In 1919, British scientist Arthur Eddington embarked on an historic expedition to the island of Principe, off the west African coast, to observe a total solar eclipse. David Tennant played Eddington in a recent BBC drama Stars in the Hyades cluster were behind the Sun during the eclipse, and appeared to shift from their true positions. This discovery provided the first experimental confirmation for Albert Einstein's theory of general relativity. On 29 May, the island of Principe marked the 90th anniversary of the experiment, described by many as one of the most important scientific achievements of the 20th Century. Astronomer Richard Massey, from the Royal Observatory Edinburgh, has been in Principe with colleagues and is writing a diary for BBC News. ANNIVERSARY CELEBRATIONS The visitors received a warm welcome. From l to r: Pedro, Gisa and Richard

After a long journey, we reached the Sundy plantation where Eddington observed the 1919 eclipse. We received a huge welcome: the residents stayed up dancing, drumming and singing in our honour. And their delicious but potent pineapple homebrew had quite the effect on my tired legs. Excitement on the island has mounted during the past week, with visits to nearby schools and reports on the radio. In the morning, a convoy of almost a hundred visitors returned to the plantation. For a population of less than 5,000, this shows how the islanders have taken to their anniversary celebration. The day is conveniently sandwiched between Africa Day earlier in the week, and Children's Day on Monday. Both are already excuses for a party here, and this bridges the gap. Just as in 1919, heavy clouds cleared right on schedule. The president of Principe unveiled the Royal Astronomical Society plaque, and launched a new set of postage stamps featuring Eddington. The anniversary has attracted lots of interest from the island's residents

I was pleased to see people gathering around and discussing the plaque's text during the day. This island has become quite the centre of general relativity! It's also a simply beautiful place. Island-hopping back by small plane shows the golden sandy coves slicing between bright blue sea and verdant rainforest. And I've found only the friendliest people, with more time to help a lost or confused stranger than I am used to expect. The island is poor but staggeringly rich. The challenge faced by this small nation is to maintain its currently undiscovered paradise while developing responsible tourism and exploiting recently discovered oil reserves. WARPED SPACE AND BENT LIGHT During this week in 1919, Sir Arthur Eddington profoundly changed the way we view the Universe. Eddington watched stars near the Sun during a total solar eclipse on the African island of Principe. When the Sun was in front of the stars, they appeared to move away from their true positions, which Eddington had recorded in Oxford, three-and-a-half months earlier. This was the first observational proof of Einstein's theory of general relativity. Interestingly, Einstein's first celebration was, like Eddington, to write to his mum. There must be a moral to take from this common theme. General relativity describes how any massive object, such as the Sun, produces gravity by bending space and time around it. Everything in that space is also bent: even rays of light, and these get deflected. Light doesn't always travel in straight lines. Gravitational lensing occurs when the light from a distant object is bent around a massive object in between it and the observer. The massive body distorts the spacetime (represented as the yellow grid) making the distant object look distorted

Distant sources behind the massive object can therefore appear to move - or also get brighter or change shape. Einstein's prediction of this effect was used to determine the nature of gravity. But now that it is well understood, the effect of "gravitational lensing" has become one of the most powerful tools used by astronomers to probe the Universe. Gravitational lenses work effectively like glass lenses, focusing and magnifying light - but on a huge scale. They let us see objects otherwise too far away or faint for even the largest telescopes on Earth. Scientists have recently discovered that most of the Universe is in the form of invisible dark matter and dark energy. But because it still has gravity, it is revealed by gravitational lensing. Images of distant galaxies that appear grossly distorted and magnified in Hubble Space Telescope photographs are a sure sign that a lot of dark matter is sitting in front of them. To mark this anniversary, and the special place that this island holds in the history of science, we will, on Friday, unveil an informational plaque at the site on Principe, amid a series of talks from us and local people. SETTING OFF What was arguably the most important scientific experiment of the 20th Century, happened in a slave plantation on a remote African island. In 1919, the Royal Astronomical Society (RAS) launched an expedition to observe a total solar eclipse from Principe, to prove or disprove Einstein's theory of general relativity. This week, we are going back. Astronomers Pedro Ferreira and myself, with Oxford University anthropologist Gisa Weszkalnys, are paying homage to the original expedition by Sir Arthur Eddington. The island holds a special place in the history of science. In concert with the local and Portuguese governments, we shall be giving a series of public talks, unveiling an informational plaque at the plantation, and installing an exhibition in the capital. The team will unveil an informational plaque on Principe

Our expedition left on Thursday 21 May, and will reach a climax on 29 May, the anniversary of the eclipse. In the aftermath of World War One, Eddington's journey to Principe took six weeks by steam ship. He frequently wrote home and his archives at Trinity College, Cambridge, tell an adventure from a forgotten age. Eddington got stuck on Madeira for three weeks, when all the ships were full of returning troops. Amid ongoing rationing in Britain, the sugar and exotic fruits were welcome relief. But he seems some sort of fanatic for Madeira bananas, eating about a dozen every day. Most of his letters went to his mother; somehow he failed to mention the island's casino. Principe is still not easy to reach. From Edinburgh, it will take me four flights, some of which run only once a week. Getting all of the equipment there on time is also proving only slightly less of a challenge than in 1919. To a modern scientist, Eddington's archives also provide a fascinating snapshot into the frustratingly slow pace of early 20th Century research. We now rely totally on the internet to maintain the pace of international collaborations. Wi-fi permitting, I will continue the story next week from Principe. Richard Massey is an astronomy fellow at the Royal Observatory Edinburgh and is a visiting associate in astronomy at the California Institute of Technology (Caltech). Gisa Weszkalnys is an anthropologist at the University of Oxford who studies the economics of Principe island. Pedro Ferreira is a professor of astronomy at the University of Oxford



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