The Day We Found the Universe - Marcia Bartusiak [79]
At the time Einstein was working on general relativity, he had early on suggested a specific test that astronomers could perform to confirm his predicted curvatures in space-time: Photograph a field of stars at night, then for comparison photograph those same stars when they pass near the Sun's limb during a solar eclipse. A beam of starlight passing right by the Sun would be gravitationally attracted to the Sun and get bent, making it appear that the star has shifted its standard position on the celestial sky, the position it would have if the Sun were in another part of the heavens. In 1911 he computed a bending of 0.83 arcseconds, the same arising from Newton's laws alone. But a few years later, once his final theory was in place, Einstein doubled his predicted bending. The extra contribution, Einstein figured out, occurs due to the Sun's enormous mass warping space-time. He calculated that a stellar ray just grazing the Sun would get deflected by 1.7 arcseconds (a thousandth the width of the Moon).
Three solar-eclipse expeditions were launched prior to 1919 to detect this light bending but were unsuccessful due to either bad weather or the ongoing war. The results of a fourth effort, an American endeavor led by Lick astronomers W. W. Campbell and Heber Curtis, were plagued by data comparison problems and so were never published. That was a fortunate turn of events for Einstein. The shaky American results went against him, and some of the other expeditions were carried out when his theory, not yet fully developed, was predicting that smaller, incorrect deflection.
That's why scientists paid keen attention to British astronomers when they announced they would give it a try in 1919 during a favorable solar eclipse whose path crossed South America and continued over to central Africa. The eclipse was taking place against a particularly rich background of stars, the Hyades cluster, which offered excellent opportunities to detect a star's shift. Sir Frank Dyson, Great Britain's astronomer royal, first pointed out this fortunate occurrence more than two years earlier. “This should serve for an ample verification, or the contrary, of Einstein's theory,” he noted at the time. And as the victors in World War I, the British had the necessary funding to organize and carry out the intricate venture.
On the evening before sailing, Eddington and his eclipse companion, E. T Cottingham, joined Dyson in his study. The discussion turned to the amount of deflection expected from classical Newtonian theory compared to Einstein's predicted value, which was twice as great. “What will it mean,” asked Cottingham playfully, “if we get double the Einstein deflection?” Dyson replied, “Then Eddington will go mad and you will have to come home alone!”
The next day Eddington and his assistant began their journey to the tiny isle of Principe, situated 140 miles off the coast of West Africa, a favorable site in the path of the eclipse. And to improve the venture's chances for a clear-weather view, two other astronomers traveled to the village of Sobral in the Amazon jungle of northern Brazil. On the day of the eclipse, May 29, a violent morning rainstorm almost doomed the Principe crew's operations. But by noon, the deluge ended and an hour and a half later they got their first glimpse of the Sun, already partially covered by the Moon. Too busy changing plates during totality, Eddington had only one chance, halfway through, to view the Sun's dark visage. “We are conscious only of the weird half-light of the landscape and the hush of nature, broken by the calls of the observers, and beat of the metronome ticking out the 302 seconds of totality,” he later recalled of the adventure.
The astronomers in Sobral were more fortunate. There they had two instruments and better weather. With their astrographic telescope they clicked off sixteen photos, and eight more were taken with a 4-inch scope. On Principe, Eddington and Cottingham, too, took sixteen photographs, but most ultimately turned out useless because of the intervening