Worse, the conditions on Príncipe turned out to be much grimmer than Eddington and his assistant Edwin Cottingham had anticipated. Given that an arcsecond is 1/3,600th of a degree, such extremely small differences would be very hard to detect. Could he detect any star-position changes caused by the Sun? More to the point, could he do it accurately enough to determine whether they were occurring in response to the physics of Newton or the science of Einstein? According to the former, star images would be deflected by about 0.8 of an arcsecond while Einstein said they would be moved by about 1.8 arcseconds. “Einstein’s theory predicted a greater deflection.”Įddington therefore faced a daunting double problem. “Newtonian physics also forecast that star positions could be shifted during an eclipse – but less so,” says Crawford. If the apparent positions of these stars moved compared with standard, night-time photographs of the region, this would indicate that the Sun’s mass was causing space to curve.
Both expeditions were organised by the British astronomer royal Frank Watson Dyson and both would study stars of the Hyades cluster in the constellation Taurus – in front of which the eclipsed Sun would pass. Later he found that only two of them contained enough starsĪnd that is what Eddington set out to prove – along with a second group of British astronomers who were sent to Sobral, in northern Brazil, which also lay under the eclipse path. Eddington worked feverishly and managed to make 16 plates. By comparing existing photographs of a particular cluster of stars with images of them taken during an eclipse, it should be possible to discover whether the latter have shifted position because space is being bent by the Sun as it passes in front of them. This blots out its blindingly bright rays and allows astronomers to study the relatively dim light of background stars. The May 1919 eclipse provided that opportunity.”ĭuring a total solar eclipse, the disc of the Moon passes in front of the Sun. What was needed was a specific testable prediction to show his theory was right. “But these were post hoc rationalisations. “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. Even a beam of light would bend as it passed along this section of curved space.
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. Instead, he maintained, it was the result of an object’s mass causing space to curve. In his 1915 theory, Einstein argued that gravity was not a force that acted at a distance between objects, as Isaac Newton had stated. 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.
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