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the moon

27 august 2018

Like Sheehan & Hockey's Jupiter, Bill Leatherbarrow's Moon for Reaktion Press delivers a concise history of the development of knowledge about a celestial neighbor, with some tips for how amateurs can contribute to that knowledge, even today, via home telescopes.

A great deal of what is known about the Moon still comes via pictures taken from Earth or from lunar orbit. This seems odd, because people have been to the Moon and brought home rocks and stuff. But only 12 people have walked there (four of them survive as of this writing), and they only visited altogether for a handful of days. The total extent of the lunar surface visited by astronauts, even with their awesome moon buggies, remains a negligible fraction of the whole. And nobody's going back in the foreseeable future.

The Moon is unique, of course, as the only object in the sky that presents much detail to the naked eye. Only the Sun is apparently as large (a coincidence that enables eclipses), but you can't look right at the sun, and wouldn't see many features if you could. The Moon offers vivid contrasts of light and dark, the origins of which are still not fully understood. The Moon is also quite stable in its appearance, which means that you can study a given feature patiently for months on end. Though this stability also means that the Moon has bored many astronomers (who need "something to draw them back to the eyepiece night after night," 68) into specializing in other objects. If you are lucky you can see ephemeral, dynamic storms on Jupiter, which is in a constant whirl. The moonscape only apparently changes, an illusion of the varying angles of sunlight that reveal new aspects as the Moon takes its slightly eccentric path through the sky.

Leatherbarrow moves briskly and illuminatingly through several centuries of the history of selenography. As telescopes got better and better from the time of Galileo into the early 20th century, the study of the moon was pre-eminently cartographic. Observers sketched night after night in competition to see who could record the most features on their lunar maps. Leatherbarrow quotes one of the doyens of the field, Walter Goodacre, who wrote in 1933:

One of the chief sources of pleasure to the lunar observer is to discover … details not on any of the maps; … if in the future a map is produced which shows all the detail visible in our telescopes, then the task of selenography will be completed. (60)
This must have been a depressing thought. And classical moon-mapping didn't even get that far. Leatherbarrow reproduces a section of a mind-blowing map by Hugh Percy Wilkins (1896-1960) that measured 7.5 meters in diameter. It looks like the doodlings of a madman, and though one supposes an even stronger telescope could reveal even more detail for an even larger map, the results would have been spectacularly impractical.

Science shifted, in the run-up and aftermath to Apollo, towards analysis and interpretation, particularly in a long-running debate over the processes that created the Moon's surface features. On the one hand, volcanists saw the craters and "seas" of the Moon as bubbling up from within; on the other, the "impact" school saw them as the result of huge rocks pelting the Moon and leaving it pockmarked. Both schools were partially correct, but many items remain mysterious.

The cultural importance of our satellite is unaddressed in The Moon, and would need an entire second volume to chronicle. "Our sentimental friend," T.S. Eliot called it, "an old battered lantern hung aloft / To light poor travellers to their distress." Like many ultra-familiar natural phenomena, the Moon is both comforting and chilling, a good omen and a bad, a guardian spirit and a fickle, malevolent influence. Like the famous pareidolic Man in the Moon himself, it becomes whatever we want to see in it.

Leatherbarrow, Bill. The Moon. London: Reaktion, 2018.