The Sun & major nine planets are the main bodies of our Solar System. In order of increasing distance from the Sun these are Mercury, Venus, earth, mars, Jupiter, Saturn, Uranus, Nepyune & Pluto. Sir William Herschel discovered Uranus in 1781 during a review of the entire sky which was so systematic as to be virtually certain to reveal any such object. A planet more distant from the Sun than Uranus was predicted independently by John Adams in 1843 and by Urbain Le Verrier in 1846. These predictions enabled Johann Galle and Heinrich d’Arrest to find and identify the planet which is now known as Neptune. In 1915, Percival Lowell published calculations predicting another planet beyond Neptune; this ninth planet was found in 1930 by Clyde Tombaugh and named Pluto.

Several of the planets have systems of satellites, or moons, in orbits round them which is several ways mimic the system of Suns and Planets. There is also a whole host of lesser objects: minor planets or asteroids, comets, meteorids & dust, as well as the solar wind.

Thw word planet is derived from a greek word which means wanderer. This name arose because of the way in which the planets wander against the backcloth of distant stars. According to Ptolemy, the Earth lay at rest at the centre of the Universe, while the moon, Sun & planets moved in orbits around it. This view was generally accepted until the middle of the sixteenth century when Nikolaus Copernicus argued that it was the Sun which should be at the centre of the Universe. This view was bitterly opposed at the time but gradually became accepted. Copernicus followed Ptolemy’s views in one respect; he built planetary orbits up from circles. The theories of both Copernicus and Ptolemy suffered from a great defect, they didnot predict the positions of the planets with sufficient accuracy. The true nature of planetary orbits was eventually elucidated by Johannes Kepler in the early seventeenth centuary when he published three relationship describing planetary motion.

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