Johannes Kepler published Book IV of the Epitome of Copernican Astronomy in 1617. In Book IV, Kepler formally presents his three laws of planetary motion that resolve the problems associated with the epicycles of Copernicus’ heliocentric model. Kepler’s First Law is called the Law of Ellipses. It states that “the orbits of the planets are ellipses, with the Sun at one focus.” Kepler’s Second Law is called the Law of Equal Areas in Equal Time. It states that “the line between a planet and the sun sweeps out equal areas in the plane of the planet’s orbit over equal times.” Kepler’s Third Law is called the Law of Harmony. It states that “the time required for a planet to orbit the sun, called its period, is proportional to half the long axis of the ellipse raised to the 3/2 power.” Although Kepler discovered these Laws, he did not know how they worked. Newton solved this problem with his Theory of Gravity.
Great Books of the Western World 