Kepler's Laws

The three laws of planetary motion discovered by Johann Kepler.

1. Planets orbit on ellipses with the Sun at one of the focii of the ellipse
2. Equal area is swept out by the planets motion as it moves around the ellipse (a planet moves fastest when it is nearest the Sun),
3. the square of the period of the orbit is equal to the cube of the semimajor axis (half the long axis of the ellipse). This is written as P² = a³, where P stands for the period of the orbit (the sidereal period) and a is the size of the semimajor axis of the ellipse.