A moon is a ball of rock that orbits a planet. Mercury and Venus are the only planets that do not have moons. Jupiter and Saturn each have at least 16 moons. Our Moon is our closest neighbour in space. Moons may be lumps of material that were left over when the planets formed. Like planets, moons can only be seen when they are lit up by the Sun.
We know more about our own Moon than any other member of the Solar System, apart from the Earth. Astronauts have even landed on the Moon. The Moon is a bare, dead place where nothing lives or grows. Its surface has hills and mountains as well as flat plains and deep craters. There is no water or air on the Moon and it is covered with a layer of fine dust. From the Moon, the sky always looks black and the Earth seems to change shape and go through phases.
Phases of the Moon
Our Moon takes 
days to orbit the Earth. During this time it seems to change shape. This is because only the side of the Moon that faces the Sun is lit up. And as the Moon orbits the Earth we see different amounts of this lit-up side.
When the Moon is between the Sun and the Earth we cannot see it. This is called a ‘New Moon’. A ‘Full moon’ is when the Earth is between the Sun and the Moon. The diagram shows phases of the Moon seen in the Northern Hemisphere.
Seen from the Moon, (foreground), the Earth appears to change shape.