State Kepler's law of period in planetory motion.

Explanation

1. Law of Orbits: Each planet moves around the Sun in an elliptical orbit with the Sun atone of the foci.
2. Law of area: The radial vector (line joining the Sun to a planet) sweeps equal areas in equal intervals of time.
3. Law of period: The square of the time period of revolution of a planet around the Sun in its elliptical orbit is directly proportional to the cube of the semi-major axis of the ellipse.
` T^2 prop a^2`
` T^2/a^3 ` = constant