The Moon is the only natural satellite on Earth. With an equatorial diameter of 3476 km, it is the fifth largest satellite in the Solar System, while in terms of size proportional to its planet it is the largest satellite: a quarter of the Earth's diameter and 1/81 of its mass. After Io, it is also the second densest satellite. It is in synchronous relationship with Earth, always showing the same face towards the planet. The visible hemisphere is marked with dark lunar seas of volcanic origin between the brilliant ancient mountains and prominent impact craters.
