Kepler's Third Law of Planetary Motion

Topics: Astronomy

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Kepler’s third law of planetary motion tells us that the square of a planet’s orbital period is proportional to the cube of the length of the semi-major axis of its orbit.

Mathematically, given:

• $T$ the time required for a given planet to make one orbit around the Sun
• $2a$ the length of the major axis of its elliptical orbit

Kepler’s third law tells us that $T^2=k{a}^3$, for some constant $k$. That is, $T^2$ is proportional to $a^3$.