Description
The Moon (Latin name: Luna) is Earth's only natural satellite. It is fifth-largest and the second-densest (after Jupiter's satellite Io) natural satellite in the Solar System. The Moon is exceptionally large relative to Earth: a quarter its diameter and 1/81 its mass. It is the largest moon in the Solar System relative to the size of its planet (though Charon is larger relative to the dwarf planet Pluto, at 1/9 Pluto's mass). Earth and the Moon are nevertheless still considered a planet-satellite system, rather than a double planet, because their barycentre, the common centre of mass, is located 1700 km (about a quarter of Earth's radius) beneath Earth's surface[1].
Formation
The Moon is thought to have formed approximately 4.5 billion years ago, not long after Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia.
Visibility from Earth
The Moon is in synchronous rotation with Earth, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters. It is the second-brightest regularly visible celestial object in Earth's sky (after the Sun), as measured by illuminance on Earth's surface. Although it can appear a very bright white, its surface is actually dark, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art, and mythology.
Distance and apparent size
The Moon's gravitational influence produces the ocean tides and the slight lengthening of the day. The Moon's current orbital distance is about thirty times the diameter of Earth, causing it to have an apparent size in the sky almost the same as that of the Sun. This allows the Moon to cover the Sun nearly precisely in total solar eclipse. This matching of apparent visual size is a coincidence. The Moon's linear distance from Earth is currently increasing at a rate of 3.82 ± 0.07 centimetres per year, but this rate is not constant.
- ↑ This article was adapted from Space Engine's ingame wiki.
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