"In his 1619 book The Harmony of the World he tells us that he discovered a harmonic law while delivering a lecture on astronomy to his students. Kepler found that for each planet, the cube of the average distance from the sun is proportional to the square of the period of revolution. Kepler later found a similar law for the satellites of Jupiter. Today we know that such a law holds for any system of bodies that circulates around a central parent body. There are many applications of Kepler's law; for instance, half a century later it gave Isaac Newton the clue to his discovery of the law of universal gravitation."