How is Kepler's Law used to find the period of orbit for earth?
Kepler's 3rd Law of Planetary Motion states that the square of the period T of revolution for any planet (mass M) around the sun is directly proportional to the cube of the semi-major axis R of the orbit.
If one uses physics principles and derives the equation and fills in the relevant constants of proportionality, we may then make the period T the subject of the formula to obtain
(Let me know if you want a precise derivation of this formula using physics methods and I will upload it for you as well).
G is Newton's Universal Gravitational constant and has value