How a total solar eclipse in 1919 left physicists ‘more or less agog’

The following is an extract from our monthly Launchpad newsletter, in which resident space expert Leah Crane journeys through the solar system and beyond. You can sign up for Launchpad for free here.

It was 1919 when the moon did a perfectly natural thing – blocked our view of the sun – and changed our understanding of the universe forever. Astronomer Arthur Eddington was watching from the African island of Príncipe, observing the positions of stars and planets that became visible during the eerie daylight darkness. With most of the sun’s light dimmed, he was able to see how light from distant stars warped as it was deflected by our sun’s gravitational pull, an effect called gravitational lensing.

He confirmed his sightings with those of another expedition in Brazil, and these observations offered some of the first proof for Albert Einstein’s relatively new theory of general relativity. This description of how massive objects warp the fabric of space-time is now considered foundational, but at the time it was a revelation. It changed everything about how we think about gravity and the cosmos.

It also resulted in my favourite newspaper headline of all time, published in The New York Times later that year: “LIGHTS ALL ASKEW IN THE HEAVENS; Men of science more or less agog over results of eclipse observations. EINSTEIN THEORY TRIUMPHS Stars not where they seemed or were calculated to be, but nobody need worry.”

“Nobody need worry” might seem a bit over the top, but watching a total solar eclipse can indeed make you feel inexplicably nervous. I saw my first one in 2017. It was absolutely unforgettable. You might think that an eclipse is just like an overcast day with a cloud drifting in front of the sun – after all, what’s happening is simply the moon passing in front of the sun and casting a shadow on Earth – but it’s astonishingly different.

The first thing you’ll notice during a total eclipse is the shadow of the moon rushing over the ground towards you at speeds in excess of 2400 kilometres per hour. The area of shadow for April’s eclipse will be about 185 kilometres wide, but this can change slightly based on the exact orientations of the sun and moon. As the shadow grows near, the moon appears to take a bite out of the sun, and there’s a strange quality to the light, as if a fog has fallen.

Then, suddenly, it goes dark. This is totality. Temperatures drop by up to 10 degrees. The only light comes from the sun’s outermost layer, called its corona, which ripples beyond the silhouette of the moon. It becomes so dark that some stars are visible in the sky. Many animals, including birds and insects, understandably seem to think that it’s nighttime, so the otherworldly twilight goes quiet except for the chirping of nocturnal insects that have awoken. I can’t say how you will feel, but for me it was a mix of awe and a strange, primal terror – the sun disappeared, and while my mind knew why, my body panicked at its loss.

Solar Eclipse 2024

On 8 April a total solar eclipse will pass over Mexico, the US and Canada. Our special series is covering everything you need to know, from how and when to see it to some of the weirdest eclipse experiences in history.

This seems to be a fairly common reaction, and not only in humans. Researchers studying animals during past total eclipses have found that while some simply went about their evening routines early, many of them showed signs of anxiety, running aimlessly or huddling together during totality.

Then, after just a few minutes, totality recedes just as quickly as it arrived. The shadow rushes away, the sun comes back out, and the birds and insects resume their chirping. The astronomers look up from their solar telescopes, groggy but excited at the treasure trove of data they’ve gathered.

Over the thousands of years humans have been observing solar eclipses, we’ve learned some pretty fascinating things. With the disc of the sun covered by the moon, its faint corona becomes visible, making an eclipse the perfect time to study the outer reaches of the sun. For example, scientists first discovered helium during a total solar eclipse. Eclipses are also the best times to observe the plumes of radiation and matter emanating from the surface of the sun through the corona. The corona itself is quite strange, and there’s plenty left to unravel about how it works – despite being far from the sun’s central fusion, the corona is millions of degrees hotter than the sun’s surface, and we still don’t know why.

Even if you’re not studying the sun’s mysterious layers, seeing a total solar eclipse is completely worth it. Those newspaper editors had it right more than a century ago: it’ll leave you more or less agog.