It is very difficult​ to describe what I witnessed on Monday, 8 April, standing in a field in Ohio a little after three in the afternoon. As the shadow of the Moon swept across the surrounding cornfields, engulfing the crowd that had gathered to watch the total solar eclipse, we were transported, briefly, to a place unlike anywhere else on Earth. The transition from partial to total eclipse is as sudden and shocking as a jump cut in a horror film. With even 99 per cent of the Sun covered, the landscape retains a familiar aspect. In totality, there is a black hole in the sky where our star should be, surrounded by a pearly white corona stretching away into a purple sky, an orange glow on the horizon.

A total eclipse happens when the new Moon comes between the Sun and the Earth, creating a syzygy, an alignment of three celestial bodies, and casting its shadow onto our planet’s surface. As the Moon moves in its orbit, the shadow moves rapidly along a track that may be only a few miles across. Observers within this path of totality see the Sun completely obscured. Those on either side see a partial eclipse, with the Moon covering only part of the Sun’s disc.

If the Moon orbited in the same plane as the Earth, there would be an eclipse every month. Instead, the Moon’s origins, more than four billion years ago, in a violent collision between a Mars-sized body sometimes called Theia and the still-forming Earth, placed its orbit at an angle which means that, most months, it passes unseen above or below the Sun in the daytime sky. Only when everything lines up precisely is there an eclipse, but even then totality is not certain. When the Moon is close to apogee, the point in its elliptical orbit furthest from the Earth, it doesn’t appear large enough to cover the whole Sun and you get an annular eclipse, with a ring of sunlight surrounding the lunar disc.

Predicting eclipses requires keeping track of the rhythms of the lunar and solar orbits. Ancient astronomers knew that an eclipse would almost always be followed by another, near identical eclipse, 223 lunar months (eighteen years, eleven days and eight hours) later, a period known as the saros. This year’s eclipse, for example, was preceded by one in 2006, which I saw from a beach in Turkey. The last American eclipse, in 2017, fell one saros period after the August 1999 eclipse that many in the UK remember for the cloudy Cornish weather that accompanied it. At any given time, around forty different saros series are in progress.

The use of these cycles to make eclipse predictions goes back at least to Babylonian observations in the sixth century bc, but the task of predicting exactly where on the Earth an eclipse will happen is much harder. As the Chinese astronomer Guo Shoujing wrote in the 13th century, ‘the test of an astronomical system’s exactitude is its treatment of eclipses. In this art of pacing the celestial motions, exactitude is hard to come by.’

Even once the route the shadow will take has been determined, the dedicated eclipse watcher has to deal with the far less predictable weather, as those watching from the West Country in 1999 found out. With totality lasting only a matter of minutes, even a single passing cloud can ruin everything. Some who travelled for this year’s event chose to chase clear skies, heading to Mexico or Texas; others scrutinised the constantly changing weather patterns before plumping at the last minute for Arkansas or the far eastern Canadian provinces.

There’s a baffling variety of weather models available. Even restricting myself, on the advice of my printed Road Atlas for the 2024 Solar Eclipse, to US and Canadian forecasts, I found myself clicking between different websites in the week before the eclipse, each of them detailed and convincing. I eventually decided to pick a spot and stick with it regardless of the outlook. I have a friend whose family home is in Farmersville, a hamlet twenty miles or so southwest of Dayton, Ohio, surrounded by stubble-strewn cornfields. The atmosphere the night before the eclipse was festive. Someone lit a bonfire (many of the visitors were camping). We drank cans of lager and watched lightning flash across the horizon. No one mentioned the thick clouds that had gathered overhead.

The next morning dawned with bright blue skies. The high cloud predicted by many of the weather models, due to roll in as the eclipse began, proved to be nothing more than a few scraps of cirrus. Excitement spread through the crowd, as we congratulated ourselves on having had the good sense to be in just the right spot on the Earth’s surface at just the right time.

The start of an eclipse is easy to miss. Our eyes are very good at adjusting to changes in light levels, and it’s possible to be under a 90 (or even 95) per cent eclipsed Sun and not realise. We watched the slow march of the Moon across the Sun’s disc through eclipse glasses, which block out all but a tiny fraction of light, and used colanders to project images of the crescent Sun onto the ground.

As the Sun’s disc narrowed, things became distinctly odd. The drop in temperature had been obvious for a while, but now colours, especially red and green, faded, giving the world a washed-out aspect (one of the pre-eclipse press releases that landed in my inbox warned that wearing bright colours wouldn’t help you stand out during totality). Shadows sharpened, and then the south-western horizon darkened and the umbral shadow was upon us.

The last sunlight shone through a valley on the Moon’s limb, producing a dazzling diamond-ring effect. Once this vanished, the corona – the Sun’s outer atmosphere – popped into brilliant view. For most of history, the corona could only be seen during a total solar eclipse. It is made up of plasma, heated to millions of degrees by the Sun’s powerful magnetic field and incredibly tenuous. Exactly how this happens was a mystery for many years, despite such efforts as an experimental Concorde flight in 1973 with a team of astronomers on board to chase the Moon’s shadow, extending their totality to an unprecedented 74 minutes. Now satellites carry instruments capable of making artificial eclipses; and last year Nasa’s Parker Solar Probe swooped just a little more than seven million kilometres above the visible surface of the Sun, travelling at nearly 400,000 miles per hour, faster than any other human-made object.

The complexity of the corona adds to its mysteries. Most photographs struggle to show the detail visible in the long streamers reaching away from the Sun, and pictures don’t capture the contrast between the bright white coronal light and the blackness of the central disk during totality. They do show the changing shape of the Sun’s atmosphere from eclipse to eclipse, however, reflecting the eleven-year cycle of solar activity that shows up in counts of sunspots and studies of flares.

Given the centrality of the corona to the experience of an eclipse today, its absence from descriptions written before the 18th century is striking. As late as 29 March 1652, discussion of a short totality that crossed the British Isles concentrated on the loss of the Sun; the day was known as Black Monday, or Mirk Monday in Scotland. Further back, the award for pithiest short description of an eclipse goes to the scribe who described an 1133 event as ‘miserabilis, horribilis, nigra, mirabilis’: wretched, horrifying, black, remarkable.

Many cultures have tried to ward off the bad luck associated with the Sun’s disappearance. Sometimes this was mere bureaucracy, as in the record of a petition to the Chinese emperor Wen to dismiss his prime minister after an eclipse in 221 ad (he refused). Others took more drastic action. The Assyrian ruler Esarhaddon responded to a report of a partial solar eclipse in 669 bc by installing a substitute king on his throne, to take on the evil from the event before being killed soon afterwards. The real king was sequestered in a modest house and referred to only as ‘the farmer’.

Eclipse warnings today tend to be limited to guidance on the dangers of staring at the Sun. Six inmates at Woodbourne Correctional Facility, New York, sued to be allowed to watch the eclipse (on religious grounds) rather than be confined inside for their safety. I’ve heard stories of school classes being kept inside as a precaution. Road signs warned of eclipse-related traffic, a modern curse. But millions of people were granted the blessing of clear skies. In Farmersville our view of the corona was complimented by a bright orange prominence, a loop of hot gas the size of several Earths reaching up from the Sun’s surface. Venus and Jupiter appeared on either side of the eclipsed star. Birds made what the ornithologist next to me said might well be evening calls, and a confused bat swooped past the crowd, enjoying the shortest night of its life.

Then, as suddenly as it started, totality was over. The reappearance of the diamond ring brought cheers, and we paid scant attention to the partial phases that ended the eclipse. The next total eclipse to cover Farmersville will be centuries distant; in the northern hemisphere, any particular spot encounters totality only once every 330 years.

Nowhere else in the Solar System sees such a spectacle. Mercury and Venus are moonless, and though cameras on Mars rovers have sent back images of its small, asteroidal moons, Phobos and Deimos, crossing the Sun’s face, they are too small to block it completely. The outer planets have plenty of moons, but see the Sun as a much smaller disc, easily covered. Despite a recent flood of discoveries that have revealed planets to be extremely common in the galaxy, we have yet to find convincing evidence for even a single moon beyond the Solar System.

Since the Moon stabilises the Earth’s axis, and hence its climate, and probably also reduces the number of asteroids that hit the planet, it has been argued that the presence of such a large satellite might be a prerequisite for life. If so, a scarcity of such moons might explain the apparent absence of abundant alien intelligence from the cosmos. My Oxford colleague Steven Balbus goes much further. The coincidence of geometry that makes the Sun and Moon roughly the same size in the sky also means that they both contribute to the tides we experience on Earth. The beating of the solar and lunar tidal cycles against each other brings the difference between more extreme spring tides and lesser neap tides. This means there are liminal places on Earth that are covered with water occasionally but dry most of the time: ideal habitats for an oft-stranded species of fish to learn to breathe air or take its first steps onto land. If this idea is correct, then we should look to planets that have moons capable of producing total stellar eclipses to find our fellow land-dwelling intelligences.

The tides cause friction which slows the Earth’s rotation, and causes the Moon to recede from us by nearly four centimetres a year: in about half a billion years’ time, the Moon’s shadow will touch down on Earth for the last time. Meanwhile, I’m looking for hotels at Keflavík Airport in Iceland, or in Majorca, under the track of the next total eclipse, on 12 August 2026.

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