like a circle, and as they foci move farther apart the ellipse becomes flatter. The eccentricity of an ellipse is measured on a scale from zero to one, where an ellipse with an eccentricity of zero has the foci occupying the same region, also known as a circle. An ellipse with an eccentricity of one would be a straight line. Therefore, all ellipses have an eccentricity between zero and one. An ellipse also has two axises, the longer one which is called the major axis and the shorter one which is called the minor axis. The semi-major axis is half the size of the major axis and is called the radius of the ellipse or the size of the ellipse. (http://csep10.phys.utk.edu/astr161/lect/history/kepler.html)Once Kepler obtained Brahe’s data and knew that the orbits were not perfect circles, but instead were ellipses that were only slightly flattened, he was able to formulate his three laws of planetary motion. In order they are:1. The orbits of the planets are ellipses with the Sun at one focus of the ellipse2. The line joining the planet to the Sun sweeps out in equal areas in equal times as the planet travels around the ellipse3. The ratio of the squares of the revolutionary periods for the planets is equal to the ratio of the cubes of their semi-major axis(Baker and Fredrick, 183)The first law was probably most simplest and easiest to understand, however, it was also the most important because up until that time all theories and models had the Sun at the center with the orbits of the planets being perfect circles. Kepler instead showed that the orbits were ellipses and the Sun was not at the center, but instead it was one of the foci of the ellipse while the other foci was usually just an empty region. Knowing that the Sun is one of the foci also meant that the distance between the planets and Sun is constantly changing. This law is demonstrated the following diagram: (http://csep10.phys.utk.edu/astr161/lect/history/kepler.html)Kepler&...
