The moon circles the Earth in 27 days. Our closest neighbour in space is, therefore, not a planet but a satellite, the first known to man. The distance from the Earth to the moon is nearly 400,000 km (248,500 miles). Both are lit by the sun but are otherwise quite different from each other. The moon is much smaller. It is a world of dust, with no trace of life. Other planets have satellites. However, none of them is as large in comparison with its planet as our moon, whose diameter (3476 km or 2160 miles) is a quarter of the Earth's.

The moon has always fascinated man. It can be studied with the right equipment and, since 1969, it has even been possible to walk on it.

Once, scientists believed that the moon came from our planet. This could have been true because when it was forming the Earth was an unstable mass. But since the recent lunar journeys, the scientists now think that the moon was formed by crystallized gas more than four million years ago.
Two American astronauts have stood on the moon. They were Neil Armstrong and Edwin Aldrin.