The Moon is Earth's only natural satellite and orbits Earth at an average distance of 384,400 km. It has a diameter of 3476 km and is composed of materials like titanium, sodium, hydrogen, iron, copper, nickel, and manganese. The Moon's phases are caused by its orbit around Earth as both orbit the Sun. The Moon causes Earth's ocean tides and also influences eclipses. Humans first explored the Moon during the Soviet Sputnik 1 launch in 1957 and then with the American Apollo 11 mission in 1969, when Neil Armstrong became the first person to walk on the lunar surface.