**Answer: **Johannes Keppler, studied the data available about planetary positions and motion and noticed that the motion of planets follows certain laws. According to him, there are three laws describing planetary motion known as Kepler’s laws which are given here.

Kepler’s first law states that the orbit of a planet is an ellipse with the Sun at one of the foci. Take the image given below:

This image shows the elliptical orbit of a planet revolving around the sun with the position of the Sun indicated by S.

Kepler’s second law states The line joining the planet and the Sun sweeps equal areas in equal intervals of time. Distances covered by the planet in equal time, are AB and CD, i.e. After equal intervals of time, the positions of the planet starting from A and C are shown by B and D, respectively.

Then there is Kepler’s third law that states the square of its period of revolution around the Sun is directly proportional to the cube of the mean distance of a planet from the Sun.

Hence,

if r is the average distance of the planet from the Sun and

T is its period of revolution then,

T^{2} a r^{3} i.e. T^{2}/T^{3}= constant = K.