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The Inner Planets

The four inner planets of our Solar System lie within the orbit of the giant planet Jupiter. They are Mercury (which is nearest to the sun), Venus, the Earth, and Mars (which is the furthest of them from the Sun).

According to the famous laws of Kepler, the further a planet is from the sun, the longer is the period of revolution; that is to say, the greater is the time it takes to orbit the Sun. This law is born out by observation. Mercury orbits the Sun in the shortest time of all the planets, and has always been associated with speed. You can see from the illustration that Mercury keeps overtaking even Venus, as it races around the Sun. Similarly, the Earth continually overtakes the planet Mars, and it is at these moments that Mars is most conveniently situated for astronomers to view it from the Earth.

The size of the Sun has been exaggerated compared to the planetary distances, as have the diameters of the planets in comparison to the diameter of the Sun. However the orbits of the four planets have been assumed to be circular, instead of elliptical, as discovered by Kepler. The diameters of the planets shown here and their periods of revolution are in the correct proportion to one another, as are the sizes of their orbits.
© Keith Gibbs/John Bourne 2008