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Tycho Brahe (1546-1601) made a series of observations regarding the motion of planets and his assistant Johannes Kepler (1571-1630) abandoned the theory of circular orbit for planets and proposed the elliptical orbits for planetary motion.He formulated three laws known as Kepler's laws of planetary motion.

 

1.The orbit of a planet relative to the Sun lies in a plane containing the Sun.It is an ellipse with Sun at one of the two foci.This law is called law of orbit. 

2.The radius vector from the Sun to the planet sweeps out equal areas in equal intervals of time.This law is called law of areas.

3.The square of the planet's time period of revolution is directly proportional to the cube of the semi major axis.

                       

The law is called the law of period.