One hundred years ago, a group of scientists from Trinity College led an expedition to Brazil and the West Coast of Africa to observe the total eclipse of the sun at 2.13 p.m. on 29 May 1919. On 6 November in the same year they presented their findings to the Royal Society and immediately made news headlines across the world by overturning the theory of universal gravitation which Sir Isaac Newton had first presented in his Principia Mathematica.
This blog commemorates the work of Sir Frank Dyson, Sir Arthur Eddington and Dr Andrew Crommelin in proving Einstein’s General Theory of Relativity, and thereby ushering in a new era in the scientific understanding of the universe.
Newton had predicted that starlight can be deflected by the gravitational pull of the sun, but in 1915 Albert Einstein suggested in his General Theory of Relativity that the deflection of light would in fact be much more pronounced than Newton had calculated.
One of the major proponents of Einstein’s theory was Arthur Stanley Eddington, who came to Trinity as a brilliant mathematician in 1902 and rose to become one of the most significant astronomers of the twentieth century. Eddington realised that Einstein’s theory could be tested by comparing the daytime appearance of the stars during a solar eclipse with their position at night: measuring the change in relative position would enable a calculation of the gravitational pull of the sun in bending the light of the stars.
At 6 minutes 51 seconds, the solar eclipse of 29 May 1919 was the longest total eclipse for more than 500 years, and it therefore provided a perfect opportunity to test Einstein’s theory. Eddington collaborated with two other members of Trinity College both then working at the Royal Observatory in Greenwich: the Astronomer Royal, Sir Frank Dyson, and Dr Andrew Crommelin, who is best remembered today for his work on comets.
Dyson calculated that the 1919 eclipse would provide an especially good opportunity not only because of its long duration, but also because the group of stars known as the Hyades would be nearly in line with the sun. The Hyades is a particularly bright star-cluster, allowing the bending of light to be more easily observed.
The eclipse could only be observed in its totality in a few locations on a path passing through Africa and South America, and Dyson and Eddington obtained funding through the Royal Society and the Royal Astronomical Society to enable the necessary equipment to be shipped to two locations: the island of Principe off the West coast of Africa, and the town of Sobral in Brazil. Eddington travelled to Principe with his Cambridge colleague Edwin Turner Cottingham, and Andrew Crommelin undertook the Brazilian observation with Charles Rundle Davidson, an assistant from the Royal Observatory.
The 1919 expedition
The expedition to observe the 1919 eclipse set sail from Liverpool on 8 March and arrived at Madeira on the 12th, where the two groups parted. Crommelin and Davidson went on to Brazil aboard the Anselm, while Eddington and Davidson were obliged to stay at Madeira until 9 April, when they recommenced their journey aboard the Portugal. They arrived at S. Antonio in Principe on the 23rd. After inspecting various possible sites on the island, they settled on Roça Sundy, the headquarters of a plantation owned by Senhor Carneiro, and their baggage was transported there on the 28th. They spent a week preparing the equipment, before returning to S. Antonio for the week 6-13 May; they then went back to Sundy to continue their preparations. The eclipse took place on 29 May. On 12 June the observers left Principe on the steamship Zaire. After changing ships at Lisbon, they arrived back at Liverpool on 14 July.
The most detailed account from the time of the expedition itself is provided in Eddington’s letters home to his mother. In a letter written during the return home on the S.S. Zaire, approaching St Vincent, Eddington recounts many more details of the observation. They began taking check photographs about 16 May. The last heavy rain fell on the 9th and soon afterwards the cool season began. On the day of the eclipse Carneiro and others came over. The eclipse was preceded by a rainstorm and it remained cloudy during the event. Afterwards they spent six days developing the photographs. The one good plate he has measured gives a result agreeing with Einstein. He then describes a monkey-hunting expedition and visits to Gola, Lapa, and Bombom.
The announcement of the findings was made in a lecture by Frank Dyson, Arthur Eddington and Charles Davidson, who had accompanied Crommelin to Brazil, in a lecture to the Royal Society in November 1919. Eddington’s formal account of the expedition was subsequently adapted for publication in the Philosophical Transactions of the Royal Society. It includes a plate of the most successful photograph of the eclipse taken on the expedition.
The news spread very widely, and rapidly established Einstein’s international fame. The headlines – Lights All Askew in the Heavens: Einstein’s Theory Triumphs – from the New York Times on 10 November 1919 is characteristic of the reports. Eddington continued to work at Trinity on what he called ‘fundamental theory’. He received a knighthood in 1930 and the Order of Merit in 1938. He died in Cambridge in 1944.