Solar eclipses — analysis

by Tamara Kachelmeier and Biodun Iginla, Science News Analysts, The Economist Intelligence Unit, New York

Once portents of terror, eclipses are now an excuse for a party

This month’s eclipse is a cultural as much as a scientific event

Print edition | Briefing

Aug 21st, 2017

FEW things in life are certain, but eclipses are among them. If you are on the coast of Oregon at 09:06 local time on August 21st, and if the sky is clear, you will see the sun’s disk start to develop a small, black dimple. Over the next hour this incursion will grow until, at 10:19, it will engulf the disk completely. A strange not-quite-night will descend, illuminated by the ghostly glow of the solar corona surrounding what looks, for all the world, like a hole punched through the sky into the cosmic blackness beyond. Behind you, arrayed along a ribbon-like track about 100km wide stretching all the way to the Atlantic, millions will await their own moment of witness.
As total eclipses go, that of August 21st — the first to be visible from the continental United States since 1979 — will be a short one. Even those standing at the most favoured spot, which is in southern Illinois, will enjoy just 160 seconds of totality, about a third of the maximum possible. A little under three minutes of darkness may not sound such a big deal. But many who have experienced a totality will tell you that it is the most awe-inspiring sight you will ever see in your life.


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In the past, this awe was often manifested as terror. Twenty-six centuries ago the hosts of Lydia and Media were busy butchering each other in what is now Turkey when an eclipse darkened the sky. Both armies threw down their weapons and called the whole thing off. Ancient cultures blamed wolves, dogs or dragons for trying to eat the sun. People would bang pots and pans together, or fire arrows at the darkening orb, trying to scare the offending beasts away. Eclipses were omens usually, auspices occasionally, but always signs of great import. In 1133 an eclipse marked King Henry I of England’s departure for France; he was to die there two years later, having never returned to his kingdom. The logical conclusion, at least for William of Malmesbury, a historian of the time, was that “the elements themselves manifested their sorrow at this great man’s last departure.”
This month’s eclipse, though, will mostly be an excuse for a party and a bit of amateur astronomy. Around 12m people live directly in its path. Millions more are expected to travel there. Where ancients worried about poor harvests or the death of the sun god, moderns will worry about car-parking spaces and whether any cheap hotel rooms are left.
Scientists will be interested, too, though not as much as once they were. In 1919 observations of an eclipse shook the world of physics. Albert Einstein’s then-new theory of general relativity described how gravity could bend light, and thus how the positions of stars would seem to shift when they were close to the sun in the sky. In normal circumstances, observing stars close to the sun is impossible. But the darkness of the eclipse let the astronomers check the apparent positions of stars near the sun against measurements they had made at night, a few months earlier. On examination, the stars were held to have moved as Einstein had said they should.
Modern eclipses, sadly, do not offer such paradigm-shifting possibilities. Today’s astronomers are far more interested in the almost imperceptible obscurations of other stars as planets pass in front of them — a crucial tool for discovering and studying such “exoplanets” — than they are by the visual drama of the moon and sun. That said, some professionals will be taking an interest. Eclipses offer opportunities for telescopes on the ground and mounted in aircraft to study the sun’s corona — which, for reasons still not properly understood, is getting on for a thousand times hotter than its brilliant surface.

Every now and then

If eclipses have become the epitome of predictability, though, they still reveal something about the capriciousness of the universe. The very fact that it is possible to see a total eclipse at all is a happy accident of time and space.

Crowning glory

It is an accident of space because the distances from the Earth to the sun and moon are such that, seen from Earth, the little, nearby moon and the vast, distant sun look as wide as one another. The former can thus just cover the latter. This correspondence is not always exact. When the moon is at its farthest from Earth (its orbit is not circular) it does not take up quite enough of the sky to obscure all the solar disk. The result is an annular eclipse, in which a thin ring of the sun’s surface surrounds the blackness created by the moon.
It is an accident of time because the moon has slowly been receding from Earth ever since its creation 4.5bn years ago. To start with, it would have blotted out the corona as well as the disk, robbing eclipses of their silvery beauty. Millions of years hence, when it has receded farther still, it will never come close enough to create a total eclipse, and any inhabitants Earth then has will have to make do with annular ones. Human beings are lucky enough to live in the sweet spot in between.
In a further stroke of good fortune, nature has conspired to make eclipses rare enough to be noteworthy but common enough that a motivated individual can see plenty in a lifetime. If the moon’s orbit around Earth were circular, and in the same plane as Earth’s around the sun, then eclipses would be monthly events. In fact, the lunar orbit is elliptical and tilted. The result is that, on average, two total eclipses are seen from Earth every three years. Each is, however, visible from only a tiny fraction of the planet’s surface — its own equivalent of the narrow ribbon laid across America this month. At a random point on Earth’s surface you can expect to hang about for more than 300 years before seeing a total eclipse.
This mix of exoticism, predictability and rarity leads some people to devote their lives to seeing as many eclipses as possible. Francisco Diego, of University College, London, is one of their number. He has observed 20 in his time, he says, in locations as far apart as Easter Island, one of the most isolated inhabited places on Earth, and the south coast of Cornwall, which is merely the most isolated place in England. Dr Diego is a professional astronomer, but many eclipse-chasers are smitten amateurs. (The Economist’s science editor was also on Easter Island when the eclipse happened in 2010; in 2003 he made his way to Antarctica in search of a few minutes of horizon-grazing totality visible over the ice cap.)

A big part of the appeal, says Dr Diego, is the sheer drama of the event itself. “By the time the sun is around 70–80% obscured, you start to notice it’s getting dark.” Just before totality, “the light level really plummets. You can see the moon’s shadow rushing across the ground towards you.” Even if you know exactly what to expect, Dr Diego says, the experience can be frightening, for the shadow moves across Earth’s surface at several thousand kilometres an hour. “People can’t help themselves — they start shouting and screaming.” Animals get just as confused. He recalls that, during an eclipse-watch in Zimbabwe in 2001, the cicadas began to chirp as the sunlight faded and — so they thought — night fell. Hours later, with the shadows long passed and daylight restored, they were still going, presumably in a state of some confusion.
The moment of totality, according to Dr Diego, is its own phenomenon, similar to night-time but different, with the landscape bathed in the corona’s faint light. These days, when even professional astronomers — who mostly book time on distant telescopes from the comfort of their air-conditioned offices — can go for months without actually having to look at the night sky, such a display is a welcome reminder of the cosmic spectacles which lie behind the equations on the whiteboard.
Non-scientists can benefit from this sense of perspective, too. NASA has calculated the time, date, location and maximum duration of every eclipse between 1999BC and 3000AD. The results are available to all on its website. Whatever humans get up to, the celestial clockwork will tick on with supreme indifference. That may sound a depressing thought. But it can also be a reassuring one. As George Orwell put it, in a different age: “The atom bombs are piling up in the factories, the police are prowling through the cities, the lies are streaming from the loudspeakers, but the Earth is still going round the sun, and neither the dictators nor the bureaucrats, deeply as they disapprove of the process, are able to prevent it.”

Posted by Biodun Iginla at 3:25 PM

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Labels: Albert Einstein, astronomers, nasa, Oregon, science, Solar eclipses, southern Illinois, Tamara Kachelmeier and Biodun Iginla, the economist intelligence unit, time and space




Biodun is a political news analyst with major news organizations and the author of 57 books of fiction and nonfiction.

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Biodun Iginla

Biodun Iginla

Biodun is a political news analyst with major news organizations and the author of 57 books of fiction and nonfiction.

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