Eddington wrote an article, Report on the relativity theory of gravitation, which announced Einstein's theory of general relativity to the English-speaking world. Because of World War I, German science was not known very much in England. This same war caused problems for Eddington himself when he was called up for military service. Being a Quaker and a pacifist, he refused to serve in the army, and wanted to be allowed to do alternative service instead, but such a thing was not possible at the time. Scientific friends of his solved the problem by successfully arguing to relieve him from military duty because of his importance for science.
After the war, Eddington travelled to the island of Principe near Africa to watch the solar eclipse of 29 May 1919. During the eclipse, he took pictures of the region around the Sun. According to the general theory of relativity, stars near the Sun would appear to have been slightly shifted because their light had been curved by its gravitational field. This effect is noticeable only during an eclipse, since otherwise the Sun's brightness obscures the stars. This shift was indeed found.
Eddington also investigated the interior of stars, and calculated their temperature based on what would be necessary to withstand the pressure of the higher-laying layers. He discovered the mass-luminosity relationship for stars, he calculated the abundance of hydrogen and he produced a theory to explain the pulsation of Cepheid variable stars. He was also the first to suggest that stars obtained their energy from nuclear fusion of hydrogen to helium, a theory which was later shown to be correct, but over which he had a long running argument with James Jeans.
He was a superb populariser of science, writing many books aimed at the layman. He is also attributed with introducing the Infinite Monkey Theorem with the 1929 phrase "If an army of monkeys were strumming on typewriters, they might write all the books in the British Museum"
The Eddington limit is named in his honour.