Keplers’s Laws of Planetary Motion, Kepler's Law, Law of Orbits, Law of Areas, Law of periods

 Keplers’s Laws of Planetary Motion:

1^st law ( Law of Orbits ):

Every planet moves around the sun in elliptical orbit with the sun at one of the foci of the elliptical orbit.

2^nd  law ( Law of Areas):

The line joining the planet to the sun sweeps over equal areas in equal intervals of time.

                                                           Area of SAB = Area of SCD 

2^nd  law tells us that a planet does not move with constant speed around the sun. It speeds up while approaching the nearest point called ‘ perigee’ and slows downs while approaching the farthest point called ‘ apogee’. therefore, distance covered on the orbit with in the given interval of time at perigee is greater than that at apogee such that areas swept are equal.

3^rd Law ( Law of periods):

The square of the time periods of revolution of a planet around the sun is directly proportional to the cube of the mean distance of a planet form the sun.

The law is mathematically expressed as

Thought kepler gave the laws of planetary motion, he could not give a theory to explain the motion of planets.

Only Newton explained that the cause of the motion of the planets is the gravitational force which the sun experts on them.

Newton used kepler’s 3^rd law to develop the law of universal gravitation.