The Moon is Earth’s natural satellite and the brightest object in the night sky. It is the fifth-largest natural satellite in the Solar System and orbits the Earth every 27.3 days. The Moon has no atmosphere and a thin exosphere, and is composed of minerals and rock. On the Moon’s surface, a solid crust and lowlands exist, along with maria (dark patches surrounded by lava plains). The Moon’s gravitational influence produces the ocean tides and amplifies the length of a day on Earth. The Moon has been visited by astronauts and robotic spacecraft from multiple nations.