Planetary Orbits and Kepler’s Third Law

Theory

Universal gravitation $F=G\frac{Mm}{r^2}$ provides the centripetal force of orbits. For circular orbits, $v=\sqrt{GM/r}$ and the period obeys Kepler’s third law: $T^2\propto r^3$.

What to observe

• Inner planets are faster.
• The moon orbits its planet while the planet orbits the star.

Try this

• Select a planet and compare the predicted and observed period.
• Give a planet a push (drag it during the simulation).

Also available in: Órbitas planetárias e a 3ª lei de Kepler (Português) · Órbitas planetarias y la 3.ª ley de Kepler (Español)