Who Built the Moon? by Christopher Knight, Alan Butler

circles, was that there were mysterious vapours between the Earth and the Moon. The images, it was suggested, were present in sunlight and were merely being reflected from ‘the vapours’. But the most popular theory, probably because it didn’t impinge on Christian doctrine, was that there were variations in the density of the Moon and that these created the optical illusions we see as markings on the Moon’s surface. This unlikely explanation was safe, though it probably did little to convince early scientists, and certainly would not have impressed Galileo.
    After Galileo’s time, telescopes improved markedly and it was obvious to anyone who studied the Moon that it was a sphere with a rocky and undulating surface. As the Church gradually lost its power to direct scientific thought, many of the earlier ideas regarding the Moon became unthinkable. But no one had any idea how the Moon had come into being and why it occupied the orbit it did around the Earth.
    It didn’t take long for the subject of the Moon to become very important to astronomers. Empires such as those created by Britain, France and Spain, were expanding. This necessitated long sea voyages and led to that most urgent of searches – a way to plot ‘longitude’ whilst at sea. It is quite easy to establish one’s position on the planet in a north–south line (latitude) but it was impossible to know where you were in terms of east–west (longitude). In the northern hemisphere, for example, latitude can be quickly gauged by measuring the angular distance between the horizon and the Pole Star. This angle also defines one’s position north of the equator.
    The longitude problem was eventually solved by having an extremely accurate clock on board a ship that was set to the time at one’s point of departure. It wasn’t difficult to work out the difference between local time, say at midday, and the time at the home port. It was then simply a matter of adding or subtracting to discover one’s true position on the Earth’s surface. This was fine but it took many decades before a suitably accurate clock could be created. In the meantime, astronomers sought for other methods to determine longitude, not least of all because there was a fabulous prize on offer for anyone who could crack the problem. And the place where many of them turned to establish longitude was the Moon.
    Astronomers proposed that if really accurate tables were kept of the Moon’s position relative to the background stars it would be possible to assess the true time of day in one’s home port. The reason this could work was that the Moon, being very close to the Earth and orbiting quickly, moved across the heavens by around thirteen degrees of arc per day. Using the Moon it was a fairly simple matter to establish ‘local time’ and then to do the necessary computations to discover one’s position.
    The lists of tables necessary to accomplish the task were not so simple, however, and as soon as good chronometers were available the Moon was abandoned as a means for longitude assessment. However, the desire to solve this problem, and the potential profitability of doing so, meant that the Moon was receiving a great deal of attention during the seventeenth century and very accurate maps of its surface began to appear.
    It wasn’t until the nineteenth century, however, that probably the first reasoned explanation as to the Moon’s origin was put forward. George Darwin, the son of Charles Darwin, the controversial Englishman who first proposed the theory of natural selection, was a known and respected astronomer who studied the Moon extensively and came up with what became known as the ‘fission theory’ in 1878. George Darwin may have been the first astronomer to ascertain that the Moon was moving away from the Earth. Working backwards from his knowledge of the rate the Moon was receding from the Earth, Darwin proposed a time that the Earth and the Moon could have been part of the