The Physics Of The Universe

# Kepler's Third Law Calculator

To calculate the semi-major axis or orbital period of a planet using Kepler's third law, input the known variable and the gravitational constant.

##### Kepler's Third Law

• Semi-Major Axis: The average distance between a planet and the sun, measured in astronomical units (AU).
• Orbital Period: The time it takes for a planet to complete one orbit around the sun, measured in Earth years.
• Gravitational Constant (G): A fundamental constant of nature that appears in the law of universal gravitation, with units of (AU^3)/(Earth years^2) and a value of 39.5.

Kepler's third law relates the semi-major axis of a planet's orbit to its orbital period:

Orbital Period2 = Semi-Major Axis3 / G

This means that given the semi-major axis of a planet's orbit (in AU), you can calculate its orbital period (in Earth years) using this formula. Conversely, given the orbital period of a planet (in Earth years), you can calculate its semi-major axis (in AU).