A Moon of Two Halves

20 July is International Moon Day

Tomorrow, July 21, is the 55th Anniversary of humanity's first step on a world other than our own.

In the early hours of July 21, 1969, Neil Armstrong spoke the most famous eleven words in history when he became the first man to set foot on the moon.

“That's one small step for [a] man, one giant leap for mankind”

It was a succinct summary of the Apollo 11 crew’s extraordinary feat. The Apollo 11 landing was ostensibly the result of four days of spaceflight and a few hours in the Lunar Module. But the achievement Armstrong's words marked was the culmination of thousands of years of observation and decades of hard science, not to mention countless centuries of artistic speculation through poetry, song, science fiction and fantasy. It’s hard to view the lunar landing as anything less than the apex of human achievement, the crowning glory of an ancient fascination, a preoccupation so primal it lies at the base of human consciousness. In those terms, the first giant leap for mankind was, in a sense, not walking on the moon, but noticing it in the first place.

The Moon Marketing Board

It isn't hard for us to notice, of course. In the modern world, the moon is ubiquitous, even when it isn't in the sky. It looms out at us from advertising hoardings, is sung and spoken of with great affection in every language on earth, has cameo appearance in films and on television and its progress and daily movements are endlessly followed - as if the moon was a star - by daily newspaper columns around the world. It makes its way, several times a day, into every nook and crevice of our life; it is almost as if the unseen hand of the Moon Marketing Board is at work to fill our lives with lunacy.

Maybe that's why we all feel that we have a passing familiarity with the moon and its appearance, in the same way that we say hello to newsreaders we spot in the wild from time to time, even though they haven't the faintest idea who we are. As with our brief moment of familiarity with a journalist from the telly, there's much we don't see behind the public face of the moon. I've no idea how much Emily Maitlis is hiding from us, but with the moon it's around 41% – the reason for the observational deficit is tidal locking. It's well known that the moon causes the tides on earth, by dragging a bulge of sea water around in its gravitational wake, but the earth also has a tidal effect on the moon. Because there are no oceans to pull, the earth tugs only at the moon's crust and that tidal pull has locked the moon's rotation to its orbit around the earth. As a consequence, the moon always presents the same face  to us and we see little of the far side. Ironically, the moon never shows us its backside.

The limited views we have of the far side are caused by eccentricities in the moon’s orbit and axis which, with a sprinkling of parallax, add up to an apparent wobble that astronomers call libration; over time, an observer on earth sees around 59% of the moon’s surface.

The moon’s two sides are as different as the opposite faces of a chocolate Hob-Nob. The side that we see, with its pattern of blotches suggestive of the man in the moon, a rabbit, a dragon, somebody carrying sticks or whatever else your cultural conditioning dictates, differs wildly from the b-side – an evenly pock-marked disc that looks like a cartoon planet from a children's television programme. There are no large dark blotches on the far side and science awaits a convincing explanation why that is so.

On the near side, the blotches were once thought to be expanses of water and early astronomers called them maria, the plural of mare, the Latin for sea. The areas of the moon that appear brighter are the highlands or terrae, from the Latin for earth. Seventeenth century science was so convinced that the blotches were sea, they graded them and labelled them by size as bays, lakes and, the largest, an ocean. There’s a whole nautical almanac of names for things on the moon and hardly a drop of water in any of them. We now know that maria are not oceans, but basalt plains formed from the flood lavas of ancient volcanic eruptions. The volcanic activity that formed the maria happened  around 3 billion years – not recent but still more recent than the formation of the lunar highlands. The face of the moon, like those of humans, can be aged by appearance; in the case of the moon, the more craters a surface has, the older it is, but how old is the moon?

The moon is almost as old as the earth or the earth is almost as old as the moon, depending on your perspective and where you think the story begins. The story starts in the solar nebula,  the spinning disc of dust, gas and rock from which planets coalesced. The first moments of a planet formed in a swirling cloud of dust, rocks, asteroids and planetesimals, is as precarious as it sounds and, indeed, the earth was perhaps only as young as 23 million years old  when it is believed that it was hit by a planet the size of Mars. That hypothetical planet – named Theia by scientists – came off worse in the impact. Smashed into countless pieces, it lost most of its iron core to the earth but then formed an orbiting ring of dust and rock which gradually reassembled what remained of itself from millions of fragments and began to orbit the earth as the moon.

The next time you catch yourself idly gazing at the moon, do consider the possibility that the moment isn’t romantic or wistful and that you are neither nostalgic, plaintive or forlorn. Depending on your emotional peccadilloes, you might find a long, cold stare at the moon, while not being in possession of a poorly-expressed yearning for something or other, rather difficult– all of which brings us to the question of why that might be so. Why might a large rock in the sky inspire longing and a reflective pensive mood? The answer, of course, lies in the layers of meaning we have imbued the moon with over the thousands of years we have been looking at it.

As our closest neighbour in space and the most obvious thing in the night sky, it’s perhaps not much of a surprise that the moon has loomed large in our lives for millennia. Our fascination with our constant companion has blessed the moon with more-or-less equal billing to the sunin mythology, in folklore and nature worship - a cooler, reflective, evening analogue to Sol, a yin to the yang of the sun.

It’s an enduring theme, recycled in one form or another across many cultures. Classical Greek mythology had a number of pairings of god and goddess to represent the sun and moon. Brother and sister Apollo and Artemis were god and goddess of the sun and moon, as were Phoebus and Phoebe, Helios and Selene and their later Roman counterparts. The Aztecs and Mayans had similar, though largely unpronounceable, names while there are an equal number of moon gods with associated sun goddesses - even though the dominance of Mediterranean myth in Western culture rather blinds us to the fact. There's a modern pantheon also; the neopagan and wiccan traditions now practised quietly in groves and on windy hill tops are reconstructed interpretations of what their advocates imagine to be the indigenous ancient religion of Britain and northern Europe. Their principal deities are the Horned God, who is venerated in many forms including the sun and the Triple Goddess, whose aspects include the moon and who I once spent a merry evening in the company of in a West country field.

Moon Minutiae

1. While staring at the moon might put you in a reflective frame of mind, the moon itself is not really that reflective at all. It may be 100,000 times as bright as the brightest night-time star, but its surface is about as reflective as asphalt. If you put a car park in space, it would shine as much light back as the moon.
2. The similar standing of the sun and moon probably stems from their near-identical size in the sky; although the sun is 400 times larger, it is also 400 times further away, a cosmic fluke that means the disc of the new moon perfectly covers the sun during a total solar eclipse. This coincidental arrangement of the heavens is temporary, a window of time where everything is ‘just-so’– the moon is slipping away from us by 4 centimetres a year and in a hundred million years solar eclipses will come and go with only a brief, hyperborean chill to remark upon.
3. You will never witness a true full moon – that is, one that occurs when the sun, earth and moon are in precise alignment – because at that exact moment, there will be a lunar eclipse.