The laws describing the motions of the planets in the solar system are termed as Kepler's Laws.

Kepler's three laws of planetary motion can be stated as follows : (1) All planets move about the Sun in elliptical orbits, having the Sun as one of the foci. (2) A radius vector joining any planet to the Sun sweeps out equal areas in equal lengths of time. (3) The squares of the sidereal periods (of revolution) of the planets are directly proportional to the cubes of their mean distances from the Sun.

Knowledge of these laws, especially the second (the law of areas), proved crucial to Sir Isaac Newton, when he formulated his famous law of gravitation between Earth and the Moon and between the Sun and the planets, postulated by him to have validity for all objects anywhere in the universe.

Newton showed that the motion of bodies subject to central gravitational force need not always follow the elliptical orbits specified by the first law of Kepler but can take paths defined by other, open conic curves; the motion can be in parabolic or hyperbolic orbits, depending on the total energy of the body. Thus, an object of sufficient energy – e.g., a comet–can enter the solar system and leave again without returning. From Kepler's second law, it may be observed further that the angular momentum of any planet about an axis through the Sun and perpendicular to the orbital plane is also unchanging.

It may be noted that Kepler's laws apply not only to gravitational but also to all other inverse square law forces.
Kepler's 1st law : “The orbits of the planets are ellipses, with the Sun at one focus of the ellipse”. The Sun is not at the center of the ellipse, but is instead at one focus (generally there is nothing at the other focus of the ellipse). The planet then follows the ellipse in its orbit, which means that the Earth Sun distance is constantly changing as the planet goes around its orbit.
Kepler's 2nd law : “The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse”. The line joining the Sun and planet sweeps out equal areas in equal times, so the planet moves faster when it is nearer to the Sun. Thus, a planet executes elliptical motion with constantly changing angular speed as it moves about its orbit. The point of nearest approach of the planet to the Sun is termed perihelion; the point of greatest separation is termed ‘aphelion’. Hence, by Kepler's second law, the planet moves fastest when it is near perihelion and slowest when it is near aphelion.
Kepler's 3rd law : “The ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their semi–major axes”. Kepler's Third Law implies that the period for a planet to orbit the Sun increases rapidly with the radius of its orbit. Thus, we find that Mercury, the innermost planet, takes only 88 days to orbit the Sun.

The following table gives the Time periods and distance values for all the Eight planets from the sun:

The above Table also shows the greater the distance, the slower the motion, which leads to the overtaking of outer planets by the Earth, making them (for a while) seem to move backwards relative to the fixed stars in the sky.

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