The Eddington Experiment: How a Solar eclipse changed physics

Solar eclipses are fascinating on their own: the moon completely obstructs the sun and the entire sky turns dark for a while. This is how a couple scientists turned this astronomical event into a groundbreaking science experiment

What is the Eddington Experiment?

On 29th May 1919, a total solar eclipse gave scientists a rare opportunity. British astronomers Frank Watson Dyson and Arthur Stanley Eddington used it to measure the gravitational deflection of starlight near the Sun. During a total eclipse, the Moon completely blocks the Sun’s glare, allowing scientists to see stars even at ‘day time’. The results of their observations confirmed Einstein’s theory of General Relativity and changed our understanding of gravity forever.

What is General Relativity?

Newton described gravity as a force: objects with mass pull on one another, causing things to fall and planets to orbit. Einstein offered a different approach to how gravity works. His key insight, known as the Equivalence Principle, was that being in free fall feels identical to floating in empty space with no gravity at all. Einstein realized that if you were to fall from a roof, you wouldn't feel your own weight while dropping. For those few seconds, gravity would seemingly disappear. This led him to the radical idea that free fall isn't a state of being 'pulled' down, but rather the most natural way to move through space.

Einstein formalised this in his 1915 theory of General Relativity, proposing that gravity is not a force but the result of massive objects warping the fabric of space and time around them. Everything then follows the straightest possible path through this curved space-time, which we experience as gravitational attraction. A useful analogy: place a bowling ball on a stretched rubber sheet and a marble rolled nearby will curve toward it. This doesn’t happen because the bowling ball ‘pulls’ the marble. In fact, the marble, relatively, moves in a straight-line. The curved surface is what makes it orbit.

What Did the Experiment Test?

Both Newton’s and Einstein’s theories predicted that starlight passing close to the Sun would be deflected from its straight-line path. Crucially, they disagreed on the amount: Einstein’s prediction was double Newton’s. This made the eclipse a decisive test. The plan was to photograph stars close to the Sun during totality, then compare their positions to reference images taken months earlier when the Sun was elsewhere in the sky. Any apparent shift in a star’s position would be caused by the Sun’s gravity bending the light, and the size of that shift would reveal whose theory was correct.

How Was It Conducted?

Two teams were dispatched along the eclipse path. Eddington led one to the island of Príncipe, off the west coast of Africa, while a second team travelled to Sobral in northern Brazil. Both photographed the Hyades star cluster, which conveniently lay very close to the Sun during the eclipse, making it an ideal target. The images were recorded on glass photographic plates, then carefully developed and measured back in England. The results pointed to one theory: the observed deflections matched Einstein’s predictions far more closely than Newton’s, providing a landmark confirmation of General Relativity.

A picture of the 1919 solar eclipse

Legacy

The 1919 measurements were not without limitations: cloud cover and the precision of the era’s instruments introduced some uncertainty, but subsequent eclipse expeditions with improved equipment consistently supported Einstein’s predictions. Today, General Relativity is not just a theoretical triumph; it has tangible real-world consequences. GPS satellites must account for the curvature of space-time to deliver accurate location data, and the theory underpins our understanding of black holes, gravitational waves, and the large-scale structure of the universe. The Eddington Experiment was the moment the world took notice of Einstein’s revolutionary vision of gravity, and its legacy continues through every branch of modern physics and astronomy.