Kepler’s Law of Planetary Motion

·        Kepler’s Laws describes the motions of the planets in the solar system.

·        They were proposed by a German Astronomer Johannes Kepler.

Kepler’s three laws of planetary motion can be stated as follows:

·        First Law:

All the planets of solar system revolve around the sun in elliptical orbits having the Sun as one of the foci.

Second Law:

A radius vector joining any planet to the sun sweeps out equal areas in equal period of time.

·        Third Law:

The square of the orbital period of a planet is directly proportional to the cube of the semi major axis of its orbit.

The elliptical orbits of the planet were indicated by calculations of the orbit of Mars. From this, Kepler inferred that other bodies in the solar system, including those farther away from the Sun, also have elliptical orbits. The second law helps to establish that when a planet is closer to the sun it travels faster. The third law expresses that the farther a planet is from the sun, the longer its orbit, and vise versa.

Sir Isaac Newton showed in 1687 that relationships like Kepler’s would apply in the Solar system to a good approximation, as a consequence of his own laws of motion and law of universal gravitation.