The Sun is a nearly perfectly spherical star that makes up over 99% of the mass in the Solar System. It has a diameter over 100 times larger than Earth and consists mainly of hydrogen and helium plasma interwoven with magnetic fields. The Moon is Earth's only natural satellite, about a quarter the diameter of Earth but only 1/81 its mass. It always shows the same face to Earth due to synchronous rotation and has many dark basaltic plains and prominent impact craters visible on its near side.