20 August: The Moon's birthday

The Aztecs celebrated the Moon's birthday on this date, so here are 10 things you may not know about the Moon.

1. On average, the moon is 384,400 km away from the Earth. It would take a Boeing 747 26 days to get there. Apollo 11 made it in three days.
2. The currently accepted theory as to where the moon came from is that about 4.5 billion years ago, Earth collided with another planet, about the same size as Mars, called Theia, and the moon is made up of the debris.
3. The adjective usually used for the moon is lunar, but a less common word is selenic, derived from the Ancient Greek Selene, as in selenography.
4. The scientific name for moon dust is regolith. Moon dust smells like spent gunpowder.
5. The Moon is exceptionally large relative to Earth: a quarter its diameter and 1/81 its mass. It is the largest moon in the Solar System relative to the size of its planet unless you count dwarf planets, in which case Pluto's Charon beats it at 1/9 Pluto's mass. The moon is bigger than Pluto. The moon is about 400 times smaller than the sun, but is 400 times nearer the Earth - so sun and moon appear the same size from Earth. A popular belief, stretching back at least to Aristotle in the 4th century B.C., is that the Moon appears larger when it's near the horizon due to a real magnification effect caused by the Earth's atmosphere. This is not true: the atmosphere does change the perceived colour of the Moon, but it does not magnify or enlarge it. In fact, the visual image of the moon is about 1.5% smaller when it is near the horizon than when it is high in the sky, because it is further away.
6. Twelve people have visited the moon - all American white males. The first was Neil Armstrong, of course, but lesser known is that the last one was Gene Cernan on the Apollo 17 mission in 1972. There is a dead guy up there, though. Dr. Eugene Shoemaker, a Geological Surveyor, educated the Apollo mission astronauts about craters, but medical problems prevented him from going to the moon himself. After he died, his ashes were sent up to the moon, according to his wishes, on the Lunar Prospector spacecraft in 1999.
7. The moon isn't round and it doesn't have a dark side. It's shaped like an egg, and the side we don't see gets as much light from the Sun as the one we do see, as the moon rotates on its own axis.
8. We all know the moon causes the tides on Earth, but the gravitational pull of the Earth affects the moon as well, causing moonquakes, which can register 5.5 on the Richter scale and last up to 10 minutes.
9. Every year, the moon moves about 3.8cm away from us. In a few billion years, it will take 47 days to orbit the Earth instead of the current 27.3.
10. The features we can see on the moon are called "maria" or "seas" because it was once thought they were full of Water. Various cultures have interpreted them as looking like a face, a Rabbit, a buffalo or a toad. The largest feature isn't visible from Earth. It's the South Pole–Aitken basin, which is the largest known crater in the solar system at 2,240 km (1,390 mi) in diameter. At 13 km (8.1 mi) deep, its floor is the lowest point on the surface of the Moon. There are also some quite mysterious features which have been photographed during various missions which are said to look like obelisks which appeared to be arranged like the three great pyramids, a bridge over one of the craters, towers that are miles high which could not have been formed by any known natural process and even a floating castle. Naturally, conspiracy theorists have had a field day with those, suggesting that the moon is really an ancient alien space ship.