Twenty-four: Light All Askew in the Heavens
The solar eclipses of 1919 and 1999 and their lessons
Arthur Eddington was that rare breed of scientist, one capable of conveying difficult concepts accurately with literary flair. (“Shuffling is the only thing which Nature cannot undo,” he wrote of entropy.) By popularizing astronomy in a time of spectacular discoveries, the quiet, diffident scholar followed a grand British tradition of science popularization begun by “Darwin’s Bulldog,” Thomas Henry Huxley.
Eddington’s fame is tied up with the legendary solar eclipse of May 29, 1919 - a story we’ll return to shortly, after I reminisce about the solar eclipse of 1999.
Last seen in Giggleswick
A solar eclipse occurs when the moon passes between the sun and Earth, casting a shadow on Earth that either fully or partially blocks the Sun's light in some parts of the Earth.
On August 11, 1999, there was a big one in the works for Europe. The moon’s shadow began it’s stately progress across the Atlantic Ocean and traversed the southern United Kingdom before moving on to northern France and parts of Western/Eastern Europe. I had arrived in Cambridge a week earlier, with a plan to motor with relatives toward the best viewing area, in Cornwall.
That’s when I discovered a UK road map is something of a Pythonesque document. Towns and villages include clerical skin conditions (Bishop’s Itchington), swamp apes (Marsh Gibbon), larval apocalypse (Maggot’s End), hits below the belt (Nether Wallop), and fashion suggestions (Matching Tye).
I once read somewhere that Brits consider it bad form to ask their place of birth, and now I think I understand. I’d be a bit taciturn myself if I spent my formative years in Mousehole or Ratney. But I digress.
The previous solar eclipse in Britain took place in 1927. (I noted with satisfaction that viewing was said to be best in Giggleswick.) This time around the British media was in a lather about this impending sky show. Lacking a believable Apocalypse in the ancient, moon-eating-the-sun sense, they went with…traffic. Supposedly every other Brit from Shellow Bowels to Puddletown was panning to squeeze into their dinky vehicles and head for a little corner in southwest England, where complete darkness from the moon’s shadow was to fall: the “path of totality”.
The traffic chaos would unimaginable, reporters predicted, though they did their best to imagine it.
The predicted chaos from a mass migration of British eclipse-hunters failed to pan out. The broadcast Cassandras had the reverse effect: the anticipated lemming-like rush for the southwest coast resulted in mass avoidance.
The overcast skies helped greatly, nixing a last-minute exodus of Peugeots and Volvos. That was my cue. and I hopped into a rental car with relatives and we made from Cambridge to Cornwall on the morning of the 11th, hoping for a sight last granted to the gobsmacked residents of Giggleswick.
Back to 1919
Arthur Eddington was born in 1882 in Kendal, Cumbria. His father, headmaster of a Quaker school, died of typhus in 1884 when Arthur was only two. The widowed mother managed to raise her son and daughter on a small income. Luckily, at the age of sixteen, young Eddington won a scholarship to the college that would become the University of Manchester.
In 1913 the Eddington landed a professorship in astronomy at Cambridge. He continued his studies at Cambridge, followed by employment at the Royal Observatory at Greenwich. The Great War, commencing in July of 1914, silenced communications between the scientific communities of England and Germany. The young astronomer only knew of relativity theory through a smuggled copy of Einstein’s paper.
Einstein had a thing for unification. In a 1915 paper, the frizzy-haired physicist spot-welded gravity and inertia into one geometrical field, warped by the presence of matter-energy. (As I mentioned here, cosmologist John Wheeler succinctly described the generalized version of relativity: “Matter tells space-time how to curve, and curved space tells matter how to move.”)
Initially, Einstein’s radical ideas were entirely theoretical, and unsupported by scientific evidence. It was Eddington’s colleague, Frank Watson Dyson, Astronomer Royal of Britain, who first conceived a test for Einstein’s theory, which predicts that light is bent in the presence of gravitational fields. It’s twice as much as Newtonian physics predicts, but the amount either way is very small.
Dyson’s brainstorm was this: the sun is an extremely massive body, and rays of light skirting the edge of the sun on the way to Earth would bend by a measurable amount. This is impossible to measure in normal circumstances because during the blinding disc of the daytime sun blots out all starlight. An entirely different situation is presented during a solar eclipse, when the disc of the sun is blocked by the moon in transit, and the stars briefly come into view when there’s a clear sky.
Astronomers can calculate eclipses many years in advance, and Dyson had determined that on May 29, 1919, a total solar eclipse would be viewable throughout South America, across the Pacific, and into Africa. The area of totality — the region where the moon’s blockage of sunlight is complete — would travel across the remote island of Príncipe, in the Gulf of Guinea off the west coast of Africa.
The stars that are normally invisible would shine in the brief darkness of the eclipse, and those closest to the disc of the sun would have measurably altered positions in the sky, due to the predicted bending of starlight.
Critically, it was the longest solar eclipse since May 27, 1416, and it would occur just as the sun was crossing the bright Hyades star cluster: perfect for measuring bent light. (It’s one of those happy accidents in science that as viewed from Earth’s surface, the moon’s disc precisely blocks the disc of the sun, no more and no less than 100 percent. This allows witnesses to see the solar corona, a hot gas of filaments rising from the sun’s surface.)
Fast forward to 1999
Just outside of Cornwall, we found a spot just inside the path of totality and parked the car. Making our way up a rise in Greater Buttocks (or wherever we were), we discovered a storybook setting of rolling hills and thatched cottages. Grey skies as predicted, but several times the disc of the sun poked through the cloud cover, with the moon gnawing into its side. Feeble sunlight from the crescent sun resulted in a surprising drop in temperature. We shivered in our t-shirts.
But disappointment was not in the cards. It turned out the clouds acted as a projection screen for the shadow of the moon. A huge dark shape, dozens of miles across, appeared on the western horizon. The disc slowly moved above us like an alien mothership, and in seconds everything went from twilight to night, as if on a dimmer switch. Swallows emerged as lights in the hamlet below blinked on.
After about a minute of not-quite total darkness, the process played in reverse, but to the opposite horizon. Seagulls screamed their morning cries, with a particularly crazed edge. The cows looked stunned — pretty much unchanged, in other words. I felt a little stunned myself. It was one of the most uncanny, spectacular things I’ve ever witnessed, with a weird hint of menace. Two teenage girls were watching behind us, and one burst into tears.
You can imagine the power wielded by those priest-kings of ancient empires with astronomers capable of predicting solar eclipses. It would be like “take all our our women, anything! Just don’t let the lights go out again in Swineshead, that scared us silly!”
Solar eclipses don’t require scientific equipment and charts to amaze; protective viewing glasses are all that’s required. I was just inside the path of totality on a cloudy day in 1999, so I didn’t witness the event in its full power. But I’ll never forget my partial experience.
(Unfortunately, I didn’t get a chance to witness the solar eclipse of August 21, 2017, which tracked across the continental US. Yet I and millions of others were offered press photos of a US president squinting at the sun without protective eyewear — and that was amazing enough.)
Back to 1919
Eddington’s involvement in the 1919 eclipse mission was nearly nixed by political matters. Like his father, he was a lifelong Quaker. Representative beliefs of the faith, including pacifism and internationalism, worked their way into Eddington’s words and deeds. With Britain’s introduction of military conscription in March of 1916, the young astronomer followed his conscience and prepared to apply for an exemption as a conscientious objector.
Cambridge spared him confrontation with the state by requesting and receiving an exemption, on the grounds that Eddington’s work was in the national interest. The Ministry of National Service later appealed against the exemption, and Eddington claimed conscientious objector status before a tribunal in June of 1918. Frank Dyson intervened with a written statement insisting on Eddington’s importance for the astronomical expedition to Africa. The tribunal granted a further twelve months of exemption from military service under the condition Eddington pursue his astronomical work in preparation for the 1919 expedition. The war ended before Eddington’s time ran out.
Reading a smuggled paper from an enemy scientist? Objecting to military service? In some quarters today, Eddington would be pegged as a dupe, a terrorist sympathizer, or an agent for a hostile power.
Or at least fodder for an over-the-top BBC drama…
But I digress (again).
With the onset of the Great War on July 28, 1914, wartime production tied up all the factories capable of producing the heavy-duty astronomical equipment necessary for any eclipse-viewing expedition to Africa. The end of hostilities on November 1918 left only a few months for things to get underway. Orders were placed, and an assortment of large telescopes, astronomical devices and motorized reflective mirrors were duly cranked out for the mission.
With the 1919 solar eclipse reliably predicted down to the second, Cambridge made arrangements for Eddington and his team to travel to Principe. A second team, led by Andrew Crommelin and Charles Rundle Davidson of the Royal Greenwich Observatory, were sent to another target area on the path of totality, in Sobral, Brazil.
It turned out that view conditions were less than perfect in Principe, with the skies clearing only partly during the actual eclipse. The team in Sobral had a much better view. However, they later found to their disgust that all 19 images taken by their main telescope were out of focus. The instrument had been warped by tropical temperatures. Luckily, they had with them a smaller 10 inch telescope, which is said to have provided the best data from the joint expedition.
On the evening of November 6, 1919, Dyson, Eddington and Crommelin stood before a full house at the Royal Society in London, and revealed their findings: Einstein was right.
The news quickly went global. The New York Times offered this celebratory headline:
Four years after Einstein’s paper on general relativity, the validation by Eddington’s team signaled the transition from the stately Old World of Science, populated with billiard-ball like atoms and eternally ticking clocks, to a chancy New World populated with black holes, quasars, quarks, and things that go bump in the lab.
You know it was a good job there wasn't a mass exodus across country to view the eclipse, it could have tipped Great Britain (Gawd bless her) right over and into the sea. Then that girl stood behind you would have had something to cry about.