Kepler's teacher, Tycho Brahe, observed the motions of the planets for many years, and he wrote everything in notebooks.
Later, after Tycho's death, Kepler himself put a lot of work into studying Tycho's notebooks. He showed that the SIMPLEST, most likely explanation for what we actually see in the sky is:
-- Earth is not the center of the universe (choice-B), and neither is any other planet.
-- Earth, and all the other planets, travel around the sun in paths that are ellipses, with the sun at one focus of the ellipse.
-- A line from the sun to any planet sweeps over equal areas in equal amounts of time. This means that a planet travels faster when it's closer to the sun, and slower when it's farther away from the sun.
-- For all of the planets in the solar system, the ratio of (the planet's orbital period) to (the planet's distance from the sun) is the SAME NUMBER. This means that a planet's "year" only depends on how far it is from the sun. Nothing else matters, like for example its mass. It also means that once you decide how long you want a planet's orbit to take, there's only one place you can put it !
A century after Kepler, Newton showed that IF his laws of Motion and Universal Gravity are correct, then planets MUST behave the way Kepler described.
Notice that Neither Kepler nor Newton PROVED anything.
400 years after Kepler, and 330 years after Newton, all of the work of both of them is still "just a theory", like the unproven theories of viruses, air, atoms, and germs.
