On a remote African island in 1919, photographs of a solar eclipse proved that gravity bends light. Turning Einstein from an obscure physicist into a global celebrity overnight.
ESO/Landessternwarte Heidelberg-Königstuhl/F. W. Dyson, A. S. Eddington, & C. Davidson, CC BY 4.0, via Wikimedia Commons
A test on a tropical island
In 1915, Albert Einstein published his theory of General Relativity, which proposed that gravity is a distortion of space and time. One of the theory's predictions was that light itself should bend when passing a massive object. The calculated deflection of starlight grazing the Sun was 1.75 arcseconds—a tiny angle, double the amount predicted by Newtonian physics. The theory was untestable under normal conditions because the Sun’s brightness makes nearby stars invisible. A total solar eclipse offered the only opportunity to see those stars.
The Astronomer Royal, Sir Frank Dyson, identified the eclipse of May 29, 1919, as a perfect chance for a test. The Sun would pass in front of the Hyades, a bright star cluster, providing clear points of reference. He organized two British expeditions to the path of totality: one to Sobral, Brazil, and another to the island of Príncipe, a Portuguese colony off the coast of West Africa.
The Cambridge Observatory's director, Arthur Eddington, led the Príncipe expedition. He was accompanied by Edwin Cottingham, a clockmaker and instrument specialist. They set up their equipment—a telescope mounted horizontally and fed by a coelostat mirror—at Roça Sundy, a large cocoa plantation on the island's northwest side. Their task was to take photographic plates of the stars around the Sun during the brief minutes of totality. By comparing these to reference plates of the same star field taken at night, they could measure any apparent shift in the stars' positions.
Two plates and a new universe
The day of the eclipse began with heavy rain and clouds. The sky only cleared moments before totality began. During the eclipse, which lasted just over six minutes, Eddington and Cottingham managed to expose 16 photographic plates. Most were ruined by cloud cover, but two plates captured a small number of stars clearly enough for analysis. Developing the plates on-site, Eddington measured the positions of the stars.
The photographic evidence showed the starlight had indeed shifted. The measurements from the two usable plates indicated a deflection of 1.61 arcseconds, with a margin of error of 0.30 arcseconds. This result was consistent with Einstein's prediction of 1.75 arcseconds and clearly inconsistent with the Newtonian prediction of 0.87 arcseconds. The data from the Sobral expedition in Brazil further corroborated the findings.
On November 6, 1919, the results were announced at a joint meeting of the Royal Society and the Royal Astronomical Society in London. The confirmation of General Relativity was a worldwide news event. It transformed Albert Einstein into a household name and changed our understanding of the universe. The site at Roça Sundy, once a plantation, is a site in the history of science.
💡Fun Facts
The expedition to Príncipe was a parallel effort; a second team of astronomers from the Royal Observatory, Greenwich observed the same eclipse from Sobral, Brazil.
Edwin Cottingham, Eddington's assistant on Príncipe, was an expert clockmaker responsible for the observatory's instruments, not a trained astronomer.
The successful verification of a German physicist's theory by British astronomers just after World War I was seen as a moment of post-war scientific healing.
The original photographic plates that Eddington took on Príncipe have been lost to history.