Its surface area is 37.9 million square kilometres, about a quarter of the area of the Earth’s continents. That makes it smaller than Asia, a bit larger than Africa, and significantly larger than all the others. A straight tunnel between the Moon’s poles would be 2,474km long—in terms of Africa, the same length as a tunnel from Cairo to Nairobi. A road around its equator would be 10,921km long—equivalent to driving from Cape Town to Addis Ababa and back. Imagine Africa stretched, snipped, spindled, stuffed and sewn back up to form a sphere around that Cairo-Nairobi axis and you have something of a sense of it.
About 17% of its surface is made up of dark, low-lying plains called maria: the plural of mare, which is Latin for “sea”. Almost all these plains are on the Earth-facing nearside. The largest, in the west, is Oceanus Procellarum, the ocean of storms. It is half the size, more or less, of the Sahara. Above it lies Mare Imbrium, which covers about 2 million square kilometres, an area roughly the size of the Congo Basin.
To the east of Imbrium, names are less nautical and more allusive; here we find not rain and storms but serenity, tranquillity and fertility. Going east, Mare Serenitatis, roughly the size of Nigeria, gives directly into the slightly larger Mare Tranquillitatis, roughly the size of Chad. Two further, less circular maria branch off Tranquillitatis: Mare Nectaris and Mare Fecunditatis. To cultures that see a rabbit in the Moon, the recumbent Y of these seas is a pair of floppy ears.
Mare Crisium, the sea of crises, sits to the north of these other seas, a dark eye surrounded by brightness: as the sunlight spreads across the surface in the early days of the waxing Moon, Mare Crisium is the first feature to become distinctly visible; as the Moon wanes, it is the first to be lost. Luna City, the setting for Robert Heinlein’s “The Moon Is a Harsh Mistress”, the most influential novel ever to be set on the Moon, sprawls through tunnels beneath it.
On the far side of the Moon, the side that the Earth never sees, there are just two small seas: Mare Muscoviense and Mare Ingenii, the seas of Muscovy and cleverness. They cover an area slightly larger than that of the two islands of New Zealand.
The brighter parts of the Moon are called the highlands. Whereas the maria are not seas, just plains, the highlands are, for the most part, high. Where they meet the maria there are often mountain ranges, mostly named after counterparts on Earth; those around Mare Imbrium are Montes Alpes, Montes Jura, Montes Carpatus, Montes Caucasus and Montes Apenninus. The Apennines, 400km long, are perhaps the most impressive, their peaks rising as much as 5km above the non-rainy plains—similar in stature to the Rwenzori Mountains in East Africa, known in antiquity as the “Mountains of the Moon”.
The most striking highland feature, when the Moon is full, is the crater Tycho, in the nearside’s southern highlands. It is the nexus for a starburst of bright linear “rays” that spreads across half the hemisphere. There are also bright craters in some of the maria, such as Copernicus, south of Mare Imbrium. The brightest of all is Aristarchus, in Oceanus Procellarum.
Tycho is the youngest major feature, probably about 100m years old. It dates from a time when, on Earth, the rift which created the Atlantic had just started to pull South America out of what is now the Bight of Benin. Copernicus is about 800m years old, which makes it older than all the fossilized animals of the Earth. And Copernicus is still, in terms of lunar geology, a recent structure. Most of the maria are four times older than Copernicus. The highlands are more ancient still, well over four billion years old.
All but the very oldest rocks still extant on Earth are younger than all but the very youngest features on the Moon.
The great bright craters and dark maria discernible to the naked eye have not changed one bit over the time that there have been humans to look at them. Every person who was ever born who could see, and who lived long enough to focus their eyes and walk outside into the night, has seen the same Moon as all who look at it today. More humans have gazed in wonder at the surface of the Moon than at any other solid object in the universe.
- II -
THE FACE OF THE MOON
WHEN ALBERTUS MAGNUS, ONE OF THE GREAT ARISTOTELIAN schoolmen of the Middle Ages, looked at the patterns on the face of the Moon, he saw a beast with its head in the west and something on its back resembling a tree. In the east, there was a man who might have been leaning on that tree. Some saw that eastern homunculus as Cain, the first murderer, or as Judas Iscariot; others said he was just a peasant who had been banished from Earth for taking wood to which he was not entitled from some lord’s lands.
It is possible that the dog and thornbush Shakespeare gave to Robin Starveling so that he could present the Moon in “A Midsummer Night’s Dream” referred to the features Albertus saw as a beast—Oceanus Procellarum and attendant smaller maria—and the crown of a tree—Mare Imbrium. It is hard to tell, though. As far as is known, at that point no one in the Western world had actually made, let alone preserved and labelled, any drawings of the features they saw when they looked at the Moon, or given them settled names.
Today, in an age saturated with images, this seems remarkable—I certainly found it so when I first learned that it was the case. But no one remarked on it at the time. Symbolic representations of the crescent Moon—with a nose and a face added in profile if personification was in order—seem to have been entirely sufficient and were widespread, not least in coats of arms and the flags of Islam. Pictures of its features, as opposed to its form, were not. If you wanted to know what the spots on the Moon looked like, you could look at the Moon. Why record things all could see, which did not matter and which no one was in danger of confusing with anything else?
The first answer seems to have been this: because recording the world as it is matters in and of itself. It is the answer you might expect of a scientist. In fact it was given, in deed not word, by an artist, the Flemish Renaissance painter Jan van Eyck. There are five pictures attributed, with differing degrees of certainty, to van Eyck in which a realistic Moon adorns daylit or twilit skies. The clearest, and most moving, is a painting of the crucifixion dated between 1420 and 1425. A gibbous Moon sits low in the afternoon sky. The dark patches of the maria are clearly visible, as is the fuzziness of the night-edge.
One of the things that set van Eyck apart from his contemporaries was a commitment to recording the specific contents of his world as they were, even when such detail was incidental. The textures of his limestones show realistic weathering patterns; his mountains are topologically precise; his clouds pass meteorological muster. His Moon is, like them, a thing in the world, rendered not as allegory or icon but as it really is.
Not, though, as it really is in the afternoon. A gibbous Moon visible in the afternoon—the time of the Christ’s death—would have to be waxing, not waning like van Eyck’s.* This lapse seems to attest van Eyck’s lack of interest in the Moon as an astronomical object; he just wanted to record how it looked. He probably made a preparatory sketch in the morning, when waning gibbous Moons set, and worked from that. If what matters to you is capturing the look of the thing in itself, Moon is a moon is a moon.
Why a moon at all? Perhaps, as it is for many of us, it was simply a sight that he liked. Perhaps he saw it as a technical challenge. There may be another reason, though. The Moon had long been associated with death. In “Concerning the Face in the Disk of the Moon”, Plutarch talks of the “substance of the soul being left on the Moon”, where it “retains certain vestiges and dreams of life”. He called the unseen farside the “Elysian Plain”, the nearside the “Plain of Persephone Antichthon”. Souls could travel from one side to the other through long “gulfs”, or suffer in the “Chamber of Hecate” (probably Mare Imbrium).
Less learnedly, the Moon has the paleness of a skull, and dark sockets for eyes. In van Eyck’s crucifixion, it is about the size of a skull, too, close to—and about as big as—the head of the impenitent thief to the Christ’s left. Read like this, the painting inevitably brings to mind the greatest moonrise picture of the twentieth cen
tury, “Moonrise, Hernandez, New Mexico”, by Ansel Adams, in which the brightness of the Moon in the evening sky matches that of the still-sunlit crosses in the high-country cemetery below.
Despite this echo across the centuries, van Eyck did not start a trend. The next artist to draw the Moon with features, as far as is known, was Leonardo, who sketched it with his unpublished notes on earthshine. But he saw no need to include its face in any finished art. The only other extant images of the Moon’s features made before the advent of the telescope were made by William Gilbert, who was physician to Queen Elizabeth, and by Adam Elsheimer, a German artist living in Rome.
Gilbert’s drawing, which dates from around 1600, is neither a sketch nor a work of art. It is a quite crude map, drawn on a grid, with various maria clearly delineated and rather prosaically named (“Southern continent”, “Northern island”, “Middlemoon Sea”, etc). It is something which could, in principle, have been drawn at any time—had anyone wished to inspect the Moon that closely and record what they saw in a way that would allow others to refer to the same features. Before Gilbert, it seems no one did.
Gilbert’s unprecedented interest in the Moon’s face was like that of Galileo, Maestlin and Kepler in its ashen light: he, like they, was one of the small band that believed the Earth and the Moon were both bodies in motion. He had concluded that the Moon and the planets were not fixed in spheres, as Aristotelians like Albertus Magnus had taught, but that they shared a material, knowable and changeable nature with the Earth. He suspected, correctly, that the Moon rocked slightly in its orbit, sometimes showing the Earth a little more of its eastern hemisphere, sometimes of its western—a change which, in and of itself, would have been enough to give the lie to the idea of it being held fast in a crystal sphere centred on the Earth. It was to look for that change and others that he mapped the Moon; he was interested not in its features but in the chance that those features, or the perspective from which they were seen, might alter over time. Indeed, rather than expressing pride at being the first to map the Moon, he lamented that no one had done so before. If they had, past changes in its aspect might have been discoverable.
His way of seeing the universe provided Gilbert with a new reason to look closely at the features of the Moon. The same, I think, is true of Elsheimer, whose full Moon in “The Flight into Egypt” (1609) is the first since van Eyck to have distinctly dark maria and bright highlands. Elsheimer was associated with the thinkers of Rome’s Accademia dei Lincei, who prided themselves on an intense interest in the details of the natural world (they took their name from the lynx because of its famously keen eyes) and whose number would later include Galileo. Some have suggested that Elsheimer’s painting was based on observations made through Galileo’s telescope—or perhaps even someone else’s—but the case is weak, at best. However, if not telescopic, it is, like Gilbert’s map and Galileo’s observations, Copernican in intent: it clearly sees the Moon as a worldly thing, not a celestial orb.
It was a way of seeing that was soon to spread far beyond a small and discreet coterie of scholars. Looking at the Moon was about to be confirmed as a new way of seeing the world.
GALILEO WAS NOT THE FIRST PERSON TO SEE THE MOON THROUGH a telescope. But “The Starry Messenger” made him the person whose telescope changed the way that others saw it. As with the question of earthshine, it was his painterly attention to illumination that mattered: specifically, to shadow.
It is often said that Galileo’s telescopic studies of the Moon revealed that it had features like the Earth’s. John Milton, who met Galileo in 1638, said as much in “Paradise Lost”, in which the “Tuscan artist”, looking at the Moon “through optic glass”, seeks
to descry new lands,
Rivers or mountains in her spotty globe.
But this is not quite right. Galileo did not find that the Moon had features like the Earth’s. He found that the Moon, like the Earth, had features: it was the physical fact of those features’ existence, not any particularly Earth-like character they might have, that mattered.
One indication of this is that Galileo showed no interest in mapping the Moon. He only showed its partial disk obliquely lit; he never named any features. His aim was to demonstrate the existence of distinct highs and lows on its surface, and his drawings and his analysis thus centre on the night-edge, where the rising or setting Sun provides the shadows that make relief most obvious.
Because the Moon’s relief is dominated by craters, they dominate Galileo’s analysis. He draws the reader’s attention to the way that, at the night-edge, their rims, lit by the Sun, produce horns of light that pierce the night. His drawings exaggerate the effect substantially, at the same time making the craters larger than they in truth appear; their aim was to instruct the eye, not to mimic the sky.
In his text he laid particular stress on the way the darkness within some of the cavities persists into daytime, showing them to be low-lying. It is like early morning in the mountains, he says, when the Sun lights the western side of a valley first, its illumination subsequently sliding down to the valley floor and only reaching the eastern slopes when the Sun is quite high in the sky.* But Galileo did not say the craters were valleys or their rims, mountains: he preferred terms with less of the landscape about them, such as prominences and cavities.
Nor did he say that the “great and ancient spots” now called maria were seas. He said only that the shadows showed them to be smoother than the brighter surface, “like frosted glass”, in which they were embedded, and also to be lower.† But that did not show they really were seas, just that, if anyone sought to “revive the old opinion of the Pythagoreans, that the Moon is another Earth… the brighter portion may very fitly represent the surface of the land, and the darker the expanse of water.”
This part of his analysis set Galileo against others, such as Gilbert and Leonardo, who when seeing the Moon as a world in the sky had believed that it was the bright bits which corresponded to seas. Galileo saw them to be wrong. To see the sea as bright is to be dazzled, often literally so, by specular reflections. Look sunwards out to sea and the shifting surface provides a million mirrors aimed straight towards you; images of the Sun distinct, perhaps, close to merge into an ever more coherent band the further your eye rises towards the horizon. To either side of the path, though, the surface is darker, reflecting only empty sky. Seen from above and from a distance it is this darkness that dominates.
The passage in which Galileo makes this point seems to me one of the most extraordinary in the whole work: “I have never doubted that if the sphere of the Earth were seen from a distance, when flooded with the Sun’s rays, that part of the surface which is land would present itself to view as brighter, and that which is water as darker in comparison.” What sort of person, then, there, would have troubled himself to think about the Earth seen from a great distance—never mind reaching undoubted conclusions as part of the exercise?
Galileo taught his readers to see the Moon’s third dimension as evidence that it was made of mundane matter—but not that it was, in fact, another Earth, with Earth-like features. On that he presented himself as agnostic. Having demonstrated the Moon’s mundane nature, “The Starry Messenger” and its author moved on to other things. Galileo never published any further observations of the Moon.
The first part of the lesson was quickly heeded. Thomas Harriot and William Lower, two Englishmen, had both looked at the Moon through telescopes before they read “The Starry Messenger” but had not grasped what they were seeing. Describing his observations to Harriot, Lower wrote: “In the full she appears like a tart that my cooke made me last weeke; here a vaine of bright stuffe, and there of darke, and so confusedlie all over.” After reading Galileo, they knew how to see what they were looking at: heights and cavities, roughness and smoothness.
As quickly as people learned to see the Moon as Galileo saw it, though, they also started to draw and talk about it in ways that he didn’t—interpreting its features according to Ear
thly analogues, naming them and marking them down on maps. And here they ran into problems. Moonlight can be deceptive—even contradictory.
The fine shattered rock of the Moon’s surface contains lots of small bits of glass that prefer to reflect light back in the direction it came from rather than off to one side; the same retroreflective effect is used in making cinema screens. This is why the Moon is much brighter when it is full, or close to full, than at other times. It is not just that more of the surface is lit; the over-your-shoulder way in which the surface is lit makes it more reflective than it is at other times.
These retroreflective particles are seen all over the Moon; but their distribution is not even, and its pattern does not reflect the underlying relief. The “rays” which extend out from young craters like Tycho, and which I take to be Lower’s “vaine[s] of bright stuffe”, are particularly densely populated with such particles and are thus striking when the Moon is full. But they are hardly discernible at other times. They are purely superficial features—not ridges or gullies—and thus cast no shadows. They have no more relevance to the shape of the surface they sit on than the trace of a kiss has to the contours of a cheek.
This is why the Moon looks so different when shadowless and full to the way it looks at its obliquely lit quarters. Mappers had to reconcile these aspects to produce images which both showed the relief imputed from shadows and did justice to what the full Moon looked like to the eye, and no two of them did so in quite the same way. The Moon that in the sky looks the same to everyone became idiosyncratically individualised when transferred to the page.
Then there is the matter of nomenclature. To begin with, different astronomers named the features on these maps according to different schemes. But the naming system used today was established in “Almagestum Novum” (1651; “The New Almagest”) of Giovanni Riccioli, a Jesuit. Although the church had reservations about the physical truth of Copernicanism—and, more profoundly, about the idea that the authority to define truth in such matters should rest in other hands than its own—it also had many fine astronomers.
The Moon Page 5