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Earth's Moon is the largest satellite in the solar system. Its day is a month long.

    Vital statistics:
  • Mass: 7.3 x 1022 kg = 0.012 Earths
  • Diameter: 3476 km = 0.273 that of Earth
  • Density: 3.35 tonnes/m3 = 0.606 Earth's density
  • Surface gravity: 1.62 m/s2 = 0.1654g
  • Distance from Earth: 0.356 - 0.405 million km (0.384 Mkm semi-major axis)
  • Period of orbit: 27.32 Earth days
  • Atmosphere: Practically none, with day/night variation in quantities of trace gases, including helium, argon, neon, sodium, potassium, hydrogen and radon.
full Moon
Earth and planets
Earth compared to the smaller planets and large moons

History of the Moon

The Moon is the second densest of all the moons in the solar system (Io has a density of 3.53 $t/{m^3}$, but some moon densities are still uncertain), and is the largest satellite relative to the diameter of its primary (the planet it orbits).


The current theory is that the Moon formed soon after the Earth, about 4.5 billion years ago (the solar disk started coalescing about 6 billion years ago). Due to the lower density of the Moon (3.35 compared to Earth's 5.51 $t/{m^3}$, or 61%), it is thought that the Moon did not coalesce in the same region of space in the same way as the inner planets, but was broken off from the Earth by a collision. The cause of this eruption of material would most likely to have been a collision with a planetoid, no bigger than Mars. This defunct cosmic visitor has been nicknamed 'Theia'.

The Moon rock samples brought back by Apollo astronauts have been analysed, and they reveal that rather than a glancing blow, Theia gave Earth a head-on punch. The debris from this punch-up formed two moons, which later combined to the one we see today. Theia's mantle and core added to the Earth's core, making it disproportionately large.

Content © Andrew Bone. All rights reserved. Created : October 25, 2015 Last updated :February 14, 2016

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