In February 1919 two teams of astronomers from the Greenwich and Cambridge observatories set out for Sobral, Brazil, and Príncipe (an island off the coast of Africa), respectively, with sophisticated equipment that would allow them to photograph a solar eclipse as it cut across South America, the Atlantic Ocean, and Africa on May 29. The purpose of the expeditions, arranged by Frank Dyson of the Royal Greenwich Observatory and Arthur Eddington of Cambridge University, was to test Albert Einstein’s theory of general relativity, which had been published in 1915 and was still regarded with skepticism by many scientists.
The eclipse presented a rare chance to verify one of the essential consequences of general relativity, the bending of light by gravity. Einstein’s theory predicted that rays of light passing near a massive body in space would be visibly bent as they followed the curve in space-time created by the body’s mass. In the case of a ray of light originating from a distant star and passing near the edge of the Sun, Einstein calculated a deflection of about 1.75 arc seconds.
Under normal conditions, Einstein’s prediction was impossible to test, for the simple reason that sunlight drowns out the light from nearby stars, making them invisible to observers on Earth. The darkness of an eclipse, though, would allow the astronomers to observe and photograph the field of stars around the Sun. By comparing the photographs with reference images taken at night, it would be possible to measure how much the presence of the Sun had bent the stars’ light. Conveniently, a cluster of bright stars known as the Hyades would appear near the Sun during the eclipse.
On the day of the eclipse the Príncipe team struggled with intermittently cloudy skies, and the team in Brazil had to resort to a lower-quality backup telescope when the photographs from the main telescope were out of focus. Both teams, though, were ultimately able to produce images.
After several months of analysis, Eddington and Dyson announced in November that their findings supported the theory of general relativity. The media reported the news breathlessly. The Times of London announced “Revolution in Science. New Theory of the Universe. Newtonian Ideas Overthrown.” “Lights All Askew in the Heavens,” blared The New York Times. The announcement immediately elevated Einstein, a moderately well-known physicist, to international celebrity. Media coverage tended to dwell on the recondite nature of Einstein’s work, emphasizing that there were only a handful of people in the world who could understand it. It could be argued that 1919 was the year when Einstein’s name became a byword for superhuman intellectual ability—making possible the small industry of Einstein-themed merchandise that still exists today. He embarked on the first of several world tours in 1921.