On the Moon, it is also slow. At the equator the night-edge slides west at just 16 kilometres an hour; to the north and south, it is slower.
After two weeks the night-edge has eaten the sunlit part of the Moon away to almost nothing. A thin crescent arches between the polar horns where limb and night-edge touch, the six-to-nine-to-midnight rim of a clock. As it has waned, the Moon has drawn closer to the Sun. When full, it rises around dusk and sets around dawn, opposite the Sun in the sky. But the waning crescent Moon rises shortly before sunrise and sets in the afternoon. This Moon is of the day, not of the night.
Eventually, there comes a point when it sits so slim and slight in the Sun-soaked sky it can no longer be seen by the naked eye. Perhaps a day after that, the terminator meets the limb and there is nothing there at all. Every speck that is sunlit is invisible from Earth; every part that faces the Earth is in darkness. The Moon is new.
When the day-blind Moon returns, it trails the Sun down the evening sky reversed as in a poletopole mirror: its crescent runs from noon through three to six. The night-edge still crawls from east to west, but now it is chasing night away, leaving behind brightness as a squeegee leaves clean glass. The crescent fattens—waxes—until the night-edge stands straight, marking the first quarter. Then the Moon is gibbous again, slowly filling itself with borrowed light until, for a moment, it is whole again, all day again, a perfect circle.
Thus the phases of the Moon: full, waning gibbous, waning crescent, new, waxing crescent, waxing gibbous, full.
This regular cycle—from one full Moon to the next takes 29 days, 12 hours, 44 minutes and 3 seconds—has defined time since time was first defined. In the Islamic calendar a year is always 12 of these months, with each month beginning on the day when the waxing crescent is first seen in the evening sky. This means months can be either 29 or 30 days long. If the moment of the new Moon is early in the morning, then the crescent Moon may be seen the morning of the following day. If the moment of the new Moon comes late in the day, another whole day may pass with no visible Moon and the next month will begin the evening after. When the month thus drawn out is Ramadan, the month of fasting, that last day is long.
Months that are never more than 30 days long mean that the years of the Islamic calendar are shorter than solar years—the time it takes for the Earth to move round the Sun. In calendars where both the phase of the Moon and the season of the year are taken into account—lunisolar calendars like the Chinese and Hebrew ones—steps, such as introducing an intercalated month every few years, are taken to keep the lunar year and the solar year aligned. In Islam this is forbidden.
In solar calendars, such as the Gregorian calendar used in the West, the months and the phases of the Moon are not aligned at all. Nevertheless, there will be 12 or 13 full Moons a year, one in each month except, occasionally, February. There are various traditions for naming them. January’s full Moon is the Wolf Moon, February’s the Hunger Moon. March’s Lenten Moon is sometimes called the Worm Moon or the Sap Moon. If the Lenten Moon falls after the spring equinox, then Easter is celebrated the following Sunday. If, as is more common, the first full Moon after the equinox is April’s Egg Moon, then Easter comes the Sunday after that instead. May has its Hare Moon or Flower Moon; June, its Strawberry Moon.
Summer storms bring the Thunder Moon—which is also the Hay Moon, rising slow and low over evening meadows of grass kissed gold in the day. August brings the Grain Moon. The Harvest Moon is the full Moon closest to the autumnal equinox; it normally rises in September, sometimes in early October. After it come the Hunter’s Moon, and then the Frost Moon, which if it rises particularly late in November or early in December becomes the Mourning Moon. The last Moon is the Cold Moon; then the Wolf returns.
Every few years or so, there are two full Moons in the same month. The second has recently come to be called a Blue Moon, whatever month it sits in.
Many names, many relations to the other signposts of the year. But always constant to its own rhythm: full, waning gibbous, waning crescent, new, waxing crescent, waxing gibbous, full. The Earth’s skies offer no other regularity comparable.
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REFLECTIONS
ON JUNE 24TH 2001, JUST AFTER THE SUMMER SOLSTICE, A telescope at the Observatoire de Haute-Provence in the South of France swung through the evening sky to fix itself on the waxing-crescent Moon sinking in the west. It may have been the first time that the telescope’s mirror, 80cm across, had ever set out deliberately to gather moonlight. These days astronomers do not much care for the Moon.
In the 17th century its features fascinated them. Galileo Galilei’s first telescopic observations of the Moon, and the deductions he drew from them, helped change astronomers’ conception of what might lie beyond the Earth—and of what the Earth itself might be. Unlike the other bodies of the sky, the Moon had a physiognomy you could map, like an island, like a palm, like a face. And map it the telescopic Moonwatchers did.
By 1892 spectacular maps had been made, and great globes—or, rather, given that only one side is seen, half-globes—graced museums. The American geologist Grove Karl Gilbert, chief scientist of the US Geological Survey, was able to announce, with some pride, that the Moon was in some ways perhaps better mapped than the continent on which he stood. None of it was as well known as the best-mapped bits of North America. But nor was any of it terra incognita, as some of central Canada or Alaska still was. All that was exposed to inspection—all of the Earth-facing nearside—could be recorded.
To a geologist like Gilbert, this panoply of strange landscapes formed by mysterious processes was a wonder; he owned himself “a little crazy on the subject of the Moon”.* Many have followed in his craziness. Despite now having maps of far more distant, complex and dynamic bodies, some astrogeologists and planetary scientists remain in thrall to the faraway nearby of the Moon’s surface. To astronomers, it was a bit of a bore. As the Victorian astronomer Richard Proctor put it, “The principal charm of astronomy, as indeed of all observational science, lies in the study of change,—of progress, development and decay.… In this relation the Moon has been a most disappointing object of astronomical observations.” If astronomers had wanted to stare at unchanging rocks, they would have gone into geology themselves, or become stonemasons.
Worse than dull, it was actually damaging. When the Moon rides high at night, some of its bright borrowed sunlight is smeared across the sky, overwhelming the fainter, cosmically distant lights astronomers have come to prize. So by the 20th century almost all astronomers had come to shun the Moon, at least in their professional lives. They avoided the nights when it was at its brightest, shielding their instruments from its insistent stare as they waited for the dark of its departure. Only the amateurs kept Moonwatching, running their eyes and instruments over the raised relief of its features for the pure pleasure of its beauty and mystery, or in the persistent faith that at some point they would, indeed, see something change.
IT WAS THUS VERY UNUSUAL TO FIND A PROFESSIONAL TELESCOPE following the Moon as it slid down over the hills of Provence on that evening in 2001. More unusually still, another fine telescope was trained on the Moon just a few hours later, at Kitt Peak in Arizona. In France it had set behind the Luberon hills, named for the wolves that once lived in them, animals which were absent for most of the 20th century but which have now, I believe, returned. In Arizona, with a pleasingly symmetric regard for the role of nocturnal animals in lunar folklore, it had risen over the granite of the Coyote Mountains.
The Moon’s light is important to the lives of many creatures on the Earth. The Peruvian apple cactus, for example, opens its enormous flowers only when the Moon is full. But as far as any researcher can tell, neither wolves nor coyotes care all that much about it. They howl on moonlit nights and moonless ones alike. Humans associate howling with the Moon only because they associate the Moon with the night, and howling with the night, and thus the howlers with the Moon.
Coyote, according to a story tha
t has been told in Arizona since before it was Arizona, howls at the Moon because he used to be the Moon. He took over that office from Crow, who the people had chosen before him but who had been too dark for the job. Crow had taken the job on from Fox, the people’s first choice, who had proved too bright. The quality of Coyote’s light proved superior to that of Crow or Fox. But his presence in the sky proved troublesome. He would use his vantage point to peek at women bathing, to reveal petty crimes, to spoil games of chance. So the people called Coyote back down, as they had Fox and Crow, and sent up another animal of similar colour: Rabbit. Rabbit curled up in the Moon, lay still, and didn’t misbehave. He has stayed there ever since. Look up and you can see him, body curled up, ears drooping down.
Those visible features of the Moon, though, were not what the astronomers in France and Arizona were interested in. They were drawn to the part of the Moon where the Sun was not shining, the part where features could scarcely be seen—the part that is lit only by earthshine.
Just as the Moon reflects the Sun’s light to the Earth, so the Earth reflects the Sun’s light to the Moon. And it does an impressive job of it. The Earth is larger than the Moon, and a better reflector; a full Earth, seen from the Moon, provides almost 50 times as much light as a full Moon does on Earth. And some of that earthlight, having travelled from the Earth to lighten the night of the Moon, bounces back whence it came.
It is when the Moon is a crescent that this light, sometimes called the ashen light, is best observed. At these times the Moon is between the Sun and the Earth; it is day on its Sun-facing farside, night on most of the Earth-facing nearside. That night is lit by the bright gibbous Earth. The earthlight means you can see the whole disk of the Moon quite plainly. The earthlit bit looks very dark—paradoxically it often looks slightly darker, to my eyes, than the surrounding sky. But it is clear that there is a whole Moon there, not just the bright shaving on one side. People sometimes call it the old Moon in the young Moon’s sunlit arms.
The first person known to have understood the ashen light to be the light of the Earth was Leonardo da Vinci, in the early 16th century. Leonardo advised those who would be artists that “the mind of the painter must resemble a mirror”; the world was there to be reflected. And, sometimes, to reflect itself, as the Moon reflected the Sun to the Earth.
What was the reflecting surface? Choppy water, Leonardo thought. If the Moon were a true, smooth mirror, he pointed out, Earthly observers would see an image of the Sun glinting off just one point on its surface; he compared it to the highlight you might see as sunlight strikes a gilded ball decorating the gable of a high building. The fact that there is no such single highlight, he argued, must mean that the Moon was a set of mirrors all reflecting the Sun in slightly different directions—like a gilded mulberry (I love this image) or the waves of a fishing-boat-bobbing sea. Liquid seemed more likely than fruit. The lunar surface, Leonardo supposed, must be mostly sea—an idea that went with the flow of a long-running association between the Moon, raiser of tides and companion to rain clouds, and water.
The Earth, too, is largely covered by water. So it, too, must reflect the Sun. “If”, Leonardo wrote, “you could stand where the moon is, the sun would look to you as if it were reflected from all the sea that it illuminates by day, and the land amid the water would appear just like the dark spots that are on the moon which, when looked at from our earth, appear to men the same as our earth would appear to any men who might dwell in the moon.” The night-time Moon was lit by light reflected from the seas of the Earth just as the Earth’s nights were lit by the seas of the Moon. It was an instance of the phenomenon of “secondary light” much discussed by artists in the Renaissance—the fact that there can be light when there was no visible source of it, as when light bounces off the wall of a sunlit room to illuminate a neighbouring room with no windows. Leonardo took the way artists thought about the painting of interiors and applied it on a scale larger than the Earth.
Galileo, who first brought the ashen light, and its explanation, to the attention of a broad public, was, like Leonardo, interested in the technical aspects of painting—indeed, he had taught a course in them. He also, though, had an interest that Leonardo lacked. He wanted to convince the public that the cosmos was not as they had thought it. And for this purpose the ashen light was particularly useful.
Most of the observations that Galileo wrote about in “Sidereus Nuncius” (1610; “The Starry Messenger”), a short but remarkably influential book, were made possible by his then-brand-new telescope, with which he had started observing the Moon and other heavenly bodies the previous year. Since telescopes were rare, most of his readers had to take him at his word—and his illustrations, which show the artistic talent with which he had once wanted to make his name—as to what he saw. But seeing and understanding the ashen light required no such high technology. Galileo assured his readers they could easily see the effect for themselves if, when the crescent Moon was low in the sky, they so positioned themselves that the sunlit sliver was hidden by a chimney or wall. Once thus seen, the ashen light was easily, even naturally, understood as light that had bounced first off the Earth and then off the Moon, like sunlight reaching an inner room.
For most of his readers, this must have been a strange new thought. For Michael Maestlin, an astronomer at the University of Tübingen, and his pupil Johannes Kepler, then the court astronomer to Holy Roman Emperor Rudolph II, this part of “The Starry Messenger” came as no surprise. They had come to the same conclusions about how the Moon was lit with no need of telescopes. So had Paolo Sarpi, a priest and statesman in Venice whom Galileo knew and with whom he may have discussed the matter. It is no coincidence that these men were, like Galileo, members of the small band of Moonwatchers which took seriously the idea that Nicolaus Copernicus, a Polish cleric, had published more than half a century before: that the Earth orbited the Sun.
That belief is not directly related to understanding the ashen glow as earthlight. The ability of the Earth to reflect sunlight on to the Moon is independent of who is going around whom; it just requires that sometimes the Earth and Sun be on opposite sides of the Moon. A contemporary of Kepler’s and Galileo’s who believed the Earth to be the centre of everything could have embraced the same explanation for the nearly-not-light the crescent Moon held in its arms. But as far as is known, none of those Earth-centred contemporaries did; it was something only the Copernicans took note of.
Why did this way of understanding the Moon to be lit by earthshine fit with one conception of the universe, but not the other? Because understanding the Moon and the Earth as having the same powers of reflection required you to see the Moon and the Earth as the same sort of thing. That was part and parcel of being Copernican: the Moon and the Earth were planets like the other planets that orbited the Sun. It was a high conceptual hurdle for everyone else. The mediaeval world had followed Aristotle in believing that the Earth was of a fundamentally different substance from the Moon, or any of the other bodies that orbited it. The Earth was made of dull matter; they were made of crystal, or fire, or some other rarefied substance. The Earth changed, but they did not. They moved, but the Earth did not.
Seeing the Moon lit by the Earth in just the way that the Earth is lit by the Moon went against that understanding. In Galileo’s words, it drew the Earth “into the dance of stars”. That choreographed companionship was at least as much a part of the Copernican revolution as the details of who orbits whom. The Earth became a planet—in the original sense of a star that wanders—and the planets became Earths, bodies as real as the world around you. Indeed, they might well be inhabited by people who saw them as worlds and for whom the Earth was a distant moving dot. They almost had to be: what would be the point of God creating uninhabited worlds? As the historian of art and science Eileen Reeves writes, there came to be “a nearly axiomatic connection, at least in the popular mind, between the theory of secondary light, the Copernican worldview and a belief in extraterrestrial life
.”
THE QUESTION OF EXTRATERRESTRIAL LIFE HAS DANCED WITH the science of astronomy ever since, as the Earth dances with the Moon and Sun: sometimes the ideas have been in opposition, sometimes in alignment. In the past 20 years they have sat in a striking conjunction. A great deal of astronomy is now justified to the public which pays for it as a search for life elsewhere.
And that is why, having ignored the Moon for decades, astronomers in Provence and Arizona found themselves peering so intently at its reflected earthlight at the beginning of this century. They were looking at it to learn what the signs of life on an Earth would look like if seen from afar.
In 1995, after decades of false alarms, astronomers started discovering planets around other stars. The light from such “exoplanets” was so faint that these bodies could be detected only indirectly—by their shadows as they moved across the face of their stars or by tiny shifts they caused in the spectrum of the stars’ light. But those interested in life in the universe—astrobiologists, as they were becoming known—believed that, in time, bigger, better telescopes would let them see some of those exoplanets directly. And when that happened they would be able to look for signs of life.
Light from an exoplanet is light from a distant star that has gone into its orbiting exoplanet’s atmosphere, been reflected or refracted back out into space and travelled on to the Earth. The many years of the last leg of that journey make no difference to the light; the fraction of a second in the exoplanet’s atmosphere and bouncing off its clouds or surface leaves its mark. The molecules of the exoplanet’s atmosphere absorb some wavelengths more than others. If astronomers could spread an exoplanet’s light out, wavelength by wavelength, in one of their spectrographs, like a dealer spreading a pack of cards across green baize, they could pick out such effects; some cards would be missing, because some wavelengths had been absorbed in the exoplanet’s atmosphere.
The Moon Page 2