The Moon, is the only natural satellite of the Earth. The Moon does not emit light, but it reflects the sun's light, which makes it look bright. We can see that the Moon’s visual image is changing daily as it orbits our Earth. That is called the phases of the Moon. In early time, human can only observe the moon with naked eyes. With the invention of the telescope in the 17th century, people can observe the moon in more detail. After the 1950s, humans began send probes to the Moon.