Ex Luna, Scientia - "From the Moon, Knowledge"
Selenology - the Science of the Moon
Selenology began when Man first looked up at the Moon and wondered what it was. It was given a boost with Galileo's first use of the telescope to study the Moon in 1610, and again when the first lunar samples were returned to Earth in 1969.
The Moon is Earth's only natural satellite. I has been in existence for 4.5 billion years, having been most likely formed from debris left over from an impact of the Earth with another proto-planet about the size of Mars soon after the Earth formed (about 50 million years later). The Moon is slowly receeding from the Earth at a rate of 38mm per year due to tidal slowing down of the Earth's rotation by the oceans drag on the continents. This has not been constant throughout geologic time due to the shifting continents of Earth. The loss of energy is being transfered to the Moon orbit making it receed from the Earth to conserved the total angular momentum of the Earth-Moon system. But the fact that it is moving away implies that in the distant past it was much closer to Earth than it is today.
Looking up at the Moon the first thing you can tell about it is it is made up of two distinct shades of material. The lighter parts are called the"highlands" and the darker parts the "maria" as Galileo termed them in 1610, 400 years ago. This tells us that the Moon is made up of at least two different types of material. What those material were, was in the realm of speculation until Apollo 11 brought back the rocks and soils for analysis in 1969, though we had hints a few years before with the Surveyor spacecraft soft landers.
How did the Moon get this two-toned appearance? Soon after the formation of the Moon, it's crust was totally molten to perhaps a depth of 200km. This is called the "magma-ocean" hypothesis and was first discovered from small fragments of anorthosite rock in the soil of the Apollo 11 samples that had been ejected from the lunar highlands, which represent parts of the original lunar crust. This magma ocean allowed a separation (fractionation) of the chemistry of the lunar crust. The lighter elements like aluminum and calcium, floated to the top forming "rock-bergs", the heavier elements sank to the base of the crust, with the elements such as magnesium and Iron. The radioactive elements (Uranium and Thorium) also sank and eventually through radioactive decay, heated the rock enough to melt it and flow back up to the surface along cracks formed by the large impacts that formed the basins. The lava filled these basins forming the circular lunar maria or seas such as the Sea of Rain's - Mare Imbrium.
The early period of the formation of the Moon and rest of the solar system was a violent time with many intense bombardments of asteroids with the planets (including the one that formed the Moon to begin with). The scars are still there to be seen today in the intensely cratered southern lunar highlands. The basins (huge craters) formed early on, then later were filled with the lava flows much later, 3.9 to 3.2 billion years ago or at least 600 million years after the Moon formed and maybe a couple of hundred million years after the basins formed. This can be worked out by looking at flooded ghost craters submerged by the lava flows that formed after the basin, but before the lava flows. For example Sinus Iridum in Mare Imbrium. By this stage the intense bombardments had eased and the lunar seas are much less intensely cratered. And that is the way the Moon is today, with only one or two large impacts every several hundred million years. The smaller craters form more frequently and today the Moon is peppered by micrometeorites which still erode away the lunar surface, giving the rocks and craters and eroded smoothed appearance. The average rate of erosion today is about 1mm per million years now. Hence when spacecraft when to the Moon they found none of the jagged mountain peaks of 1950's Sci-Fi paintings, just sooth mountains and eroded boulders.
Today the Moon is a quiet place, with some moonquakes do each month (max of mag 4 on the Richter scale have been detected, most are much smaller) from tidal squeezing and from the occasional impact of meteorites from space. The Moon atmosphere is so thin the molecules don't even collide with each other. There are possible gaseous emissions called Transient Lunar Phenomenon (TLP/LTP), of red glows, obscuration's and flashes that have been seen by observers over the years, and Apollo spacecraft have detected Radon gas over Aristarchus crater, a hot-spot for TLP events, so these are worth watching out for. Watching for impact flashes is a promising new area of research.
The Moon changes it's appearance with the phase angle so there is always something new to see, some fine detail that you may not have glimsed before, or some major feature overlooked since observations began, like the Shannen Ridge. So keep an eye on the Moon, you never know what you will see, even today 4.5 billion years after it formed! Welcome to the Moon.