2019The 100th Anniversary of Einstein's Eclipse - First Day Cover

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.