Before Newton with his laws of Gravity several great minds came up with astronomical theories. One of these great minds was Johannes Kepler. If you've looked up at the night sky and seen that the stars aren't in the same place every night, you understand the ideas of Johannes Kepler.

Planets don't move around randomly. If you look up in the sky you see planets moving in a fashion called retrograde motion. For years scientists could not explain this but the work of Tycho Brahe and Johannes Kepler together could put together the idea of ellipses.

The above video explains centripetal force and how the planets are kept in perpetual motion without energy.

Johannes Kepler's First Theory: Planets move in Ellipses

Johannes Kepler's Second Theory: Each planet moves such that an imaginary line drawn from the sun to the planet sweeps out equal areas in equal periods of time.

Johannes Kepler's Third Theory: The ratio of the squares of the periods of any two planets revolving around the sun is equal to the ratio of the cubes of their average distance from the sun.

Quiz:

1. What force is it that keeps the planets in ellipses?

2. What is the foci of the ellipses?