Kepler’s laws apply to objects that revolve around the Sun. Its complete form is:
p2 = k a3
where k is a proportionality constant.
When this relation is applied to the Earth for which p = 1.0 y and a = 1.0 AU, k = 1.0 .
Therefore, Kepler’s third law takes the form:
p2 = a3 but p must be expressed in years and a must be in AU and the object must revolve around the Sun rather than around a planet.