The near face of the Moon and the far face of the Moon
Click on images to enlarge
(Image credit: from the Lunar Reconnaissance Orbiter on the NASA project website)
From the Earth we see always just the one face of the Moon (apart from a minor 'rocking' of the Moon which allows a small peek around the edges of the disc). The Moon is said to be 'tidally locked', that is, the mutual gravitational field causes an attractive deformation in the radial direction and this, over a long period, pulls the Moon to rotate at a speed which matches the orbital speed of the Moon.
The Moon resulted from the collision of the proto Earth and another early, rocky, planet (Theia) which, it is thought, was about the size of Mars. This occurred about 20 to 100 million years after the Solar System formed from the coalescence of the disc of material surrounding the Sun. This so called "giant impact hypothesis" is supported by a number of observations:
* Earth's spin and the Moon's orbit have similar orientations.
* Moon samples indicate that the Moon once had a molten surface.
* The Moon has a relatively small iron core.
* The Moon has a lower density than Earth.
* Evidence exists of similar collisions in other star systems (that result in debris disks).
* Giant collisions are consistent with the leading theories of the formation of the solar system.
* The stable-isotope ratios of lunar and terrestrial rock are identical, implying a common origin.