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  • There's only one way to see the stars while the sun is out.

  • And that's during a total solar eclipse.

  • You have to be at right place at the right time.

  • Under a clear sky.

  • Standing somewhere along the narrow path where the moon aligns perfectly between the sun

  • and the Earth.

  • When the moon passes in front of the sun's disc, it darkens the sky just enough for distant

  • stars to become visible.

  • There have been many photos of total solar eclipses.

  • But this one is special.

  • It helped prove a radical idea.

  • That redefined gravity.

  • And turned Albert Einstein into a celebrity.

  • Because the stars in this photo aren't where they're supposed to be.

  • Isaac Newton laid the foundation for understanding the physical universe in the Principia, published

  • in 1687.

  • In it, he defined gravity as a force of attraction that draws massive objectslike stars

  • and planetstoward each other, and keeps them in orbit.

  • And for more than 200 years, gravity was defined this way: as an attracting force.

  • But Albert Einstein saw gravity as something completely different.

  • According to his theory of general relativity, which he published in 1915, gravity isn't

  • a force between objects in space.

  • It's the influence of objects on the shape of space itself.

  • According to Einstein, massive objects like the sun bend the space around them.

  • So when a smaller object moves in a straight line along this space, it gets diverted because

  • of the curve caused by the mass of the larger object.

  • That puts one object in an orbit around the other.

  • And if Einstein was right then the same curve would divert the path of light as well.

  • Meaning if you observed distant stars through a telescope on Earth while the sun is in front

  • of them, their light, deflected by the sun's gravity, would make them appear slightly out

  • of position.

  • It was a revolutionary idea.

  • But there was a big conflict keeping Einstein from testing it.

  • The world was at war.

  • Einstein lived Germany at the time.

  • But his work landed in the hands of a British astrophysicist: Arthur Eddington.

  • Even though they were on opposing sides of the war, Eddington, along with astronomer

  • Frank Dyson, set out to test the theory.

  • They would photograph a total solar eclipse.

  • They needed to compare the position of a cluster of stars in the night sky with a photograph

  • of the same stars during an eclipse.

  • If the stars' apparent positions had shifted, it would prove that starlight was traveling

  • through space curved by the sun's gravity.

  • The May 1919 eclipse was the ideal one for this experiment.

  • The sun would be in front of a very dense cluster of stars, the Hyades.

  • And that meant multiple bright stars would be visible during the eclipse.

  • Planning began in 1917, and a couple of years later, two expeditions departed England.

  • One led by Eddington went to the island of Principe in West Africa, and the other headed

  • to Sobral, Brazil.

  • Two locations that were in the path of the eclipse and had favorable climates.

  • Each group traveled with powerful photographic telescopes that could record detailed photos

  • of space onto glass plates.

  • Photographing the eclipse that May required transporting, and then carefully assembling

  • them, in the field.

  • With the plates tilted 45 degrees on one of the telescopes to include as many stars as

  • possible.

  • And this was the result.

  • This is one of the few successful plates from the 1919 expeditions.

  • It came from Brazil.

  • It shows the eclipse during totality, the sun's corona bursting forth, and the rarely

  • seen solar prominence.

  • Most importantly, bright stars of the Hyades.

  • Back in England, Eddington compared the position of the stars from the eclipse plate with another

  • of the night sky, using a machine that can take measurements within photos at the microscopic

  • level.

  • The comparison revealed that the stars had shifted during the eclipse by roughly the

  • amount that Einstein predicted.

  • According to Newton's calculations, starlight should bend near the sun too.

  • But if Einstein was right, that deviation would be twice what Newton predicted.

  • Eddington's result showed that the deflection of the stars came closer to Einstein's calculation

  • than Newton's.

  • It wasn't a perfect match, but it was close enough to validate the theory of general relativity,

  • and completely shift our understanding of the universe.

  • The success of the experiment was first announced in The Times of London on November 7th, 1919.

  • Almost a year to the day after the end of World War I.

  • An Englishman had gone to great lengths to prove the ideas of a German, and the news

  • that space is warped by the planets and stars excited the world.

  • Einstein, who before this moment was only known in the physics world, essentially became

  • a celebrity overnight.

  • He remained an international pop culture icon for the rest of his life.

  • And a favorite subject of press photographers.

  • Observing eclipses continued to be one way of testing general relativity for decades

  • to come.

  • With more sophisticated equipment repeatedly confirming the accuracy of Einstein's theory.

  • General relativity allowed physicists to begin to understand advanced concepts about the

  • universelike black holes.

  • Which ultimately led to this: the first photograph of a black hole, taken in 2019.

  • A century after Eddington first proved Einstein's theory with a photoand completely changed

  • our definition of gravity.

There's only one way to see the stars while the sun is out.

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The eclipse photo that made Einstein famous

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    林宜悉 posted on 2020/08/10
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