Among the spectacular nature of total solar eclipses, there’s one stands out above others— the eclipse of 1919. Observations that were made during this eclipse, by teams led by astronomer Arthur Eddington, marked perceptible shifts in the positions of starts that confirmed Einstein’s theory of relativity. The confirmation cemented the German physicist’s reputation as a superstar and forever changed human understanding of the world.

Einstein’s theory

In 1915, Einstein debuted his theory of general relativity to the world. In it, he proposed that among other things, an extension of the fixed views offered by Sir Issac Newton. To Einstein, space and time were part of a four-dimensional continuum called spacetime, which could experience change as gravitational fields of objects caused warps in it and weaved gravity into a continuum. Einstein believed that light would curve by a small amount when traveling through spacetime and nearing an object’s gravitational field.

Einstein’s ideas were met with high levels of resistance. At the time, the established scientific community was largely supportive of Newtonian physics as the most credible explanation for how things in the universe worked. The differences between the two views can be seen in how they each predict how gravity affects light. Both believed there would be a change, but Einstein predicted the impact would be larger than Newton did.

Measuring starlight to find answers

The scientific community knew that if the effect of gravity on light could be properly measured it would prove that one of the two versions were correct. They could do this by measuring the positions of background stars at the sun’s edge to evaluate change and comparing them to the positions of the stars at night. Given the blinding brightness of the sun, this was no easy task. The only way to do it would be to take advantage of a total solar eclipse that naturally leaves only the dimmer rays of the sun’s outer corona visible. At that point,  the stars would be more easily seen.

Prior to 1919, scientists had been trying to conduct measurements of light during an eclipse. Unfortunately, World War II made this difficult for a time. Following the war, there was an attempt to measure light during a 1918 total eclipse, but clouds prevented any reliable view of the sky.

Observing the 1919 total solar eclipse

Scheduled for May 29th, the eclipse was the perfect version of its type for the teams to use. At six minutes and 51 seconds, it was the longest eclipse in more than 500 years. In addition, its location meant that the sun would pass by a cluster of stars named the Hyades which would offer plenty of samples of starlight for the team to measure.

On the day of the event, the eclipse was viewed by two separate teams led by Eddington. The astronomer was personally on the remote island of Principe off the west coast of Africa. A second team under his authority was based in Sobral Brazil. The location of the two teams helped reduce the chances that cloud cover would make observing the eclipse impossible.

Each team took measurements of the deflection of starlight off of the sun’s gravitational field and then compared them to the previously established position of the stars at night. By the end of the eclipse, Eddington was able to prove that the light rays of distant stars were pulled off of their paths by the gravitational field of the sun and that their movements were consistent with the theory of relativity. His observations showed that starlight had shifted by no more than the width of a quarter when seen two miles away, but it was enough to prove Einstein’s theory.

The world responds

Though Einstein himself was calm at the news of Eddington’s proof, the world responded jubilantly. Joyful headlines celebrated the triumph with announcements like the New York Times’: “Light’s All Askew In The Heavens. Einstein’s Theory Triumphs.”  Other parts of the same headline noted that “scientists were agog over the results.”

Coming so close to the end of World War I, the news also had geopolitical implications. Eddington, who was English and a pacifist, had conducted substantial work that validated the truth of a German scientist. Given that the two countries were on different sides during the war, some believed that Eddington’s support was connected to his political views. Regardless of the astronomer’s motivations, this kind of cooperation gave the world hope for a future of peace. Even more importantly, it provided a useful verification of Einstein’s theory and changed the way humans understand the world.

In the years immediately after the measurements were released, there were a number of people who questioned the validity of Eddington’s results. Skeptics closely analyzed his work and conducted several similar tests in subsequent eclipses. In the end, Eddington’s observations held up.

A century of relativity’s impact

Today, Einstein’s theory is seen as the leading way to explain how things work. While Newton’s laws are useful, they’re generally taken as a less developed explanation. Einstein’s theory has been used in connection with an array of phenomena ranging from GPS signals to merging black holes and gravitational waves. In 100 years, there’s not been an observation that conflicts with its predictions and scientists are continuing to discover new ways that it predicts how the universe operates. As it turns out, the theory that was verified by a total solar eclipse will continue to be an integral part of how humans understand our world going forward.