The Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is 384,403 kilometres (238,857 mi), about thirty times the diameter of the Earth. The common centre of mass of the system is located at about 1,700 kilometres (1,100 mi) - a quarter the Earth's radius - beneath the surface of the Earth. The Moon makes a complete orbit around the Earth every 27.3 days (the orbital period), and the periodic variations in the geometry of the Earth–Moon-Sun system are responsible for the phases of the Moon, which repeat every 29.5 days (the synodic period).
The Moon's diameter is 3,474 kilometres (2,159 mi), a little more than a quarter of Earth's. Thus, the Moon's surface area is less than a tenth of the Earth (about a quarter of Earth's land area), and its volume is about 2 percent that of Earth. The pull of gravity at its surface is about 17 percent of that at the Earth surface.
Source: en.wikipedia.org/wiki/Moon - 5.03.2010
How brilliant: A source of light, which from ancient times is a support for the division of the time because of the constant change of its look.