Kepler's three laws are geometric relationships that describe the motions of the planets in the solar system. German astronomer Johannes Kepler derived them in the early 1600s, with the help of more than two decades' worth of detailed observations by Danish astronomer Tycho Brahe. Kepler worked as Tycho's assistant for the last 18 months of Tycho's life. Much of Tycho's work described the position of Mars in its orbit around the Sun.
Kepler was one of the first people to attempt to explain why planets move the way they do around the Sun. When Kepler joined Tycho's crew of assistants, he used Tycho's data to confirm his intuitions about the motion of Earth--that our planet is not privileged, but rather, moves about the Sun much like all the other planets. While working with Tycho's stacks of records on Mars, Kepler devised his laws of planetary motion. The first two were published in 1609, five years after he originally conceived them. Kepler published the third law in 1618. Together, these laws laid the groundwork for Sir Isaac Newton, whose laws of motion and universal gravitation were the foundation of modern physics. Newton's law of universal gravitation was mathematical proof of Kepler's laws.
The three laws of planetary motion are: 1. Planets revolve around the Sun in elliptical orbits. The Sun sits at one focus of the ellipse, rather than the center of a circular orbit as once thought. 2. Each planet moves so that a line connecting the planet and the Sun sweeps out equal areas of space in equal periods of time. In other words, a planet moves more quickly in its orbit when it is closer to the Sun. Kepler came upon this law after painstakingly calculating Mars' distance from the Sun at every degree of its orbit. He used Tycho's meticulously recorded observations for this work. From his calculations, Kepler deduced that Mars couldn't possibly revolve around the Sun in a circular orbit. Instead, the planet must revolve around the Sun in an orbit shaped like an ellipse. Thus, the first law of motion was born. (Kepler himself never numbered the laws.) 3. The square of a planet's period of orbit around the sun is directly proportional to the cube of a planet's average distance from the Sun. That is, d3 /T2 is the same for all planets.
Kepler's laws are true for any satellite that orbits a body—including artificial satellites scientists launch from Earth. As such, the laws are useful in predicting the motion of one object around another.
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