# Kepler's Laws of Planetary Motion

Written by Jerry Ratzlaff on . Posted in Classical Mechanics

## Kepler's first law

The orbit of every planet is an ellipse with the sun at one of the two foci.

## Kepler's Second law

A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

## Kepler's third law

The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

### Formulas that use Kepler's third law

 $$\large{ P^2 = a^2 }$$

### Where:

$$\large{ P }$$ = planet's distance from the sum

$$\large{ a }$$ = semi-major axis of the planet's orbit