Kepler's Laws of Planetary Motion

        Keller's Laws of Planetary Motion 
Johannes Kepler was a 16th  century astronomer who established three  laws which govern the motion of planets (around the sun). These are known as Kepler's law of planetary motion. The same laws also describe the motion of satellite (like the moon) around the planet (like the earth ). 
1.  Kepler's first law states that : The planet move around the sun, with the sun at one of the two foci of the elliptical Orbit. This law means that the orbit ( or path) of a planet around the sun is an ellipse ( oval-shaped) and not an exact circle. An elliptical part has two foci, and the sun  is at  one of the two foci of the elliptical path ( foci is the plural of focus).
2. Kepler's second law states that : Each planet revolves around the sun in such a way that the line joining the planet to the sun sweeps over equal areas in equal interval of time. We know that a planet moves around the sun in an elliptical Orbit with sun at one of its focus. Now, since the line joining the planet and the sun sweeps over equal areas in equal interval of time,  it means that a planet moves faster when it is closer to the sun, and moves slowly when it is farther from the sun.
                  The Kepler's second law tells us that a planet does not move with constant speed around the sun. The speed is greater when the planet is nearer the sun, and less when the planet is farther away from the sun. A planet could move around the sun with constant speed only if its orbit were a true circle (and not in ellipse).
3. Kepler's third law states that : The cube of the mean distance of a planet from the sun is directly proportional to the square of time it takes to move around the sun. Though Kepler gave the laws of planetary motion but he could not give a theory to explain the motion of planets.It was Newton who showed that the cause of the motion of planets is the gravitational force which the sun  exerts on them. In fact, Newton used the kepler's third law of planetary motion to develop the law of universal gravitation.
         
I am Saheb Sarkar from purnea .

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