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Johannes Kepler (December 27, 1571 to November 15, 1630) was a mathematician, astronomer, and astrologer. He was, also, a key figure in the scientific revolution. He is most famous for his laws of planetary motion. Kepler incorporated religious arguments and reasoning into his work. He was motivated to do this by the conviction that God had created the world according to an intelligible plan that is accessible through reason, referring to his view of astronomy as ‘celestial physics’.
His first major astronomical work, Mysterium Cosmographicum, was the first published defense of the Copernican system. While teaching, he realized that regular polygons bound one inscribed and one circumscribed circle at definite ratios, which might be the geometrical basis of the universe. He failed to find a unique arrangement of polygons that fit known astronomical observations, so he began experimenting with polyhedra. He found that each of the five Platonic solids(convex polyhedron that is regular) could be uniquely inscribed and circumscribed by spherical orbs. Nesting each of these solids in a sphere within one another would produce six layers, corresponding to the six planets known at the time(Mercury, Venus, Earth, Mars, Jupiter, and Saturn). Then, by ordering the solids correctly(octahedron, icosahedron, dodecahedron, tetrahedron, and cube), he found that the spheres could be placed at intervals corresponding to the relative sizes of each planet’s path, if he assumed heliocentrism to be correct.
Kepler is best known for his three laws of planetary motion. Law 1 states the orbit of every planet is an ellipse with the Sun at one of the two foci. Law 2 says that a line joining a planet and the Sun sweeps out equal areas during equal intervals of time. Law 3 states the square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.
The first law is easy to understand. Planets do not orbit in a perfect circle, but in an ellipse. In his time it was a revolutionary thought that the Earth was the the center of the universe. It was a further shock to propose that the Sun was not the center of an orbit. Kepler showed that the Sun was merely one focal point of the ellipse. Today we know that the barycenter of the Sun/planet group is the actual foci.
The second law(law of equal areas) describes the speed at which any given planet will move while orbiting the Sun. That speed is constantly changing. A planet moves fastest when it is closest to the Sun and slowest when it is furthest away, but if an imaginary line were drawn from the center of the planet to the center of the sun, that line would sweep out the same area in equal periods of time.
The third law(law of harmonies) compares the orbital period and orbital radius of a planet to those of other planets. The comparison being made is that the ratio of the squares of the periods to the cubes of their average distances from the Sun is the same for every planet.
We’ve also recorded an entire episode of Astronomy Cast all about the Johannes Kepler and His Laws of Planetary Motion. Listen here, Episode 189: Johannes Kepler and His Laws of Planetary Motion.