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100 years ago today, a solar eclipse helped confirm Einstein's theories on gravity

On May 29, 1919, British astronomers Frank Dyson and Arthur Eddington positioned themselves in the path of a total solar eclipse, on opposite sides of the Atlantic, for an experiment putting Einstein's theories on gravity to the test. Their results would prove to be one of the most important physics experiments of the 20th century.

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By
Tony Rice
, WRAL contributor/NASA ambassador

On May 29, 1919, British astronomers Frank Dyson and Arthur Eddington positioned themselves in the path of a total solar eclipse, on opposite sides of the Atlantic, for an experiment putting Einstein's theories on gravity to the test. Their results would prove to be one of the most important physics experiments of the 20th century.

The eclipse offered the rare opportunity to view stars, normally lost in the sun’s glow. Bent by the gravity of the sun, the light is deflected a fraction of degree, shifting stars left or right of where they would normally be.

Results of the Eddington Experiment closed gaps in our understanding of gravity first identified by astronomers in the mid 1800s. They noticed that Mercury’s position didn’t match what Isaac Newton’s laws of gravity had predicted. Newton’s laws predicted gravity could bend light, but failed to accurately calculate by how much.

Two expeditions set out from Liverpool months before on voyages to Brazil and West Africa. They carried with them a collection of equipment borrowed from observatories in Greenwich and Dublin along with some items created just for the experiment. A local clockmaker assembled a mechanism which tracked an object moving through the sky using a falling weight. A metronome provided timing for photographs taken on glass plates.

If you struggled to see the Great American Eclipse through clouds as it passed across the country in August 2017, you can appreciate the challenges Eddington and Dyson had. The morning of the eclipse, both locations were cloudy.

“The rain stopped about noon and about 1:30 p.m.... we began to get a glimpse of the sun,” Eddington wrote in his notes, adding “I did not see the eclipse, being too busy changing plates.” He saw a spectacular 100,000 mile tall solar prominence later when reviewing his photographs.

In the end, the team got the image they needed.

“The one good plate that I measured gave a result agreeing with Einstein," Eddington wrote to his mother during the return trip to England.

The stars were right where Einstein's formula predicted, bent twice as much as calculations using the Newtonian theory of gravity.

The Eddington Experiment has been performed again during total solar eclipses in Australia in 1922 and the United States in 2017 with similar results. Principles of general relativity are critical to designs from the cathode ray tubes that made early televisions possible and is part of the calculation your phone's GPS makes to pinpoint your location on Earth.

The next total solar eclipse in the United States will pass through Mexico into the Ohio valley on April 8, 2024. If you'd prefer the sun's shadow to come to you, a total solar eclipse will pass through North and South Carolina on May 11, 2078.

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