Tycho Brahe was a scientist who used an instrument called a quadrant to measure the angular position of stars and planets to the Earth. He got the most accuarate data at that time.After Tycho Brahe died, his assistant Johannes Kepler took those data to analyze and noticed that he could summerize the observation result using three basic mathematical statements about the solar system, known as Kepler's laws of planetary motion:

1.The path of the planets about the sun is elliptical in shape, with the center of the sun being located at one focus.

2.An imaginary line drawn from the center of the sun to the center of the planet will sweep out equal areas in equal intervals of time. 

3.The ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distances from the sun. 

The first statement is known as Kepler's First Law, also known as Law of Ellipses.It tells us that all planets in the solar system are turning around the sun in an elliptical path. The second statement is known as Kepler's Second Law and the Law of Equal Areas. If basically says if an imaginary line were drawn from the center of the any planet (in solar system) to the center of the sun, that line would sweep out the same area in equal periods of time. That is a great discovery because the planets in the solar system does not move at the same speed all the time, but this Law of Equal Areas is always true. The third statement is called Kepler's Third Law as well as Law of Harmonies. It says every planet has the same T²/R³ ratio. "T" refers to orbital period while R refers to radius of orbit of a planet in solar system.