Johannes Kepler

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Astronomia Nova -  · The Astronomia Nova, or the New Astronomy was Kepler's masterpiece. Published in 1609, it was the result of over eight years of work. Kepler spent those years trying to work out the shape of the orbit of Mars. Using Tycho's data about the motion of the planets, Kepler was finally able to determine the shape of the orbit more accurately than anyone who had come before him. This resulted in the formation of his first two laws, which were published in the Astronomia Nova.
Geocentric -  · A geocentric system is one in which the earth is at the center of the universe. For thousands of years, scientists, philosophers, and theologians believed that the universe was geocentric. They were unwilling to believe Copernicus when he challenged that assumption.
Harmonice Mundi -  · The Harmonice Mundi, or Harmony of the World was the culmination of Kepler's life-long study of the structure of the universe. Published in 1618, it described a system in which the spacing between the planets was determined by universal harmonies. The theory was wrong, but the book is nonetheless important, as it marks the first appearance of Kepler's third law.
Heliocentric  -  · A heliocentric system is one in which the sun is at the center of the universe. The system that Copernicus introduced was a heliocentric system. This was not a completely original idea – some of the philosophers of ancient Greece had imagined that the universe might be constructed in this way. However, the dominant view had always been that the universe was geocentric, so Copernicus's claims were a shock to the European system.
Kepler's Three Laws  -  · Kepler is best known today for his contribution of the three planetary laws, which were instrumental in Newton's later development of his theory of universal gravitation. They are as follows: 1. The planets travel around the sun in elliptical orbits with the sun located at one focus. 2. As the planets travel around their orbits, they sweep out the same amount of area per unit of time, no matter where they are on the orbit. 3. The distance a planet's orbit is from the sun, cubed, is directly proportional to the time it takes the planet to travel around the orbit, squared. Mathematically, this can be stated as a3/p2 = K where "a" is the distance a planet's orbit is from the sun, "p" is the period, the time it takes for a planet to revolve around the sun once, and "K" is a constant.
Mysterium Cosmographicum -  · Published in 1597, the Mysterium Cosmographicum, or Mysteries of the Cosmos, was Kepler's first major work. It described his theory of the perfect solids, which, although he never fully admitted it, was completely wrong. More importantly, the Mysterium was Kepler's first step to rejoining physics and astronomy, as he grasped for physical explanation for the structure of the universe. He was the first astronomer in centuries to do so. It is in the Mysterium that Kepler first proposes that the sun be moved to the exact, physical center of the universe, and that a force from the sun is responsible for moving the planets around their orbits. The Mysterium was also the major work in fifty years to support the Copernican system.
Perfect solid -  · A perfect solid a three dimensional figure, such as a cube, whose sides are all identical. There are only five perfect solids: the tetrahedron (which has four triangular sides), cube (six square sides), octahedron (eight triangular sides), dodecahedron (twelve pentagonal sides), and icosahedron (twenty triangular sides). Each perfect solid can be inscribed in and circumscribed around a sphere. In the beginning of his career, Kepler believed that the planetary orbits could all be inscribed in one of the perfect solids.

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