The bright Moon has been the celestial companion of humans throughout our existence. Its regularly changing phases have served as calendars for over ten thousand years.
We see these phases because the Moon shines by reflected sunlight as it orbits the Earth. It seems to change shape due to our seeing the illuminated hemisphere from different angles. At the time of the new moon - also called the dark of the Moon - we can't see the Moon at all, for it reflects the Sun's light away from us and is lost in the glare of the Sun. Rarely, the Earth, Moon and Sun are aligned at this time, in which case we might see a solar eclipse.
The Moon orbits Earth in just over 27 days and during that time it also rotates once on its axis. Therefore there is no permanently dark side to the Moon, but there is a side never seen from Earth. Our first glimpse of the far side was in 1959 when the Soviet spacecraft Luna 3 took photographs as it orbited the Moon. Nine years later the crew of Apollo 8 were the first humans ever to see it with their own eyes.
It's no coincidence that one Moon day equals one orbit. Not only does the Moon's gravity cause tides on Earth, but Earth's gravity affects the Moon in a way that creates what is called a tidal lock. This is what keeps the same face of the Moon turned always towards Earth.
The far side of the Moon was a surprise, for it consists almost exclusively of light-colored, thickly cratered terrain. Our own familiar view includes not only this kind of feature, but also extensive smooth dark areas which Galileo thought were oceans. He called them maria (singular mare, Latin for sea). They cover nearly a third of the hemisphere facing Earth and practically none of the opposite one. So far we have no completely convincing explanation for this.
There is recent evidence of frozen water in shaded craters, but no evidence that there were ever bodies of water on the Moon. The maria filled with lava which cooled to form a volcanic rock common on Earth, basalt. They are comparatively uncratered and at a lower altitude than the light-colored highlands surrounding them.
Apollo astronauts brought back about 840 lbs (380 kg) of rocks from their missions. Rocks from the highlands showed evidence of a major bombardment by Solar System debris some 3.9 billion years ago, about 700 million years after the formation of the Earth. Since the maria surfaces have much less cratering, they were obviously filled in later - perhaps up to 900 million years later.
The Moon rocks also show major similarities to Earth rocks, but they aren't identical. The evidence of the rocks underpins the currently-accepted explanation of the formation of the moon, the giant impact theory. It says that the Moon formed from the material thrown out from Earth following the impact of a Mars-sized body.
The early geological record of Earth has been erased by the constant recycling of the Earth's crust. This occurs through internal geological processes plus weathering and erosion. But it didn't happen on the Moon.
The Moon has only 2% of Earth's volume and just over 1% of its mass. Therefore, although both bodies started molten, the Moon cooled much more quickly. Even its interior processes probably stopped at least two billion years ago.
The Moon's low mass means it has only a sixth of Earth's gravity, which isn't enough to hold an atmosphere. Without air and water, significant surface changes are caused only by impacts and extreme temperature changes. At the equator, for example, the daily temperature range is from -280 to +260 degrees Fahrenheit (-170 to +125 degrees Celsius). Nonetheless these changes are slight compared to geological processes on Earth.
Apart from Earth, the Moon is the only body upon which humans have set foot and the slow pace of change on the lunar surface means Neil Armstrong's famous footprint should remain undisturbed for a long time.
(1) NASA Mini-RF, "NASA Radar Finds Ice Deposits at Moon's North Pole" http://www.nasa.gov/mission_pages/Mini-RF/multimedia/feature_ice_like_deposits.html
(2) Spudis, Paul D. "Moon." World Book Online Reference Center. 2004. World Book, Inc.
(3) "Where did the Moon come from?" http://www.nhm.ac.uk/nature-online/space/planets-solar-system/moon/origins/index.html [accessed 14.07.10]
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