A reflection on the "Great American Eclipse"—and the beauty of an unexplained phenomenon

I waited 45 years for my first total solar eclipse before finally seeing it on 11 August 1999, in Cornwall. It was cloudy.

Solar eclipses are repeated every 18 years, 10 and 1/3 days, which brings us to 21 August 2017. The one-third is important: in that extra 8 hours the Earth spins eight time-zones. So instead of Cornwall, this time I headed for the western United States and the so-called “Great American Eclipse.” I decided to take my 7 years old grandson, Max, as he would be the same age as I was, long ago, when my teacher showed me a solar partial eclipse and inspired my lifelong interest in science.

After that first experience, I was hooked, and began the long wait for that first total eclipse in 1999. Since then I have seen five more, in exotic locations ranging from the Sahara Desert in Libya to the South Pacific Ocean. I have written about them in Prospect, and in anticipation of the Great American Eclipse of August 2017, told my story in Eclipse: Journeys to the Dark Side of the Moon. That tale concluded with our preparations as we set off for the USA. How would Max react to the experience which had meant so much to me all those years ago? I could never have anticipated what actually happened.

2000 meters up in Teton Village, Wyoming, the sky was clear. Everything was in place for us to witness the beauty of a natural coincidence. For that is precisely what eclipses are: the moon is 400 times nearer to us than the sun, and is also some 400 times smaller than it in diameter, which means that if it passes in direct line of sight, it will totally eclipse the bright solar disc.

It has not always been thus. The natural chronometer of eclipses is so precise that it is possible to calculate dates and times of past examples to the second. When Edmund Halley first did this in the 18^th century, he discovered a discrepancy with the written records—and the further back in time he went, the larger the mismatch became. The reason is that Halley had assumed the Moon follows the same elliptical orbit repetitively through the ages whereas, we now know, it is in fact gradually receding from the Earth. This is admittedly occurring very slowly—a few inches per year—but it was enough, nonetheless, for the long time-span of recorded eclipses to have revealed the fact. Today we have direct proof, thanks to mirrors placed on the Moon by Apollo astronauts. These reflect laser beams from Earth and enable the distance to be measured to precisions of better than a centimetre.

Extrapolate millions of years into the future and there will come a time when the then more distant moon will appear significantly smaller than now, and be unable to obscure the solar disc completely. So we live in a golden age for total eclipses—to mix metaphors.

Today, the moon is near enough that it can by chance overfill the sun slightly and, as on this occasion, take a couple of minutes to cross the sun’s face. We were ready and knew what to watch for: the moon’s shadow gathers like a distant storm in the west; the temperature falls as the sky turns steely grey stars appear, crickets start to chirp, animals go to sleep, and bears think night is coming (don’t go hiking in the Teton Mountains during the eclipse!) In the final seconds darkness descends as if someone has turned a dimmer switch. The last sliver of sun is obscured, the solar corona appears—a million times fainter than normal daylight—and as it surrounds the moon in silhouette, there is a flash of sunlight like a diamond ring. Then follows two minutes of totality. All this happened and it took our breath away. “Its so dark” exclaimed Max; adults cheered; some cried.

What I hadn’t anticipated was something that most eclipse watchers have never seen, and is perhaps the rarest and most ephemeral phenomena associated with an eclipse: “shadow bands.” With a minute yet to go before totality, Max suddenly started jumping up and down as if standing on red hot coals: the roadway appeared to be wobbling, as if surface waves from some earthquake were moving down the street at jogging pace. In fact, this was no earth tremor but one of the most tenuous, subtle and least understood optical phenomena in nature. Alternate bands of dark and light, so faint as to be barely visible in the deepening gloom, their width similar to an adult’s hand, their motion in the same direction as moon across the fast disappearing sun.

Many veteran eclipse chasers have never seen them and, spotting them on his first attempt, grandson Max was laughing nervously at this weird sensation. The alternate bands of lighter and darker grey gave the appearance of sand ripples after the tide recedes, but their motion away from us was disorienting.

As to their cause, I believe that no one really knows. Many websites assert that it is some optical interference phenomenon caused by the sudden cooling of the atmosphere as the moon’s shadow envelopes us. Layers of relatively warm and cool air act like lenses, so the hypothesis goes, leading to these bands similar to the bright patterns visible at the bottom of a swimming pool on a sunny day. The association with the final moments before (or immediately after) totality is because the sun is then a thin slit. This causes twinkling—like the stars—but in one direction only; in the perpendicular direction the sun is still extended, if only for a few more moments.

The problem for me is that this would give a random scatter of light and dark, as in the swimming pool for example, whereas what we saw was highly ordered, a regular coherent pattern moving as if to the beat of some unseen conductor. Another idea is that they are the result of an atmospheric shock wave. The moon’s shadow is traveling far above the speed of sound. The sudden cooling of the atmosphere thus spreads forward in supersonic fashion.

Quite how this leads to coherent shadow bands, however, is not clear. Indeed, the explanation of shadow bands is presently as obscure as the phenomenon itself. There are still everyday phenomena—or, rather, every-eighteen-months phenomena—which defy explanation.

Eclipse: Journeys to the Dark Side of the Moon is out now from Oxford University Press.