It’s 45 years since the first Moon landings, yet our enigmatic lunar neighbour still seems oddly far away. Here, Katie Grocott explains just what has made, and continues to make this shimmery celestial rock so alluring.
There’s A Moon In The Sky (Called The Moon)
Life on Earth arose and evolved under the influence of the Moon. For human beings, our satellite has been a cultural, scientific and philosophical touchstone ever since people first looked up into the sky – woven into myths and religions, embedded into our calendars, part of our very language and the way we think about things. I have been aware of the Moon almost from the time I learned to speak – as a child I took it for granted that cows could, and did, jump over the Moon, and that the Moon was inhabited by a family of pink knitted mice (or a family of friendly spoons).
What is the Moon, and where did it come from? Do we need it? Will it always be there?
Far Out
The Solar System contains many moons. The giant planets Jupiter and Saturn alone boast well over 150 moons between them, Uranus and Neptune have a handful each; Mars has two. Even tiny Pluto, demoted from full planetary status in 2006, has five satellites. Earth’s Moon, though, is unique among moons. Although it’s not the largest moon in the Solar System (that accolade goes to Ganymede, the largest moon of Jupiter) it is by far the largest relative to the size of its parent planet – at 3500km, just over a quarter of the Earth’s diameter, and about 1/81 of Earth’smass. Some astronomers, indeed, feel it more proper to describe the Earth-Moon system as a binary planet. By way of comparison, Jupiter’s diameter is about 27 times that of Ganymede.
The vast majority of Solar System moons are irregularly shaped rocks a few kilometres or tens of kilometres across; these are most likely captured asteroids or comets, visitors from the outer Solar System that strayed too close to a planet to escape being caught by its gravitational pull. The larger moons of Jupiter and Saturn probably formed in situ, at around the same time they did. Our own Moon, on the other hand, was created under much more dramatic circumstances.
New Moon On Monday
Around 4 billion years ago, the Solar System was a swirling disc of dust, gas and larger chunks of rock circling the young Sun. Smaller chunks of rock, known as planetesimals, gradually accreted via countless collisions into larger proto-planets. Two of these proto-planets, the proto-Earth and a smaller, Mars-sized body called Theia, also collided. Both these proto-planets were essentially giant balls of lava, heated by the repeated impacts of many smaller collisions into a molten state. The heavier elements present in them, mostly iron and other metals, had trickled down and collected in their cores.
When the impact occurred, it was relatively gentle (anything more violent would have smashed both planets into space dust!) There was, however, enough force in the collision to fling the outer layers of both Earth and Theia out into space; there the debris remained in orbit. Meanwhile, the heavier elements present in Theia’s core sank into the Earth, adding to the existing stock of metals there.
As time and gravity took their courses, the ring of orbiting rubble cast off by the collision coalesced to form the Moon. The impact itself also altered conditions on Earth – and set the scene for life to arise. The impact knocked the Earth’s axis of rotation sideways, where it remains at an angle of around 24 degrees – this tilt is responsible for the seasonal changes that we now experience, which regulate the surface temperature and ensure that things stay within a comfortable range for biological life to thrive. Earth’s enlarged metallic core is now the source of a powerful magnetic field, which prevents our ozone layer from being stripped away by energetic, charged particles streaming out from the Sun. And it’s thought that the Moon itself encouraged tidal pools to form in Earth’s early oceans, enabling a concentration of minerals and chemicals to build up, essential for the emergence of life.
Wild Gravity
The Moon actually formed about ten times closer to the Earth than it is now – and it is still receding from us at the rate of an inch or so per year. The culprit is gravity – somewhat counter-intuitively, since gravity attracts rather than repels – but as the Earth and the Moon orbit each other they exert a mutual gravitational drag. This drag causes tides in the Earth’s oceans, but also extends to the very rocks that both Earth and Moon are made of, and the mutual pull causes the rotation of both bodies to slow (Earth’s day was originally eight hours or so). The energy lost by the rotational slowing (known as angular momentum) has to be conserved, however, and the effect of this is that the Moon’s orbit is boosted outwards, while its orbital speed actually decreases. Theoretically the Earth could end up gravitationally locked to a more distant Moon, with the effect that one terrestrial day would last about a month, and the Earth and the Moon would always present the same faces to each other – however, the Sun will come to the end of its life before that comes to pass!
The first humans to directly observe the far side were astronauts on the Apollo 8 mission; as they passed into the Moon’s shadow on Christmas Day 1968 they were entirely cut off from communications with home. The new perspective they gained of the Earth, glowing half-illuminated and utterly silent in the vastness of space, must have been accompanied by the most incredible sense of isolation.
The Dark Side Of The Moon
The fact that we always see the same face of the Moon is due to a phenomenon called tidal locking, caused by the same gravitational drag responsible for the Moon’s recession. As well as moving away from us, the Moon’s rotation and orbit have settled into a synchronous pattern, the result being that the time it takes to rotate once is the same as the time it takes to orbit once around the Earth. So, from our point of view, the Man in the Moon always gazes at us. The far side of the Moon gets its fair share of sunlight too, though, so cannot truly be said to be ‘dark’ – only in the sense that it is obscure to Earthbound observers.
It wasn’t until 1959 that we saw the Moon’s far side, courtesy of Russia’s Luna 3 space probe. The far side was revealed to be much more cratered and less varied in topography than the near side. The best guess as to why this should be is that the internal structure of the Moon itself is somewhat asymmetrical, with a thinner crust on the near side leaving ancient lava plains exposed; the Man in the Moon’s origin is still something of a geological mystery. The first humans to directly observe the far side were astronauts on the Apollo 8 mission; as they passed into the Moon’s shadow on Christmas Day 1968 they were entirely cut off from communications with home. The new perspective they gained of the Earth, glowing half-illuminated and utterly silent in the vastness of space, must have been accompanied by the most incredible sense of isolation.
Walking On The Moon
The idea that there might be life on the Moon didn’t arise with modern science fiction. Before the invention of the telescope, the ancient Greeks divided the Universe into the imperfect, dark and tainted sphere of our own terrestrial world and the perfect, luminous and pristine celestial realms, a notion that (along with that of the geocentric universe) persisted into pre-Enlightenment Christianity and dominated Western religious thought. Extra-terrestrial (heavenly) life was assumed to be correspondingly perfect; the fall of Lucifer, for example, saw the flawed angel literally banished to a place more befitting to him: the tainted Earth below.
Galileo brought our ideas about the Moon down to Earth with a bump. Through his newly-invented ‘spyglass’, in 1609, he saw that “…the Moon is by no means endowed with a smooth and polished surface, but is rough and uneven and, just as the face of the Earth itself, crowded everywhere with vast prominences, deep chasms, and convolutions.” Of course, the Moon’s demotion to an imperfect world rather like our own earned Galileo the ire of the Church and he was forced to recant under threat of execution (“a prisoner to the Inquisition”, as John Milton put it in Areopagitica). After all, his discovery took place a mere decade after the Italian philosopher Giordano Bruno was burned at the stake for his heretical beliefs – including the idea that there are many populated planets throughout the universe.
Once we accept the fact that the Earth and Moon have many similarities, it’s logical to think that there must therefore be lunar life that’s similar to terrestrial life. The Moon is the only object in the sky that we can see in any kind of detail with the naked eye; with the advent of the telescope it was possible to make ever more detailed maps, many of which relied heavily on the interpretation and prejudices of the astronomer concerned. We can see an irresistible temptation to ‘humanise’ the Moon; to extrapolate from what was already known about the Earth. Philosophers and scientists post-Galileo had no hesitation in affirming that the Moon was inhabited. William Herschel (discoverer of the planet Uranus) claimed to see evidence of lunar vegetation through his telescope in 1780, and the German astronomer Johann Hieronymus Schröter (1745–1818) was convinced that colour changes observed on the Moon and Mars were due to seasonal cultivation and industrial activity undertaken by their respective citizens.
The line between imaginative speculation and scientific enquiry was undoubtedly a blurred one at this stage in history. But as technology improved and serious astronomers became, for the most part, convinced that life on the Moon could not be possible, such speculation moved into the realms of science fiction. And, once the Apollo 11 astronauts set foot on the arid, airless Moon in July 1969, it was clear that science fiction was just that – fiction.
Moonshot
 Is the Moon still a source of artistic inspiration? Is it even relevant now that we’ve visited and found it lifeless? I’d say yes, unquestionably. The Moon still occupies a place at the heart of questions about politics and the environment. Modern-day debates about drilling in the Arctic, about whether a pristine wilderness should be sacrificed to the interests of big business and our ever-increasing need for energy, could one day serve as a test case for the Moon. The prospect of visiting space for commercial, rather than scientific, reasons (for example, tourism or mining) has been explored in films like Duncan Jones’s Moon (2009) and is fast becoming a reality. The idea of colonising the Moon has never really gone away – in the run-up to the 2012 US Presidential Election, Republican hopeful Newt Gingrich spoke of his ambition to have a US colony on the Moon by 2020. All of which leads to the question: who owns the Moon? The 1967 Outer Space Treaty and 1984 international Moon Treaty go some way towards answering this; they state that outer space is regarded in law as “global commons”, beyond any territorial or sovereign claim by any nation. As technology advances, though, it will be interesting to see how the law, and political art, evolves with it.
The Moon looms large in projects at the interface of the arts and the sciences, such as the Moon Arts project underway at Carnegie Mellon University (http://moonarts.org/), which aims to create a Cultural Heritage site on the Moon. It’s not the first time that art has been sent into space, either – both Blur and Damien Hirst contributed works to the ill-fated Beagle 2 mission, which lost contact with the Earth after landing on Mars in 2003. The Voyager 1 probe, launched in 1977 and now travelling beyond the Solar System, has on board a ‘golden record’ with samples of music from across the globe. The famous plaques on the Pioneer 10 and 11 probes, showing a graphic representation of human beings and a ‘map’ of our location in the galaxy, is a design icon. Interdisciplinary projects like these have at their heart the belief that the boundaries between art and science aren’t, after all, that rigid. Wild imagination and boundless curiosity are as important as rocket boosters and life support modules in taking humans beyond the limits of our own atmosphere.
Moonage Daydream
For all our scientific knowledge, the Moon still represents a mystery. Although it’s our nearest neighbour, by far the best-studied of all celestial objects and an integral part of the human psyche, it is simultaneously an utterly remote, hostile and alien world that is inaccessible to all but a select handful of humans; those with “the right stuff”. It occupies a liminal place between science and the imagination, utterly familiar and yet utterly mysterious, of our world and yet not of it. The human fascination with the Moon is unlikely to end any time soon.

Earthrise, one of the first images of Earth taken from beyond the Moon, by the Lunar Orbiter I probe, 1966. Courtesy of the UCL Faculty of Mathematical and Physical Sciences

Earthrise, one of the first images of Earth
taken from beyond the Moon, by the Lunar Orbiter
I probe, 1966. Courtesy of the UCL Faculty of
Mathematical and Physical Sciences

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