On 29 May 1919, British-led teams observed a total solar eclipse from Príncipe, off the west coast of Africa, and Sobral in Brazil. The eclipse gave astronomers a rare chance to photograph stars close to the Sun.

The test came from Einstein’s general theory of relativity. His theory predicted that the Sun’s gravity would bend starlight by a measurable amount. During an eclipse, the Sun’s bright disc was covered, so stars near it could be photographed. Their apparent positions could then be compared with their normal positions.

Arthur Eddington was part of the observing party at Príncipe. Another British team worked at Sobral. The work was difficult. Weather, equipment and measurement limits all affected what could be recorded. The latter story is often cleaner than the observing conditions were.

The results were announced in London later that year and treated as strong support for Einstein’s theory. They did not settle every scientific question around the measurements, and the eclipse should not be described as simply proving relativity. It did help make Einstein famous beyond specialist science.

The timing gave the event extra force. Soon after the First World War, a British-led test had supported the work of a German scientist. A technical measurement made during an eclipse became a public scientific event.