29 May 1919 was an important milestone in the history of the physical sciences and for our understanding of the universe. Albert Einstein’s theory of general relativity – which radically altered our understanding of space and time – was put to the test through observations made during a total eclipse of the Sun. During the eclipse, the shadow of the Moon was projected onto the Earth’s surface, covering a strip across the Atlantic Ocean from South America to Africa. Two teams of British astronomers stationed at two points along the narrow strip where the eclipse was total – in Sobral (Ceará, Brazil) and on the island of Príncipe (Gulf of Guinea, at the time under Portuguese administration) – recorded the phenomenon by photographing the position of the stars observed in the vicinity of the sun. The expeditions had been prepared with the help of Portuguese and Brazilian astronomers and the authorities of both countries. In Portugal, the Astronomical Observatory of Lisbon and the Lisbon Geographic Society provided valuable logistical assistance.
Although the success of the observations was threatened by clouds on the island of Príncipe, some photographs were taken there. In Brazil, even more photographic evidence was obtained. This evidence revealed a de ection of the starlight being attracted towards the Sun, making the stars appear visually more distant from the Sun than they would be without this de ection, that is, without the interposition of a large mass – in this case, the Sun. But such observations can only be made during a total eclipse, when the Moon completely obscures the brightest super cial layer of the Sun, the photosphere, darkening the sky and making the brightest stars visible from earth. Carefully measuring these deviations con rmed the predictions of Einstein’s theory. In early November 1919, when the results of the observation of the eclipse were released at an epic joint session of the Royal Society and the Royal Astronomical Society in London, Albert Einstein became the most famous scientist of the century overnight.
Nuno Crato and Luís Tirapicos
