An admirably elegant theory

On 6 November 1919, at a joint meeting of the Royal Astronomical Society and the Royal Society, held at London’s Burlington House, the ‘lights went all askew in the heavens’. That, anyway, was the rhetorical flourish with which the New York Times hailed the announcement of the results of a pair of astronomical expeditions conducted in 1919, after the Armistice but before the official end of the first world war. One expedition, led by Arthur Stanley Eddington, assistant to the Astronomer Royal, had repaired to the plantation island of Principe, off the coast of West Africa; the other, led by Andrew Crommelin, who worked at the Royal Greenwich Observatory, headed to a racecourse in Brazil.

Together, in the few minutes afforded by the 29 May solar eclipse, the teams used telescopes to photograph shifts in the apparent location of stars as the edge of the sun approached them. The possibility that a heavy body such as the sun might cause some distortion in the appearance of the star field was not particularly outlandish. Newton, who had assigned ‘corpuscles’ of light some tiny mass, supposed that such a massive body might draw light in, like a lens, though he imagined the effect was too slight to be observable.

The degree of distortion that the Eddington expedition hoped to observe was something else again. 1.75 arc-seconds is roughly the angle subtended by a coin a couple of miles away: a fine observation, but not impossible at the time. Only the theory of the German-born physicist Albert Einstein — respected well enough at home but little known to the Anglophone world — would explain such a relatively large distortion, and Eddington’s confirmation of his hypothesis brought the ‘famous German physician’ (as the New York Times would have it) instant celebrity.

‘The English expedition of 1919 is ultimately to blame for this whole misery, by which the general masses seized possession of me,’ Einstein once remarked; but he was not so very sorry for the attention.