The motion of the planets that go around the sun obeys three laws discovered by Johannes Kepler. The first law states that each planet goes around the sun in a curve called an ellipse. A chicken egg has a shape of an ellipse. The second law states that the planets do not go around the sun at a uniform speed, but move faster when they are nearer to the sun and move slowly when they are farther from the sun. Finally, the third law states that when the orbital period and orbit size of any two planets are compared, the periods are proportional to the power of the orbit size. (period = time interval it takes a planet to go completely around its orbit. size = length of the greatest diameter of the elliptical orbit. ) If written mathematically, these three laws can be written as : --> Each planet moves around the sun in an ellipse, with the sun at one foucus. --> The radius vector from the sun to the planet sweeps out equal areas in equal intervals of time. --> The squares of the periods of any two planets are proportional to the cubes of the semimajor axes of their respective orbits.