Kepler’s Laws of Planetary Motion



  • The first law states that the orbit of a planet is an ellipse, with the Sun at one focus. The second states that a line between the Sun and a particular planet sweeps out equal areas in equal times. The line is termed the radius vector. This law means that the nearer a planet is to the Sun, the faster it moves. The third law states that the square of the orbital period of a planet is proportional to the cube of its distance from the Sun. Neptune is on average thirty times as far from the Sun as the Earth, but takes 165 times as long to orbit the Sun, since 303 is 27,000, as is 1652. This law means that more distant planets have much longer years. Kepler’s laws also apply to the orbits of artificial and natural satellites, binary stars, and other celestial objects including comets and asteroids.