Kepler's Laws of Planetary Motion

                                                             

Johannes Kepler was a great astronomer is the 17th century, and he was the first to give true predictions and laws of planets' orbits and their properties. Below, there is a brief description of his laws and explanations, why they do so as they are mentioned:

1st Law:

                                                

First law states that when multiple planets are orbiting the sun, the planets' orbits are not circular, but an ellipse. An ellipse is a shape, like an oval, with two foci and the sun is present in one of those foci. If there was only one planet orbiting the sun, the planet's orbit would be exactly circular. But when another planets enters, the gravitational pull of both the planets can affect the orbit of each other. So, the orbits become ellipses, or an oval. For example, when only Earth is orbiting the sun, the orbit of Earth would be exactly circular, so therefore, there will be no such thing called seasons as earth won't go away from sun, or come too close to the sun. But when a massive planet like Jupiter or Saturn start orbiting the sun, the massive planet may pull earth away from the sun, when they are in a straight line, for a period of time. But still, earth would be orbiting the sun. But when earth exits the straight line between the sun and the massive planet, the massive planet stops pulling earth, and sun will be the only source, which pulls earth. So, in this period, the earth would move closer to sun. So, it is clear that earth sometimes comes close to sun, and sometimes goes away from sun, staying in orbit. So, the shape finally, is an oval or an ellipse.

2nd Law:

Second law states that the line joining the earth ans sun covers equal areas in equal intervals of time. This means that:

Consider the above picture. Sun is at position A, and Earth is in position B. After two months, Earth is in position C, and a line is connected from sun to earth. We can see that A, B and C form a sector. Take this sector as Y.

After another two months ( same time period between first two positions ), Earth is at position D. So, another sector is formed by A, C and D. Name this sector as Z.

As there is same time period gap between B,C and C,D ; the areas of sectors Y and Z is equal.

3rd Law:

The square of period of revolution of a planet around the sun is directly proportional to he cube of the average radius of the planet's orbit.

This means that the square of the time, taken by a planet to complete its orbit around the sun is directly proportional to the cube of the average radius of the planet's orbit, as it is not a circle. If only one planet orbited around the sun, we would take the average radius as the actual radius of the orbit. This law is commonly known as law of periods. The farthest point of a planet from the sun is called aphelion and the closest point is called perihelion. 

                                                                                                                         Article by M. Santosh

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