A daring 1919 eclipse expedition across two continents proved Einstein right and changed our understanding of gravity forever.
The Eddington experiment was organized by British astronomers Frank Watson Dyson and Arthur Stanley Eddington to test Einstein's general theory of relativity during the total solar eclipse of May 29, 1919. Two expeditions were sent — one to the island of Príncipe off West Africa and another to Sobral, Brazil.
Einstein initially predicted in 1911 that starlight would deflect 0.83 arcseconds near the Sun — the same as Newtonian gravity predicted. Luckily for him, bad weather foiled earlier eclipse attempts, and by 1915 he corrected his prediction to 1.75 arcseconds, exactly double the Newtonian value.
Multiple earlier attempts to test Einstein's prediction were thwarted by fate. In 1912, torrential rains ruined observations in Brazil. In 1914, World War I broke out just as three expeditions arrived in Crimea — German astronomers were captured by Russians, and clouds blocked the remaining teams.
Eddington was nearly drafted into WWI in 1917, but Astronomer Royal Frank Dyson argued he should be exempted to lead the eclipse expedition. As a Quaker conscientious objector, Eddington was granted a twelve-month extension — though the war ended before it mattered.
Dyson strategically chose the 1919 eclipse because the Sun would pass in front of the Hyades, an unusually bright star cluster. Brighter stars made it far easier to detect the tiny positional shifts caused by the Sun's gravitational bending of light.
On eclipse day at Príncipe, a morning storm with heavy rain nearly ruined everything. Eddington took 16 photographs largely on faith, barely glancing at the eclipse himself. On June 3, he wrote in his notebook: 'one plate I measured gave a result agreeing with Einstein.'
At Sobral, clouds initially obscured the sky but cleared just before the 5-minute-13-second eclipse. The main 13-inch telescope produced blurred images that were discarded, while a smaller 4-inch backup telescope captured the clearest images showing deflection slightly above Einstein's prediction.
The results were presented at a joint meeting of the Royal Society and Royal Astronomical Society in London. Widespread newspaper coverage catapulted Einstein from an obscure physicist to a worldwide celebrity almost overnight.
Einstein later credited Brazil's skies for confirming his theory, telling the Brazilian press in 1925: 'The problem conceived by my brain was solved by the bright Brazilian sky.' The Sobral observation site is now home to the Eclipse Museum.