Kepler’s Laws of planetary motion is based on his analysis of Tycho Brahe's long and detailed observations of the orbits of the planets.
- The laws are:
1. The planets orbit the Sun in elliptical orbits, with the Sun as one common focus.
2. The line between a planet and the Sun (the radius vector) sweeps out equal areas in equal periods of time (sometimes called the Law of Equal Areas).
3. The square of a planet's period, T, is directly proportional to the cube of its average distance from the Sun, r: T2 ∝ r3.
- ↑ The third law, also known as the Law of Periods or Harmonic Law, also applies to other orbital systems from the moons of Jupiter through to binary star systems. Astronomers use it to calculate the masses of stars in binary systems.