The Moon is Earth's only natural satellite. We usually see it in the night sky and also during the day. Some other planets also have moons or natural satellites. Our moon is about one-fourth of the width of the Earth. Because it is far away it looks small, about half a degree wide. The gravity on the moon is one-sixth of the Earth's gravity. It means that something will be one-sixth as heavy on the Moon than on Earth. The Moon is a rocky and dusty place. It moves slowly away from the Earth at a rate of 3.8 cm per year, due to the effect of tidal dissipation. For the origin of the Moon, see the giant impact hypothesis. Phases Because the Moon is round, half of it is lit up by the Sun. As it goes around (or orbits) the Earth, sometimes the side that people on Earth can see is all lit brightly. Other times only a small part of the side we see is lit. This is because the Moon does not send out its own light. People only see the parts that are being lit by sunlight. These different stages are called Phases of the Moon. It takes the Moon about 29.53...