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Kepler's Third Law

Relates orbital period to semi-major axis.

Formula

T2=4π2GMa3T^2 = \frac{4\pi^2}{GM}a^3

Variables

TOrbital period
aSemi-major axis
MCentral mass

Example

Earth’s a = 1 AU and T = 1 year satisfy T² = a³ exactly.

Did You Know?

Kepler's third law let astronomers weigh the Sun and planets long before any spacecraft flew.

keplerorbitperiod