The MOON is a natural satellite of Earth, orbiting the planet at an average distance of 384, 400 km. It is the fifth-largest natural satellite in the Solar System and the largest among planetary satellites relative to the size of the planet that it orbits. The MOON is thought to have formed approximately 4.5 billion years ago, not long after Earth. Its gravitational influence produces the ocean tides, body tides, and the slight lengthening of the day. The MOON's surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt.