# Planets and Kepler’s Laws: Explanation

The planets of the Solar System are 9 and all have an almost spherical shape. According to the order of increasing distance from the Sun they are Mercury , Venus , Earth , Mars , Jupiter , Saturn , Uranus , Neptune and Pluto . Mercury, Venus, Earth and Mars are called inner planets while the others are called outer planets.

All the planets are in motion. There are two types of motion: the rotational motion of the planet on itself around an axis of rotation and a motion of revolution around the Sun. The time taken by a planet to rotate around its axis is called day while time used to rotate around the Sun is called a year. The planets make their revolution in elliptical orbits that are almost coplanar with the Earth’s orbit with the only exception of Pluto. The plane of the Earth’s orbit is called the plane of the ecliptic. The motion that the planets make around the Sun follows three fundamental rules, called Kepler’s laws .

## KEPLER’S LAWS EXPLAINED SIMPLY

The first law concerns the shape of the orbit : the planets move around the Sun on elliptical orbits , of which the Sun  occupies one of the foci. According to this law, the orbits of the planets are not perfect circles and the center of the Sun is not the center of the Solar System. Therefore, due to the elliptical shape of the orbit and the position of the Sun, the distance from the Sun of the moving planets changes continuously. The point of the orbit + near the Sun is called Perielio, the point of the orbit + distant from the Sun is called Aphelion. The line that joins perihelion and aphelion and coincides with the major axis of the ellipse is called Line of the apses.

## KEPLER’S SECOND LAW: DEMONSTRATION

The second law concerns the speed with which planets move on their orbit : each planet moves on its orbit in such a way that the line that ideally joins it to the Sun sweeps equal areas in equal times. That is + a planet is far from the Sun + it is slow. Therefore the speed of revolution of the planets along their orbit is not constant but varies from day to day. The minimum speed is when the planet is in aphelion, the maximum speed when it is in perihelion.

## KEPLER’S THIRD LAW: EXPLANATION

The third law relates the distance of a planet from the Sun to the time necessary to travel the entire orbit : the ratio between the square of the revolution times of the planets and the cube of their average distance from the Sun is constant. That is, it compares different planets. The law shows that the average angular velocity of revolution of a planet is the less the + it is away from the Sun: the slowest planet is Pluto, the faster one is Mercury.

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