A few thousand years ago. man did not even know, whether the earth was flat or round. A few hundred years ago, he knew it was round, but did not know whether it was the centre of the Universe or just one of the many planets, orbiting the Sun. But now, he knows that the Sun is a star and the bodies orbiting the planets are moons.

Our planet earth moves through space as it orbits the Sun. This family of the Sun, together with the Sun itself is called the "Solar System" Born at least 5,000 million years ago, it includes nine planets and approximately 33 moons that we know of. It also includes many thousands of minor planets or asteroids. The Sun, the largest object, is at the centre of the "Solar System". It is an enormous ball of glowing gas and its diameter is 1,392,000 Kms and that of the earth is only 13000 kms. The Sun`s surface is yellow hot and the temperature is 6000 degrees C. Intense light and heat that it sends into space make life impossible on Mercury, the planet nearest to the Sun. Life is equally impossible on the farthest planets, because they are so far away that they are always frozen, and life exists only on the earth, as far as we know. The planets move round the Sun in paths called orbits. The orbits are elliptical, shaped like flattened circles. Most of them are very nearly circular. Except for the orbit of Pluto, they lies nearly in one plane i.e. they are not greatly tilted. Most of the planets have moons in the orbit around them.

Most of the matter of the Sun is gas hydrogen. It is being slowly turned into gas helium, releasing energy. The four inner most planets are made up mostly of the rocks and metals. They contain very little hydrogen and helium, which are most common substances in the Universe as a whole compared to other planets, Earth the third planet from the Sun, has huge amounts of oxygen and water, apart from nitrogen, carbon, helium etc. It's surface temperatures are suitable for development of life, even at it's frozen poles or in it's hottest deserts. Three fourth of Earth's surface is water and one fourth is land. Earliest known civilizations of the Earth were found on the banks of rivers and not on the beaches of seas.

The Sun itself is a Star.

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