Kepler's Laws of Planetary Motion/Third Law/Astronomical Units

Physical Law
When distance is measured in astronomical units and time is measured in years, Kepler's Third Law of Planetary Motion can be expressed as:
 * $T^2 = a^3$

These units are derived specifically from the nature of the revolution of the Earth in its orbit, for which $T = 1 \mathrm{yr}$ and $a = 1 \mathrm {au}$.