Moon
A moon is a big rock which goes around (orbits) a planet. The Earth has one moon. Some other planets have many moons, and some have none. When we talk about the moon of the Earth, we usually write "the Moon".Something that goes around a planet is called a satellite. Moons are natural satellites. People also use rockets to send machines into orbit around the Earth. These machines are called artificial satellites.
The Moon of the Earth takes about one month to go around the Earth. This is how different calendars measure the length of the month. But often a month of the calendar isn't exactly the same as the time it takes the Moon to orbit the Earth. Sometimes calendar months are a little longer or shorter than this.
Moons don't make their own light. We can see the Moon of the Earth because of the light of the Sun. The Sun always shines on half of the Moon, and half is always dark. This is why the Moon looks different at different times of the month. This change is called the phases of the Moon. When the Moon looks very big and round, the Sun is behind us as we look at it. When the Moon looks very small, the Sun is almost on the other side of the Moon as we look at it.
The same side of the Moon always faces the Earth. It is easy to understand this. If you walk around a tree, while turning so that you are always facing towards the tree, this is how the Moon orbits around the Earth.
The orbit of a moon or other satellite is affected by two forces: gravity, and the centripetal force.
Other planets in our Solar System that have moons:
- Mars, two moons
- Jupiter, 63 moons
- Saturn, 31 moons
- Uranus, 27 moons
- Neptune, 13 moons
- Pluto, one moon (Charon)