WASHINGTON-A hundred years ago this month, the British astronomer Arthur Eddington arrived at the remote West African island of Príncipe. He was there to witness and record one of the most spectacular events to occur in our heavens: a total solar eclipse that would pass over the little equatorial island on 29 May 1919.

Observing such events is a straightforward business today, but a century ago the world was still recovering from the First World War. Scientific resources were meagre, photographic technology was relatively primitive, and the hot steamy weather would have made it difficult to focus instruments. For good measure, there was always a threat that clouds would blot out the eclipse.

These risks doubtless caused worries but they were well worth facing, Eddington reckoned, for he believed his observations could prove, or disprove, the most revolutionary scientific idea to have been put forward in modern science: Albert Einstein’s theory of general relativity.

In his 1915 theory, Einstein argued that gravity was not a force that acted at a distance between objects, as Isaac Newton had stated. Instead, he maintained, it was the result of an object’s mass causing space to curve. From this perspective, a body in orbit around the Sun is actually going in a straight line but through space that has been bent by the mass of the Sun. Even a beam of light would bend as it passed along this section of curved space.

“Einstein used existing astronomical observations to support his theory – for example, known anomalies in the orbit of Mercury round the Sun,” says Carolin Crawford, of the Institute of Astronomy, Cambridge. “But these were post hoc rationalisations. What was needed was a specific testable prediction to show his theory was right. The May 1919 eclipse provided that opportunity.”

During a total solar eclipse, the disc of the Moon passes in front of the Sun. This blots out its blindingly bright rays and allows astronomers to study the relatively dim light of background stars.