The Solar Eclipse of 1919 observed in the city of Sobral
This year the centenary of the astronomical observations made during the solar eclipse of May 29, 1919 is being celebrated worldwide. The measures of the deflection of the light of the stars on the edge of the Sun, from them, constituted a fundamental proof for the confirmation of Einstein’s General Theory of Relativity. This theory has profoundly altered our view of the Universe, its structure and functioning. It supplanted the gravitational theory that Newton had formulated two centuries earlier. The decisive observations were made by British astronomers in the city of Sobral (state of Ceará, Brazil) and Príncipe Island (Africa), then belonging to Portugal. The bending of the light ray from a star, when it passes close to the Sun, had been predicted a century earlier based on Newton’s theory. In 1911, and independently, Einstein reached the same prediction. He proposed to astronomers that the deflection of light could be measured in a total solar eclipse by photographs of stars whose light passed at the edge of the Sun compared to photographs of the same stars when the Sun was no longer in front of them. An Argentine astronomical expedition, led by Charles D. Perrine, attempted to measure this deflection of starlight in 1912. Observations during a total solar eclipse would be made in the city of Cristina, state of Minas Gerais, Brazil; but it rained all the time, and nothing was measured. Other attempts, and equally frustrated, were made in Crimea (1914), Venezuela (1916) and USA (1918).In 1915, Einstein elaborated the General Theory of Relativity based on the idea that the gravitation results of the alteration of the geometry of the space-time by the presence of the matter. Matter tells space-time how to curve and space-time geometry tells matter how it should move. And he predicted that starlight, following the shorter trajectory in space-time, would suffer a deflection in the vicinity of the Sun by a value that would be twice as predicted in Newtonian theory: the deflection angle should be approximately 1.74” (seconds of arc). British astronomers organized two expeditions to observe the solar eclipse of May 29, 1919: an expedition with Arthur Eddington and Edwin Cottingham to Príncipe Island, and another with Charles Davidson and Andrew Crommelin to the city of Sobral. Sobral’s choice as a point of observation in Brazil was made by Henrique Morize, director of the National Observatory of Rio de Janeiro, who was also in charge of providing the infrastructure for the foreign expeditions that would go to Sobral. In the city of Sobral, on May 29, 1919, despite the initially cloudy weather, conditions were good at the time of the eclipse, which occurred at 08:56 and lasted about 5 minutes. Seven photos taken by British astronomers were considered very good; seven stars appeared on them. In Príncipe Island, the weather was rainy and only two photos could be used, and lead to results more uncertain than those of Sobral. The Brazilian commission in Sobral, led by Morize, made observations about the solar corona during the eclipse and took beautiful photos of an immense solar protuberance. Foreign astronomers were very grateful for the reception and support they received from the Brazilian commission, the authorities and the population of Sobral. On November 6, 1919, British astronomers presented the results of observations made in Sobral and Príncipe Island. They led to a near deflection angle, within the margin of error, of that predicted by the General Relativity Theory. Einstein was right! Regarding the importance of the observations in Sobral, they stated: “Summarizing the results of the two expeditions, the highest weight should be attributed to those obtained with the 4-inch lens in the city of Sobral. From the superiority of the images and the larger scale of the photographs, it was recognized that these would be the most reliable.” In the days that followed, newspaper headlines from around the world signaled that a revolution had occurred in science: Einstein’s theory had supplanted Newton’s theory. This event made Einstein, a scientist previously known only to his physical colleagues, become the best-known scientist of all time. When he was in Rio de Janeiro in 1925, Einstein declared: “The problem conceived by my brain was responsible for solving it the bright sky of Brazil.” The issuance of a postage stamp commemorating the centennial of the eclipse of 1919 by the Brazilian Post and Telegraph Company – a public company linked to Brazilian Ministry of Science, Technology, Innovation and Communication – does justice to this event of enormous importance for science and for humanity. The solar eclipse of 1919, observed in Brazilian lands in Sobral, contributed for Einstein to become a well-known scientist and for the consolidation of General Relativity Theory, the theory that is currently accepted by science for the description of the Universe and the movement of the stars.