Eclipse
I sat in a field and watched the Sun disappear. It was 11 August 1999 and I’d been waiting 30 years to see that day’s total solar eclipse – ever since realising as a small boy that it would be the only one visible from Britain during my entire lifetime. I’d originally intended to head for Cornwall, where the 50-mile-wide eclipse track clipped the south-west corner of England, but then I had a better idea. The whole of Europe was bisected by the path of the Moon’s shadow and clear skies with clear roads were much more likely on the mainland. The discovery that a friend’s mother lived just off the eclipse track, in the German village of Schwabniederhofen, settled matters and I invited myself and five companions to stay for the week. I should add that our gracious hostess seemed delighted to have us.
However, as I sat in that field on the morning of the eclipse, Bavaria didn’t seem such a clever idea after all. The solar disc had not disappeared behind the Moon, but, instead, had been obscured by a thick bank of cloud that had blown in from the west. Our only hope of seeing the eclipse lay to the east where the skies remained clear, and so we climbed back into our cars and weaved along German country lanes while trying to keep the Sun ahead of the advancing cloud-front and ourselves close to the centre of the eclipse path. Several hours later and with just seconds to go before totality, we pulled into a crowded field near the Austrian border and scrambled out of our vehicles. It was worth all the effort. The totally eclipsed Sun was an awe-inspiring sight with the jet-black silhouette of the Moon contrasted against the blue-white halo of the Sun’s atmosphere and the pink, cloud-like solar prominences glowing within it. It wasn’t just the eclipsed Sun that impressed. On the horizon in front of me, as I faced the Sun, I could see distant Alpine mountaintops glistening brightly in the sunshine beyond the southern edge of the Moon’s shadow, while behind, eclipse-darkened clouds falsely threatened a thunderstorm of unimaginable violence. After one minute of the two-minute eclipse, the weather front we’d been outrunning for the previous two hours finally claimed victory as its clouds covered the still eclipsed Sun. No matter, we’d seen what we’d come to see and our motorised pursuit of an ever-shrinking patch of blue sky is now as precious a memory to me as that of the eclipse itself. That was my first-ever eclipse but I hope it won’t be my last; especially as I forgot to give my wife a go with the binoculars. Perhaps I should take a spare pair next time.
Eclipses are undoubtedly among the most beautiful of nature’s spectacles but many people, including me, are also struck by their sheer implausibility – the fact that eclipses occur only because the Sun and Moon happen to look almost exactly the same size. The Sun is really 400 times wider and 400 times further away but that doesn’t lessen the impression that something very odd is going on. Nagging feelings are not usually a good basis for solid science, and most experts dismiss eclipses as a meaningless, although scientifically useful, coincidence. But there is another way to look at it. Perhaps a Moon big enough to obscure the Sun might make our planet better suited for life than it would otherwise be, and if this is the case, planets that by chance happen to experience eclipses are more likely to give rise to intelligent life than planets that do not. At first this seems an insane suggestion. How could a complex and beautiful biosphere like the Earth’s possibly be more likely on worlds that have eclipses, phenomena that occur, at any given place on the Earth’s surface, only two or three times in a thousand years? However, the bigger a moon looks, the stronger the tides it generates, and, with tides, we do have something that directly affects life. A moon is not needed for tides, since the Sun also produces them, but tides are significantly increased in strength if a planet has a sizeable moon too. Tidal forces are doubled on a planet with a moon that looks 80 per cent as large as the Sun and tripled if, as happens on Earth, the Sun and Moon appear equal in size. Critically, as I discussed when looking at the causes of ice ages earlier in this book, tidal forces indirectly affect our climate by controlling how fast and how much the tilt of the Earth’s axis changes through time. Eclipses could therefore indirectly contribute to the life-friendliness of our planet if worlds that happen to have big-looking moons, and therefore large tides, also have better weather.
This idea that there may be a direct link between our large Moon and the habitability of our world was irresistible to me. I have always been fascinated by our satellite. Whether by day or by night, whatever phase she shows and whether she sits red and bloated on the horizon or shines her silvery light from high in the sky, the sight of our Moon never fails to lift my spirits. She has also played a concrete role in shaping my career. Like many people of my age, I developed a lifelong interest in science and technology as a direct result of the Apollo programme that took men to the Moon for the first (and so far only) time in the late 1960s and early 70s. I vividly remember sneaking out of bed, as a small boy, to listen to the live broadcast from Apollo 11 as it landed. Forty-four years later I retain a crystal clear memory of the transistor radio, with its plastic red and silver buttons and silver speaker grille, which I pressed against my ear to hear Neil Armstrong announce from the lunar surface that ‘the Eagle has landed’. Later, as a teenager, I built my own telescopes in the back garden and had the love–hate relationship with the Moon that many stargazers have. It’s a beautiful alien world and the only one whose surface can be seen clearly through small telescopes, but on the other hand, it always seems to be full and bright when you want to catch a fleeting glimpse of a faint comet or count the shooting stars in a meteor shower. As I grew older my scientific career drifted away from astronomy and into the related and equally fascinating realm of geology, and so I stopped looking up and started to look down. Decades later still, after reading Barrow and Tipler’s The Cosmological Anthropic Principle, I started to think about whether the Earth might have unusual, life-enabling characteristics. It was clear to me that looking at the Moon was the best way to test that intriguing idea. Was our large Moon an example of a rare property that helps make our world highly habitable? Is it part of what makes the Earth special?
It may seem surprising that the Moon could provide the best evidence of the Earth’s life-friendliness when other factors, such as biological evolution, have had a much more direct and significant impact on our planet’s developing environment. There are several reasons why the Moon tells a more convincing story of our good fortune than many other, apparently more promising, facets of our world. For a start, the behaviour of the Earth–Moon system is a reasonably well understood one, controlled by the relatively simple equations of celestial mechanics. I say ‘relatively simple’, because the details are still a bit of a nightmare. Isaac Newton himself complained that thinking about the motions of the Moon made his head ache! Nevertheless, unlike climate evolution or the evolution of animals and plants, the changing behaviour of our satellite through time can be mathematically modelled with reasonable precision. In the jargon of these things, modelling of the Earth–Moon system is ‘tractable’, and unlike poor old Newton who only had his towering genius to rely on, we at least have electronic computers to help with the number crunching. There is yet another reason why the Moon is a good target for study when investigating the life-friendliness of our world. If we have an Earth–Moon system whose characteristics happen to make the Earth particularly habitable, then this can be used to distinguish between the three scientifically respectable explanations for our good fortune in living on a planet as benign as Earth. Let me recap these in the light of the material covered in the preceding chapters.
Firstly, it could be that highly habitable worlds are fairly common because mechanisms, such as volcanic weathering and limestone deposition or the possible Gaian properties of a complex biosphere, emerge naturally from the physics of a life-friendly universe. Thus, we shouldn’t be surprised by the Earth’s suitability for life because the laws of the Universe guarantee the existence of many such worlds. Alternatively, it may be that life is extraordinarily adaptable and will thrive under a wide range of conditions. Thus, we shouldn’t be surprised that the Earth fits life because, in fact, life has adapted to fit the Earth. Finally, perhaps well-regulated planets occur only very rarely and purely by chance, but because the huge size of our Universe allows many attempts at constructing even the most peculiar of worlds, such places are still inevitable. We shouldn’t then be surprised by our good fortune in living on one of these oddballs, because we must find ourselves inhabiting one of the lucky worlds that had the billions of years of good conditions necessary to produce a complex biosphere and, ultimately, intelligent observers.
These three possibilities are not mutually exclusive. The evidence that our Universe is surprisingly life-friendly and that life is remarkably adaptable is compelling. But I don’t think that is the whole story. Luck must play an important role, too. For one thing, it’s not necessary for highly habitable worlds to be common in the Universe from the point of view of explaining our existence. Even if only one planet in a trillion is habitable there will still be an unimaginably large number of such worlds in the Universe. I don’t think there is any evidence at all to suggest that life-friendly worlds are substantially more common than this, and so, in my view, imaginatively populating our small corner of one galaxy with hundreds of advanced civilisations is just wishful thinking. The scientifically conservative position should be that life is rare and intelligence even more so.
However, more evidence is needed to substantiate this pessimistic view, and that’s where the Moon comes in. Only anthropic selection, the unavoidable observational bias that occurs because we must live on a habitable world even if that requires some really odd features, can explain a benign Moon. If properties of the Moon, such as its size and distance, turn out to be fine-tuned to make the Earth more habitable, this cannot be the result of feedback or of Gaia; there are no mechanisms to move the Moon or modify its size to compensate for poor environmental conditions at the Earth’s surface. Lunar fine-tuning also cannot be the result of natural selection, because the Moon is obviously not a living organism. An Earth–Moon system with particularly good biosphere-enhancing properties cannot even be the result of living in a life-friendly universe, because moons with other sizes or separations are entirely possible. We can see this just by looking around our own solar system. If the Earth–Moon system is fine-tuned for life, with the size of the Moon, for example, being ‘just right’, then at least one of Earth’s chance peculiarities is a necessary precondition for the emergence of intelligent life.
With this analysis I had found a good reason for me, as a geoscientist, to investigate the Moon, and it felt like coming home. I’m probably a bit of a Moon bore, if truth be told, and I’m often astonished by friends’ misconceptions. Many people, for example, believe that the Moon comes out only at night when in reality it’s up in the daytime just as much as it is at night. I’m also easily irritated by minor Moon mistakes in novels. One book I’ve been trying to read recently has a half-moon rising at twilight. This is simply not possible. If it rises around sunset, the Moon must be nearly opposite the Sun and will therefore be full. I’ve been struggling to take the rest of the book seriously ever since, which is a pity as the mistake happens on page five. Television is often not much better. The Moon seen through the bedroom window of Peppa Pig, my daughter’s favourite cartoon character, is always a crescent illuminated on the left-hand side. But a crescent Moon in the evening is lit from the west, which will be on the right as seen from the northern hemisphere. Peppa therefore either lives in the southern hemisphere, despite the Pig family’s strong English accents, or she has an unreasonably late bedtime for such a young pig. These are trivial issues, of course, and it is childish of me to be annoyed by them, but they do illustrate how rarely modern people look at the sky. Our ancestors would never have made such mistakes.
Recently, I’ve found myself a whole new lunar misconception to correct, and this is one propagated by professional astronomers rather than cartoonists or novelists. It is widely believed by experts that the strong tidal forces generated by our large Moon help to stabilise the Earth’s axis. This fits nicely with the ‘eclipse coincidence’ with which I began this chapter. If a large Moon is needed to prevent the climatic chaos that would result from a wildly tumbling Earth, then it is no longer surprising that we inhabit a world where the Moon is both large enough to stabilise our axis and, as an entirely fortuitous side-effect, large enough to cause eclipses. However, the idea that large moons stabilise the axes of their parent worlds is simply wrong. The next chapter looks at why.