A total solar eclipse is not just an occasion for skygazing, it’s an occasion for science. The most celebrated experiment conducted during an eclipse occurred on May 29, 1919. Four years earlier, in Nov., 1915, Albert Einstein first presented his theory of general relativity, arguing, among other things, that gravity can warp space-time and bend light as it passes around a massive object like the sun. That bending would be slight. Starlight passing by the sun would, Einstein calculated, be diverted by just 1.75 arc seconds, with a single arc second measuring one=3,600th of a degree. It would be impossible to observe that phenomenon in real time, of course, since the brilliant fires of the sun would wash out something as faint as starlight. During an eclipse, however, those fires would be briefly blotted.
To take advantage of that opportunity, two expeditions of astronomers—one from the Greenwich Observatory and one from Cambridge University—set out for Sobral, Brazil and the island of Principe off the west coast of Africa, where the approaching 1919 eclipse would be visible. During the brief minutes of totality, they charted the precise positions of stars near the sun that popped into view when the lights went out. On subsequent evenings, after the sun had set, they mapped the positions of the same stars and found that they indeed differed slightly from the measurements taken during the eclipse.
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