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How did Arthur Eddington test Einstein’s prediction during the 1919 solar eclipse?

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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light. — relativity Brian Greene kicking off his *Your Daily Equation* video series with Albert Einstein's famous equation E = mc².

To test Albert Einstein’s prediction that gravity would deflect light passing near a massive body, Arthur Eddington and other British astronomers organized two expeditions to observe the total solar eclipse of May 29, 1919.1

The researchers aimed to measure the angular deflection of starlight as it passed near the Sun’s edge, which Einstein’s general theory of relativity predicted to be 1.75 arc seconds. The teams set up observation sites in Sobral, Brazil, and on the island of Príncipe.1

During the eclipse, the astronomers photographed stars in the vicinity of the Sun. They then compared these images to photographs of the same stars taken when the Sun was not present in that part of the sky. By analyzing the displacement of the stars' positions on the photographic plates, the expeditions provided evidence that gravity indeed bends light, supporting Einstein’s theory and helping to establish him as a leading physicist of the era.12