Kepler's Third Law (Period–Semi-major Axis) (T² = 4π²a³/(GM))

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T² = 4π²a³/(GM)

What each symbol means

SymbolWhat it stands for
GGravitational constant
MMass of central body (kg)
TOrbital period (s)
aSemi-major axis (m); for circle a=r

When to use this

Planet/satellite mass ≪ M; M is attracting centre. Circular orbit: replace a by r.