May 29, 1919.


By the time Albert Einstein developed his general theory, he was back in Germany, working in Berlin. But a copy of his general theory of relativity was soon revealed at Cambridge. Once the copy was at Cambridge, was read by Arthur Stanley Eddington, a well known Britain’s leading astrophysicist. Eddington realized that Einstein’s theory could be tested somehow stating that If space really was distorted by gravity, then light passing through it would not travel in a straight line, but would follow a curved path in consequence where the stronger the force of gravity is, the more the light would be curved. The bending would be largest for light passing very close to a very massive body, such as our Sun.

This was before black holes were seriously considered and stars provided knowledge of strong gravitational fields. So, in this case, the Sun was particularly useful but at the same time impossible to see how the light from weak backgrounds might be curved by the golden body gravity.

- Einstein and Eddington. German-born physicist Albert Einstein (1879-1955), at left, was famous for his theories of relativity. British astrophysicist Sir Arthur Eddington (1882-1944), at right, pionee -
Einstein and Eddington in conversation. University of Cambridge, circa 1930.


The solution  was across the corner, observe during a total eclipse when the Sun’s light is covered out giving the ability to see distant stars appearing close to the Sun in the sky. If Einstein’s theory was correct, Sun’s gravity would change the position of the stars to slightly different positions, compared to where they are seen in the regular night sky at other times of the year when the Sun is far away from them. The closer the star appears to the Sun during the total eclipse, the bigger the changes on the position will happen.

Eddington began to arrange in collaboration with the British scientific institutions the experiment. The Astronomer Sir Frank Watson Dyson realized that the 1919 eclipse was the ideal moment and a once in a lifetime opportunity, leveraging that totality will be quite long (around 6 minutes) and the Sun position will be in front of the Hyades, a cluster of bright stars.

The path of the 1919 total eclipse shadow passed from northern Brazil crossing the Atlantic Ocean to West Africa. Two expeditions were organized, the first to Sobral in Brazil and the second to the island of Principe in the Gulf of Guinea,  close to the West African coast. Eddington himself went to Principe; the expedition to Sobral was led by Andrew Crommelin from the Royal Observatory at Greenwich.

Graphical explanation of the experiment and corresponding text as of: The results obtained by the British expeditions to observe the total eclipse of the sun last May, verified Professor Einstein’s theory that light is subject to gravitation. Dr. A.C. Crommelin, one of the British observers, said: “The eclipse was specially favorable for the purpose, there being no fewer than twelve fairly bright stars near the limb of the sun. The process of observation consisted in taking photographs of these stars during totality, and comparing them with other plates of the same region taken when the sun was not in the neighborhood”.  Then if the starlight is bent by the sun’s attraction, the stars on the eclipse plates would seem to be pushed outward compared with those on the other plates…. The second Sobral camera and the one used at Principe agree in supporting Einstein’s theory…. It is of profound philosophical interest. Straight lines in Einstein’s space cannot exist; “They are parts of gigantic curves.” From the Illustrated London News of November 22, 1919.


Many problems arose during the expeditions. At the day of the eclipse on Principe island where Eddington was the leader of this expedition, a heavy thunderstorm and torrential rain took place. By mid-afternoon the sky had partly cleared and he took some pictures through the clouds.

Meanwhile at Sobral, Crommelin which was the leader of this other expedition, had much better weather, but serious human errors happened during the setup of the equipment. He focused the main telescope the night before the eclipse but without the proper adjustments to calibrate the distortions that would take place as the temperature climbed during the day. Fortunately, he had a backup telescope which was the one that provided the best results overall.

The 16-inch lens and large coelostat (on the left) and the smaller 4-inch telescope in a square box (on the right), with its much smaller coelostat on location at Sobral, Brazil. (Photo credit: The Science Museum, London.)
Eddington’s photograph of the May 29, 1919, solar eclipse. The photo was presented in his 1920 paper announcing the successful test of general relativity.   Photo credit:  Royal Society of London.

Once the eclipse ended, Eddington measured the positions of the stars that appeared near the Sun’s eclipsed image, on the photographic plates exposed at both Sobral and Principe. Then a comparison was conducted with reference positions taken previously when the Hyades were visible in the regular night sky. The measurements had to be incredibly accurate, not only because the expected deflections were small. The images of the stars were also quite blurred, because of problems with the telescopes and because they were seen through the light of the Sun’s glowing atmosphere, the solar corona.

dyson eddington
Frank Dyson (left) and Arthur Eddington (right). Photo credit: AIP Emilio Segrè Visual Archives, W. F. Meggers Collection.


Before the overall results were ready and analyzing the preliminary data provided, the Royal Society called a special meeting in Nov 6, 1919 in London. Dyson as Astronomer and the appointed spokesperson rocked the stage announcing that the measurements did not support Newton’s long-accepted theory of gravity. Instead, they agreed with the predictions of Einstein’s new theory.

Cosmic Times news 1919 Edition. Credit: NASA.

Reactions of the scientific community and the press were extraordinary and unprecedented. Einstein was immediately positioned at the main front pages of international media stating that there was more to this than purely the scientific content of his theory.

After years of the WWI, the public, the media and the scientific community embraced this moment that moved mankind from the horrors of destruction to the transcendence of the human knowledge unveiling the secrets of the Cosmos and joining two pacifists, the British Eddington and the German Einstein, which were particularly pleased claiming that reconciliation between their nations is possible through the advance of science.


Author: Jesus Padilla

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