Kepler's third law states that the square of the period of revolution of a planet is proportional to the cube of the semimajor axis of the planet's elliptical orbit around the Sun. In symbols, or . The constant can be taken as 1 for simplicity, equivalent to measuring in years and in astronomical units. In this Demonstration the ellipses have different eccentricities; the value of is chosen to equal 5, so that the period is equal to .