|Born||27th December, 1571 Württemberg|
Johannes Kepler was a German astronomer and mathematician. He is best known for his laws of planetary motion in their orbit around the Sun. Kepler was originally a follower of Copernicus' ideas, and constructed a cosmological model in which the Sun was at the center of the universe, and the planetary orbits occurred on the surface of spheres. Each of the spheres of Venus, Earth,[Fact?] Mars, Jupiter and Saturn (the planets known at the time, with the exception of Mercury) was embedded into one of the five Platonic solids. Being a deeply religious man, he saw his model as a proof of the perfection of God's Creation. He wrote: "I wanted to become a theologian, and for a long time I was restless. Now, however, observe how through my effort God is being celebrated in astronomy".
However, after he started collaborating with Tycho Brahe, who had compiled the best astronomical observations of his time, Kepler realized that his model did not agree with the data. Kepler was forced to abandon the idea of circular orbits, and create a new model based on elliptical orbits. This model did not only agree with Brahe's data, but was able to accurately predict astronomical occurrences, like the transit of Venus of 1631. (The transit of Venus is when Venus crosses directly in front of the Sun, as seen from the Earth). Although his model was a success, Kepler did not like the idea that orbits were elliptical, because he had adopted an ancient Greek idea that circles are more perfect figures than ellipses, and therefore more adequate for God's perfect creation. Ironically, it can be added that, according to general relativity, discovered some three centuries after Kepler, planets do follow the simplest (and so, in some interpretations, most perfect) possible path: a straight line. However, it is a straight line in a four dimensional curved space-time, so it looks like an ellipse in our familiar three dimensional space[Citation Needed].
Kepler Laws of planetary motion
First Kepler Law: The planets move in ellipses around the Sun, with the Sun located in one of the foci of the ellipse.
Second Kepler law: The velocity of the planets in their orbits is such that an imaginary line from the Sun to the planet always spans the same area in a given time interval. This implies that planets move faster the closer they are to the Sun.
Third Kepler Law: The period squared (period the amount of time taken to complete an orbit) multiplied by the length of the semi major to the cube (The semi major axis is the longest diameter of the ellipse) is a constant, that is, it is the same for all planets.