90 years on - the 1919 eclipse expedition at Príncipe

Abstract

The first experiment to observationally confirm Einstein's General Theory of Relativity was carried out in May 1919, on a Royal Astronomical Society expedition to observe a total solar eclipse. Sir Arthur Eddington travelled to Príncipe, a small island off the west coast of Africa, and sent another team to Sobral, Brazil, from where the eclipse would also be visible. This year, in a new RAS-funded expedition organized for the International Year of Astronomy, we returned to Príncipe to celebrate this key experiment that shook the foundations of 20th-century science. Since 1687, Sir Isaac Newton's law of gravity had been the workhorse of celestial mechanics. Newtonian gravity could be used to explain the motions of a host of celestial bodies and the heavens were reliable and predictable. There was one small discrepancy: accurate measurements of Mercury's orbit did not quite fit the Newtonian paradigm. Mercury was observed to precess around the Sun slightly too quickly, by an extra degree for every 8400 years. By the end of the 19th century, attempts to explain the anomaly with classical solutions, such as unseen moons or interplanetary dust, had failed.