In 1919, the Royal Astronomical Society (RAS) launched an expedition to the West African island of Príncipe, to observe a total solar eclipse and prove or disprove Einstein’s General Theory of Relativity. Now, in a new RAS-funded expedition for the International Year of Astronomy (IYA 2009), scientists are back.

Astronomers Professor Pedro Ferreira from the University of Oxford and Dr Richard Massey from the University of Edinburgh, along with Oxford anthropologist Dr Gisa Weszkalnys, are paying homage to the original expedition led by Sir Arthur Eddington and celebrating the 90th anniversary of one of the key discoveries of the 20th century.

Einstein first proposed his General Theory of Relativity in 1915. It describes how any massive object, such as the Sun, creates gravity by bending space and time around it. Everything in that space is also bent: even rays of light. Consequently, distant light sources, behind the massive object, can appear in a different position or look brighter than they would otherwise.

The total eclipse of 29th May 1919 gave scientists the chance to test the theory for the first time. Eddington travelled to Príncipe to observe the eclipse and measure the apparent locations of stars near the Sun. Heavy clouds parted minutes before the eclipse and, with the Sun almost directly in front of them, the stars appeared to be shifted from the positions that Eddington had recorded in Oxford 4 months earlier – direct evidence that our nearest star shapes the space around it.

“This first observational proof of General Relativity sent shockwaves through the scientific establishment,” said Professor Ferreira. “It changed the goalposts for physics.”

To mark the anniversary, in partnership with the International Astronomical Union, São Toméan and Portuguese governments, the team will gather with local people for a series of public talks, the installation of an exhibition in the capital Santo Antόnio, and the unveiling of a plaque at the plantation where the original observation was made. Dr. Weszkalnys feels it “particularly important that in 2009, the International Year of Astronomy, the dramatic role played in the history of science by a tiny island like Príncipe should not be forgotten.”

Eddington’s 1919 measurement of the bending of light was used to determine the nature of gravity. At the time, even Einstein saw no further uses. “But now that gravity is well understood,” said Dr. Massey, “the effect, known as ‘gravitational lensing’, has become one of our most powerful tools to study the Universe.”

Gravitational lenses work in a similar way to ordinary glass lenses, focusing and magnifying light – but on a huge scale. They enable astronomers like Dr Massey to see objects that are otherwise too far away or faint for even the largest telescopes on Earth.

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