that
has ever been suggested to explain how they really move.
That's what Kepler discovered. He didn't PROVE anything,
BUT ... 75 years later, when Newton published his law of universal
gravitation, he was able to show that IF his law of gravity is correct,
THEN planets MUST have orbits and move in them exactly the way
that Kepler's model suggested.
