We live on the planet Earth, a ball of rock 12,750 km in diameter. Like all the planets, the Earth rotates on its axis and orbits the sun. But the earth is not alone. It has a companion on its travels -- the moon - which orbits the Earth once a month. But the two worlds are very different. The Moon is a dead planet. It has no volcanoes or geological activity, it is airless, waterless and lifeless. The Earth, on the other hand, is lush and fertile. It supports millions of living things -- plants, insects, birds, animals and human beings. It has fascinating erupting volcanoes. Since the moon has no atmosphere to protect it, its surface is heated to 105 C during its day, and cools to -155 C at night. In contrast, the Earth is covered by an atmosphere which we can breathe, and which also keeps the temperature quite constant.

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