cal observations, did Kepler finally realize that this model of planetary motion was mistaken, and that in fact planets moved around the sun in ellipses, not circles. It was this discovery that led Newton, less than a century later, to formulate his law of gravity and gave us our modern conception of the universe.HIS LAWSThe German mathematician and astronomer Johannes Kepler (1571 - 1630) was a Platonist, and set out early in his professional career to show that the motion of the planets was circular, and that they could be described in terms of the Platonic solids. However, he was also a friend and assistant to the great Danish astronomer Tycho Brahe, who used the newly invented telescope to make precise observations of the planets and stars. When Tycho Brahe died, in 1601, Kepler inherited this enormous collection of data and studied it. After studying this data for 20 years, Kepler realized that his earlier assumptions about planetary motion were wrong, and that if an earth-centered (Ptolemaic) understanding of the universe were abandoned for a sun-centered (Copernican) model, then the motion of the planets was clearly elliptical. From this basis, Kepler generated his three "laws" of planetary motion: 1.The orbit of each planet is an ellipse with the sun at one focus. 2.The line segment joining a planet to the sun sweeps out equal areas in equal time intervals. 3.The square of the period of revolution of a planet about the sun is proportional to the cube of the semimajor axis of the planet's elliptical orbit. These laws are shown in the following diagram: :These laws imply that the speed of revolution of a planet around the sun is not uniform, but changes throughout the planet's year. It is fastest when the planet is nearest the sun (called the perihelion) and slowest when the planet is farthest away (called ...
