# Kepler's laws of planetary motion

**Kepler's three laws of planetary motion** were developed in the early 17th century by Johannes Kepler. Kepler developed the laws after studying the astronomical observations of Tycho Brahe for several years. Kepler realized that the movement of the celestial bodies whose data Brahe had recorded did not fit a circular orbit, as was believed at the time.

Kepler's three laws of planetary motion are:

1. Planets orbit the sun on elliptical orbits with the sun occupying one of the foci of the ellipse.

2. A line connecting the sun to the orbiting body sweeps out an equal area in an equal amount of time regardless of position in the orbit. In other words, the body moves faster when it is closer to the sun and moves slower when it is further away. This law was much later shown to be true because of conservation of angular momentum.

3. The square of the period of the orbit of a body is proportional to the cube of the semimajor axis of that orbit.