(1571-1630AD)
Kepler was born in Weil der Stadt (now Germany). He was among the first astronomers to think about how the planets are affected by the sun in terms of the planet's distance from it. He began theorizing that the sun was emitting a constant array of heat that hit each of the planets. And that this heat is more intense when a planet is close to the sun, and less intense when it is far from the sun.
Eventually Kepler began scrutinizing the idea brought forth from previous astronomers that the orbits of the planets are circular. Kepler began subscribing to the idea that orbits are in fact elliptical, not circular; and that a planet's orbital velocity increases when close to the sun and decreases when further away from the sun. This led to Kepler's second law, being "equal areas, equal time", meaning that no matter where a planet is in terms of its orbit, it will cover the same amount of distance for each equal amount of time it travels. This is all concluded by Kepler's third law being "the square of the orbital period of a planet is equal to the cube of the orbital radius of the planet's orbit".
A model showing elliptical orbits, as well as explaining Kepler's law "equal area, equal time".
