NAME:________________________________________ Period:__________ Kepler’s 3 Laws of Planetary Motion 1. Law of Orbits Kepler's first law states "all planets move in elliptical orbits with the Sun at one focus and the other focus empty". Applied to Earth satellites, the center of the Earth becomes one focus, with the other focus empty. 2. Law of Areas Kepler's 2nd law, the law of areas, states "the line joining the planet to the Sun sweeps over equal areas in equal time intervals". When a satellite orbits, the line joining it to the Earth sweeps over equal areas in equal periods of time. Segments AB and CD take equal times to cover. Therefore, the speed of the satellite changes, depending on its distance from the center of the Earth. Speed is greatest at the point in the orbit closest to the Earth, called perigee, and is slowest at the point farthest from the Earth, called apogee. It is important to note that the orbit followed by a satellite is not dependent on its mass. In simpler terms, as a planet moves closer to the sun its speed increases. As the planet moves further from the sun, its speed decreases. 3. Harmonic Law Kepler's 3rd law, the law of periods, relates time required for a planet to make 1 complete trip around the Sun to its mean distance from the Sun. "For any planet, the square of its period of revolution is directly proportional to the cube of its mean distance from the Sun." So, the time a planet takes to complete one revolution around the sun depends on the planet’s distance from the sun. The further a planet is from the sun, the more time it takes.