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The solar eclipse that proved Einstein right

Black circular silhouette of moon, surrounded by solar corona, during a total solar eclipse.
Einstein’s triumph. This early photograph shows the total solar eclipse on May 29, 1919. See the tick marks around stars near the eclipsed sun? It was the precise measurement of the positions of these stars that proved the sun bends starlight, in accordance with Einstein’s theory of general relativity. Image via Wikimedia Commons.

May 29, 1919, is the date of a solar eclipse that caused a revolution in science. The eclipse is famous for testing Albert Einstein’s theory of general relativity. Einstein was relatively unknown at the time. He had proposed general relativity in 1915, and scientists had been intrigued by the entirely new way of thinking about gravity – for example, the idea that mass causes space to curve – but no one had experimentally proven the theory to be correct. Then, on May 29, 1919, an expedition of English scientists – led by Sir Arthur Eddington – traveled to the island of Príncipe off the west coast of Africa to observe a total solar eclipse. If the theory were right, the light from stars should be bent by the gravity of the sun and appear displaced. An eclipse, where the moon blocks the sunlight enough for stars to be seen near the sun, was the perfect opportunity to test this.

The scientists’ measurements during the eclipse showed that, astoundingly, Einstein’s predictions were indeed correct. The locations of the now visible stars appeared displaced, due to the fact that their light had to travel to us on the curved space around the sun caused by its gravity, as described by Einstein.

From anonymity to stardom via a solar eclipse

Later that year – on November 6, 1919, in London – England’s Astronomer Royal, Frank Dyson, who had organized the expedition, presented the results at a joint meeting of the Royal Astronomical Society and the Royal Society. Dyson said “there can be no doubt” that measurements made during the May 29, 1919, solar eclipse “confirm Einstein’s prediction.”

As part of the celebration of the 100th anniversary of this legendary solar eclipse, Caltech physicist Sean Carroll explained to NBCNews in 2019:

General relativity was the poster child for being a crazy, new, hard-to-understand theory, with dramatic implications for the nature of reality. And yet you could see [the results]; you could photograph it. So people got caught up in that excitement.

And so Albert Einstein was catapulted to rock star fame, a status in popular culture he has retained ever since.

Diagram of sun, star, Earth. Straight line from us to a star's apparent location and a bent line to its actual location.
During a solar eclipse, stars normally not visible in the glaring sunlight appear on the side of the sun and are displaced from the location they’d normally be in. Why? Because – just as Einstein’s theory said it should – light bends in the presence of mass, in this case the mass of a star, our sun. Rather than traveling in a straight path, the light of distant stars is forced to travel a curved path along the curved space near the sun. Note that the bending of starlight is exaggerated in this image. In reality, the stars are displaced by up to 1.75 arcseconds (about 0.0005 degrees). Image via GSFC/ NASA/

A new perspective on gravity and the universe

Einstein’s general theory of relativity underlies our most basic modern cosmology, our way of looking at the universe as a whole. Before Einstein, scientists relied on Isaac Newton’s theory of gravity. Newton’s way of looking at gravity is still valid and is still taught to physics students. But while Newton’s formulation of gravity is more of a special case under specific conditions, Einstein’s theory is a refinement of scientists’ understanding of gravity that covers the big picture … and what a mind-blowing big picture! Einstein proposed that mass causes space to curve. So, for example, although there appears to be a “force” (as described by Newton) that causes our Earth to be pulled towards the sun by gravity, that force can “simply” be described as Earth traveling in curved space around the sun, according to Einstein.

Einstein’s general theory of relativity not only explains the motion of Earth and the other planets in our solar system. In our modern cosmology, it also describes extreme examples of curved space, such as around black holes. And it helps to describe the history and expansion of the universe as a whole.

The solar eclipse was the first proof of many

In the century and a bit since the 1919 total solar eclipse, Einstein’s relativity theory has been proven again and again, in many different ways. You might have seen the recent first-ever photo of a black hole?. It also proved, once again, that Einstein was right.

Read more: Black hole image confirms Einstein’s relativity theory

Read more: Clocks, gravity and the limits of relativity

Glowing, fuzzy orange donut with black center.
This image captured people’s imaginations when it was released in 2019: the first-ever actual image of a giant black hole, in the center of galaxy M87. It also proves Einstein’s theory, which predicted the observations from M87 with unerring accuracy. Image via Event Horizon Telescope Collaboration.

Now and then

The Royal Astronomical Society (RAS) described modern-day practical applications of Einstein’s theory:

The theory fundamentally changed our understanding of physics and astronomy, and underpins critical modern technologies such as the satellite-based Global Positioning System (GPS).

The theory of relativity is essential for the correct operation of GPS systems, which in turn are relied on in many common applications including vehicle satellite navigation (SatNav) systems, weather forecasting, and disaster relief and emergency services. However, the world had to wait decades before the applications of such a blue skies result could be realized.

Back in the day of the 1919 eclipse, Sir Arthur Eddington attended a dinner of the same organization – RAS – shortly after the successful expedition. He then showed his humorous side by reciting a verse he had written on the feat:

Oh leave the wise our measures to collate
One thing at least is certain, light has weight
One thing is certain and the rest debate
Light rays, when near the sun, do not go straight.

Black and white photo of a man with glasses.
Sir Arthur Eddington led the expedition that provided the first proof of Einstein’s theory of general relativity. Image via Wikimedia Commons.
A dark-haired man with a mustache and a dignified expression, in a suit.
Albert Einstein in 1912.

Bottom line: The solar eclipse of May 29, 1919, was the day astronomer Sir Arthur Eddington verified Einstein’s general theory of relativity, by observing how stars near the sun were displaced from their normal positions. This apparent change in position happens because, according to Einstein’s theory, the path of light is bent by gravity when it travels close to a massive object like our sun.


Via NBCNews


May 29, 2021
Human World

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