Solar System -

Earth's Moon

The Moon is very large compared to the size of the Earth, being over 27% its size, although less than 1.25% its mass.  It is also the fifth largest moon in the whole Solar System.  At its closest, the Moon is 363,104 km (c. 227,000 miles) from Earth, and at its furthest, 405,696 km (c. 353,500 miles), averaging out at 384,399 km (c. 240,250 miles).  Its average diameter is 1,737.10 km  with an equatorial diameter of 1,738.14 km and a polar diameter of 1,735.97 km.  Its surface gravity is around 16.5% that of earth.  It is in synchronous orbit around the Earth, meaning it takes  27.321582 days both to complete one orbit, and to revolve once about its axis, thus always presenting the same face to us. 

There are several theories for how the Moon formed, but most have flaws; for example, capturing a passing object.  This would reuire the Earth to have had an incredibly extended atmosphere in order to dissipate  the energy of the object  Most planetary astronomers agree that a body, perhaps half the diameter of the Earth, collided with it, and the material thrown off into space by the impact accreted to form the Moon.  This solution also has some issues that require the vaporized material of the Earth and Moon to mix post the impact to equalize their composition.  The best estimate of when the Moon formed is 4.527 0.010 billion years ago, though there is some evidence that it occurred, possibly, 50 million years later than this. 

Like other rocky objects in the Solar System, the Moon has distinct crust, mantle and core.  The core is very small, occupying a diameter of less than 700 km (435 miles), about 20% the diameter of the Moon.  Earth's core, for example, occupies nearly 55% of the Earth's diameter.  The Moon's core is probably predominantly iron with small quantities of impurities like sulphur and nickel, and it is at least partly molten. 

The Moon is, of course, covered in craters and maria or "seas".  The largest, the South Pole-Aitken basin is about 2,240 km (1,400 miles) in diameter, and is the largest crater known in the Solar System.  It is also very deep at about 13 km (~ 8 miles) below the surrounding terrain.  Maria are the result of volcanic eruptions, and can form when when lava floods a crater created by an impact.  Mare Imbrium has a diameter of 1123 km (700 miles), and it is the largest mare in an impact basin.

Description of the Moon