Definition: Kepler's laws:
- Kepler's first law: A planet orbits the Sun in an ellipse with the Sun at one focus.
- Kepler's second law: A line directed from the Sun to a planet sweeps out equal areas in equal times as the planet orbits the Sun.
- Kepler's third law: The square of the period of a planet's orbit is proportional to the cube of that planet's semimajor axis; the constant of proportionality is the same for all planets.