Short answers, pulled from the story.
Arthur Eddington is best known for his 1919 solar eclipse expedition to the island of Príncipe, which provided one of the earliest observational confirmations of Einstein's general theory of relativity. He also correctly predicted around 1920 that the energy source of stars is the fusion of hydrogen into helium, and he developed the mass-luminosity relation for stars in 1924.
During the total solar eclipse of the 29th of May 1919, Eddington photographed stars in the Hyades cluster from the island of Príncipe off the west coast of Africa, measuring whether starlight grazing the Sun was deflected by the amount predicted by Einstein's general relativity or the smaller amount implied by Newtonian gravity. His results supported Einstein. A 1979 re-analysis with modern equipment validated his conclusions.
Chandrasekhar showed mathematically that stars above a certain mass would collapse catastrophically rather than settle into stability, foreshadowing the concept of black holes. Eddington rejected this conclusion, insisting that a purely mathematical derivation could not produce such physically extreme consequences. Eddington was ultimately wrong, and the episode damaged his scientific reputation.
The Eddington number in physics refers to the value 1/137, the fine-structure constant, which Eddington argued should be exactly that value for epistemological reasons after earlier measurements had suggested 1/136 and he adjusted his reasoning. Critics at the time nicknamed him "Arthur Adding-one" for this change of position, which damaged his credibility in the physics community.
The Eddington cycling number is the maximum number E such that a cyclist has ridden at least E miles on at least E separate days. Eddington's own lifetime cycling E-number was 84. The measure is analogous to the h-index used to quantify scientific productivity.
Eddington argued for philosophical idealism, holding that "the stuff of the world is mind-stuff," as he wrote in The Nature of the Physical World in 1928. He believed that because relativity and quantum physics had discarded mechanical theories of matter, a materialist metaphysics was outdated, and that the structure of the objective world was precisely mirrored in consciousness. He also championed indeterminism in physics, arguing that the uncertainty principle reflected an ontologically undetermined component of nature rather than a limitation of measurement.