What 1919 solar eclipse expedition confirmed Einstein's theory?

The 1919 solar eclipse expedition that confirmed Einstein's theory was the Eddington expedition, also known as the Eddington experiment. It tested general relativity's prediction that gravity bends starlight passing near the Sun.

The expedition was organized by Britain's Astronomer Royal, Frank Watson Dyson, and astronomer Arthur Eddington. On 29 May 1919, observing teams travelled to two locations: Sobral in Brazil and the island of Príncipe off the west coast of Africa. During totality, the Moon blocked the Sun's bright disk, allowing the teams to photograph stars that appeared close to the Sun in the sky.

The photographs were compared with images of the same stars taken when the Sun was elsewhere. Their apparent positions shifted because the stars' light had been deflected by the Sun's gravity. General relativity predicted a deflection of about 1.75 arcseconds for light grazing the Sun, roughly twice the Newtonian prediction.

The results were presented to the Royal Society and Royal Astronomical Society in London in November 1919 and were widely celebrated as confirmation of Einstein's theory. Modern historians note that the plates were difficult to interpret and the evidence was not as decisive as popular accounts suggest. Later observations, including radio and optical measurements, provided much stronger tests. The expedition is still historically important because it made general relativity famous worldwide.

The event is sometimes called the Eddington experiment, but “Eddington expedition” is a fair canonical answer for the question. It should not be confused with the Michelson–Morley experiment, which tested the supposed motion through the luminiferous aether, or with LIGO, which detected gravitational waves a century later.