Kepler's 3rd law says what?

Kepler's third law, also known as the law of the cycle: the planets around the sun's revolution cycle and their elliptical orbit semi-long axis of the cubic is proportional. It is not difficult to derive from this law that the gravitational force between the planets and the sun is inversely proportional to the square of the radius. This is an important basis for Isaac Newton's law of gravitation. Expressed as a formula

t²:a³=K

a is the planetary orbit half-long axis, t is the planetary revolution period, and K is a constant.