Our Moon is the only celestial body visited by humans. Those voyages into space taught us many things about our satellite and inspired many of us to dream of traveling further out to visit other planets and moons in our solar system. The knowledge gained on those first steps are helping to make those dreams reality with plans to visit Mars with the next decade or two. So the question is: What did we learn and what speculations were confirmed?
The Moon, known to the ancient Greeks as Selene and to the Romans as Luna, is the largest satellite in our solar system relative to the size of its primary i.e. Earth. Both Jupiter and Saturn have larger moons but when one considers that Jupiter and Saturn are 11.2 and 9.4 times the size of Earth respectively the size of their moons is really not all that impressive. The moon is, after all the fifth largest satellite in the system. So how did we end up with such an impressive moon?
There have been a number of theories proposed but the latest is currently the most popular. That theory suggests that during the formation of Earth it was struck by another planet and a large portion was absorbed into our planet but a large piece was expelled by the force of the impact. That piece coalesced to form the moon. The support for that theory includes the fact that with an average density of only 3.34 there is a relatively small metallic core to the moon. There is the additional fact that the Moon is moving away from the Earth at about 3 cm. every year. It certain that the tidal forces are dissipating energy in the Earth-Moon system and this will eventually result in Moon being about 1.4 times its current distance from Earth; they will have equal months of 47 days and will be tidally locked facing each other. Not to worry though, we are talking millions of years into the future.
Our Moon is currently at a mean distance of 384,000 km (238,000 m.) and has a diameter of 3,476 km (2155 m.). Its spin period (the time it takes to rotate on its axis once) is almost identical with the time it takes to orbit the Earth, with the result that we only see one face (the near side) of Luna. Due to its slightly eccentric orbit, however, we do get to view 60% of the surface instead of the mere 50% we would see if its orbit was perfectly elliptical. This slight eccentricity (e = 0.055) also partially accounts for those days when the moon seems bigger to the human eye, it is not an illusion since it is closer.
The surface gravity of the moon, as demonstrated by Armstrong, Aldrin and the ten other astronauts who made lunar landings, is roughly 1/3 that of Earth. But as mentioned earlier even that gravitational attraction is enough to influence the tides on Earth due to the extraordinary size of Earth’s companion. It is not enough, however, to retain any atmosphere and the moon is truly devoid of any atmosphere of any kind.
When lover’s gaze up at the moon they are able to distinguish the face of ‘the man in the moon’ created by differing shades of light and dark. The darker areas are the maria which are areas only moderately cratered while the lighter highlands area are heavily cratered. The maria is now known to be composed of basaltic lava. Generally speaking the surface of the moon consists of rock crushed to an almost flour-like consistency and is called the regolith. (And the lunar explorers had a very difficult time with the regolith as it stuck to their spacesuits and they had to try and squeegee it off.)
The moon, it was discovered by having the astronauts lay out a network of seismographs, is subject to weak moonquakes and the data from these quakes provided a picture of the interior structure of Luna. It was found that the moon is relatively homogeneous with only a very small core of denser material as mentioned earlier. There is a crust of slightly lighter material that averages about 65 km in thickness. Due to the 4 billion years of impacts the moon has been subjected to, the outermost 10 km of the crust is shattered and heavily fragmented.
All in all the moon is not actually a very romantic place. It is a place of extreme temperatures and shadows, colourless and harsh. And personally, if offered, I would make the trip in a New York minute.