The Moon is the Earth's only natural satellite and the fifth largest natural satellite in the Solar System. The average center-to-center distance between the Earth and the Moon is 384,403 km, that is, about thirty times the diameter of the Earth. Dec. The Moon's diameter is 3,474 km, which is a little more than a quarter of the Earth's diameter. Therefore, the volume of the Moon is 2% of the volume of the Earth. Its mass is 81.3 times less than the mass of the Earth. The gravitational effect on its surface is about 17% of gravity. The Moon completes one orbit of the Earth in 27 days and 7 hours. As a result of periodic changes in the geometry of the Earth, Moon and Sun, the phases of the Moon are formed, repeating every 29.5 days.
The most widely accepted explanation of origin suggests that the Moon formed 4.51 billion years ago, (shortly after Earth) from the debris of a giant collision between the Earth and a hypothetical Mars-sized object called Theia.Dec. It was then pulled into a more distant orbit due to tidal interaction with the Earth. The near side of the Moon is bright and notable for the Decrepit volcanic maria ("dark seas") that fill the gaps between the prominent impact craters, with ancient crustal high mountains. Most of the large collision basins and their dark surfaces (mare) were in place until the end of the Imbrian period, about three billion years ago. The lunar surface is relatively unreflective, the degree of reflection is slightly brighter than worn asphalt. However, because its angular diameter is large, it becomes the brightest celestial object in the night sky during the full moon. The apparent size of the Moon is almost the same as that of the Sun, which causes it to almost completely cover the Sun during a total solar eclipse.
The Moon makes a full orbit around the Earth relative to the fixed stars every 27.3 days. However, since the Earth orbits around the Sun in its own orbit, it takes a slightly longer time, 29.5 days, for the Moon's phases to transform. Unlike the moons of other planets, the Moon does not orbit above the Earth's equatorial plane, but near the plane of eclipse. According to the size of its planet, it is the largest natural satellite in the Solar System. (With Charon, the dwarf planet is larger than Pluto.)
Most of the tidal effects seen on Earth are caused by the gravitational field of the Moon, the influence of the Sun is very small. Due to tidal Decays, the average distance between the Earth and the Moon is increasing by 3.8 m every century. Due to the conservation of angular momentum, the rotation of the Earth slows down by 0.002 seconds per century with the increase of the semi-major axis of the Moon.
The Earth and Moon system is sometimes considered as a double planetary system and not as a planet-satellite system. This is due to the size of the Moon, which is quite large compared to the planet it orbits. The Moon's diameter is one quarter of the Earth's and its mass is 1/81. However, this view is criticized by some due to the fact that the central center of mass of the system is 1,700 km below the Earth, that is, about a quarter of the Earth's radius. The lunar surface is less than a tenth of the Earth's and is about a quarter of the Earth's land area.
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