How it began
The British astronomers Frank Watson Dyson and Arthur Stanley Eddington organized two expeditions to observe the total solar eclipse of May 29, 1919. Eddington and Edwin Turner Cottingham, a clockmaker at the Cambridge Observatory, went to the island of Príncipe in the Gulf of Guinea, now part of São Tomé and Príncipe. Andrew Crommelin and Charles Davidson of the Greenwich Observatory went to Sobral in Brazil. Eddington sailed from England in March 1919 and had his equipment set up on Príncipe by mid-May. The discovery is shared between the two sites.
Before it existed
Einstein predicted in 1911 that starlight passing the edge of the Sun would be deflected, and after completing general relativity in 1915 he revised the figure to about 1.75 arcseconds for light grazing the Sun, different from the smaller Newtonian-based value. Earlier attempts to test this had failed. Three expeditions to the Crimea for the eclipse of August 1914 were disrupted by the outbreak of the First World War and by clouds, and American attempts at the 1918 eclipse were foiled by clouds and unsuitable equipment.
How it works
During a total eclipse the Moon blocks the Sun's glare, so stars close to the Sun's edge can be photographed. The positions of those stars on the eclipse plates are compared with photographs of the same star field taken at night, when the Sun is elsewhere. If the Sun's gravity bends the light, the stars appear slightly shifted. On Príncipe Eddington used an astrographic lens borrowed from the Radcliffe Observatory in Oxford. A storm with heavy rain cleared around noon, and the team took sixteen photographs through cloud, of which the last few showed usable star images.
Who pushed back
From the 1970s some scientists and historians questioned whether the 1919 measurements were good enough to support Einstein, or whether Eddington's analysis had been biased. The cloud-affected Príncipe plates were few, and historian Daniel Kennefick has argued that without the Sobral photographs the result would have been inconclusive. Modern reanalysis, including a 1979 remeasurement of the Sobral plates, suggests the original analysis was accurate.
What changed first
Eddington developed the plates on Príncipe and on June 3 noted that one plate he measured gave a result agreeing with Einstein. The combined results were presented at a joint meeting of the Royal Society and the Royal Astronomical Society on November 6, 1919, and published in the Philosophical Transactions in 1920. Press coverage in Britain, then in the United States, including The New York Times, marked the beginning of Einstein's international fame and led to a wave of popular science books on his theories.
Where it stands today
The centenary was marked in May 2019 with a scientific conference held on Príncipe, along with educational projects on the island and exhibitions in Lisbon and Rio de Janeiro. The meeting linked the 1919 measurement to current work in gravitational physics on black holes and gravitational waves.
