On this day in 1919, a solar eclipse permitted observation of the bending of starlight passing through the sun's gravitational field, as predicted by Albert Einstein's theory of relativity. Expeditions from the Royal Astronomical Society traveled to Brazil and the west coast of Africa to make measurements of the position of stars visible close to the sun during a solar eclipse. These observations showed that, indeed, the light of stars was bent as it passed through the gravitational field of the sun. This was a key prediction of Einstein's theory that gravity affected energy as in addition to the familiar effect on matter.
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