The Science of Discworld Revised Edition

revolve around the Earth. (If it didn’t rotate at all, it would always be pointing in the same direction – not the same direction relative to the Earth, but the same direction period. Imagine someone walking round you in a circle but always facing north, say. Then they don’t always face
you
. In fact, you see all sides of them.) It wasn’t always like this. Over hundreds of millions of years, the effect of tides has been to slow down the rotation rates of both Earth and Moon. Once the moon’s rotation became synchronized with its revolutions round the Earth, the system stabilized. The moon also used to be quite a bit closer to the Earth, but over long periods of time it has moved further and further out.
    Between 1600 and 1900 three theories of the formation of the Moon came into vogue and out again. One was that the Moon had formed at the same time as the Earth when the dustcloud condensed to form the solar system – Sun, planets, satellites, the whole ball of wax … or rock, anyway. This theory, like early theories of the solar system’s formation, falls foul of angular momentum. The Earth is spinning too fast, and the moon is revolving too fast, to be consistent with the Moon condensing from a dustcloud. (We misled you earlier when we said that the dustcloud theory explained the satellites too. Mostly it does, but not our enigmatic Moon. Lies-to-children, you see –
now
you’re ready for the next layer of complication.)
    Theory two was that the Moon is a piece of the Earth that broke away, maybe when the Earth was still completely molten and spinning rather fast. That theory bounced into the bin because nobody could find a plausible way for a spinning molten Earth to eject anything that would remotely resemble the Moon, even if you waited a bit for things to cool down.
    According to theory three, the Moon formed elsewhere in the solar system, and was wandering along when it happened to come within the Earth’s gravitational clutches and couldn’t get out again. This theory was very popular, even though gravitational capture is distinctly tricky to arrange. It’s a bit like trying to throw a golfball into the hole so that it goes round and round just inside the rim. What usually happens is that it falls to the bottom (collides with the Earth) or does what every golfer has experienced to their utter horror, and goes in for a split second before climbing back out again (escapes without being captured).
    The rock samples from Apollo missions added to the mystery of the Moon’s origins. In some respects, Moon rock is astonishingly similar to Earth rock. If they were similar in most respects, this would be evidence for a common origin, and we’d have to take another look at the theory that they both condensed from the same dustcloud. But Moon rock doesn’t resemble
all
Earth rock, only the mantle. The current theory, which dates from the early 1980s, is that the Moon was once
part
of the Earth’s mantle. It wasn’t ejected as a result of the Earth’s spin: it was knocked into space about four billion years ago when a giant body, about the size of Mars, struck the early Earth a glancing blow. Computer calculations show that such an impact can, if conditions are right, strip a large chunk of mantle from the Earth, and sort of smear it out into space. This takes about 13 minutes (aren’t computers good?). Then the ejected mantle, which is molten, begins to condense into a ring of rocks of various sizes. Some of it forms a big lump, the proto-Moon, and this quickly sweeps up most of the rest. What’s left doesn’t go away so easily, however, but over 100 million years nearly all of it crashes into either the Moon or the Earth, because of gravity.
    The first simulations to support this theory of the Moon’s formation suffered from some problems; in particular, they dated the impact very early in the Earth’s formation, in order to get the Moon’s angular momentum right. But if the collision had occurred that early, then the Moon would have accumulated a lot of iron from later impacts, just as the Earth did. However, there is little iron on (or in) the Moon. More recent work shows that a rather later impact could also give the Moon the correct angular momentum, and avoids this difficulty. However, it predicts that about 80% of the impacting body would have ended up on the Moon. In order for the Moon to resemble Earth’s mantle as closely as it does, the impacting body would
also
have had to