Our moon. We often see it in the sky as a bright white circle. We have only one moon, but there are planets with many more moons. Like Jupiter, the fifth planet in our solar system. Jupiter has four big moons and even more smaller moons: altogether about 50 moons. Galileo Galilei was the first to discover that we are not the only planet with a moon, over 400 years ago. He studied Jupiter through a telescope and discovered its four moons. With a good pair of binoculars you can see them yourself on a bright night. But in Galileo’s time it is a big discovery, because in his day people believed that the entire universe revolves around the earth. Our moon revolves around the earth in about a month. But Io, one of Jupiter’s moons, is much faster: it takes Io less than two days to circle around Jupiter, and Jupiter is much bigger than the earth. So a moon really is a space object revolving around a planet. That is why we call moons natural satellites. Moons come in different shapes and colours. Our moon is white and round, but Mars for example has two small moons, Phobos and Deimos, that look more like potatoes.