The moon is the fifth largest natural satellite in the solar system, and one quarter the diameter of Earth. The presence of the moon orbiting the Earth actually slows down Earth's rotation on it's axis. The effect is slight, but it's enough to make the Earth day about 2 milliseconds longer every one hundred years. Under the Outer Space Treaty, signed in 1967, the moon remains free for people of all nations to land on and explore; it is not owned by any one nation, despite pennants and flags from both the Soviet Union and the United States being planted on it's surface.