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Ellis et al.: 1919 eclipse revisited Ellis et al.: 1919 eclipse revisited 90 years on – the 1919 ecli pse expedition at Príncipe

Richard Ellis, Pedro G Ferreira, Richard Massey and Gisa Weszkalnys return to Príncipe in the International Year of Astronomy to celebrate the 1919 RAS expedition led by Sir Arthur .

he first experiment to observationally near the are confirm Einstein’s General Theory of not normally visible, TRelativity was carried out in , they would be during on a Royal Astronomical Society expedition to a total . observe a total solar eclipse. Sir Arthur Edding- Einstein attempted ton travelled to Príncipe, a small island off the to convince observers west coast of Africa, and sent another team to to measure the gravi- Sobral, Brazil, from where the eclipse would tational deflection. He also be visible. This year, in a new RAS-funded found an enthusiastic expedition organized for the International Year colleague in Erwin of Astronomy, we returned to Príncipe to cel- Findlay-Freundlich, 1: The old plantation buildings at Roça Sundy, Eddington’s base in 1919. ebrate this key experiment that shook the foun- whose expedition to dations of 20th-century science. Russia in August 1914 was scuppered by the America and Africa, but it was important to Since 1687, Sir Isaac Newton’s law of outbreak of war in the very month of the solar be as close as possible to the sub-solar point, had been the workhorse of celestial mechanics. eclipse: as a German national in Russia, he where the Earth’s rotation keeps observers in Newtonian gravity could be used to explain the was arrested. William Campbell, Director of the Moon’s shadow for longer, thereby extend- motions of a host of celestial bodies and the the Lick Observatory, was similarly unlucky in ing the observing window. heavens were reliable and predictable. There his attempts. His missed the 1914 eclipse and Eddington observed the cluster was one small discrepancy: accurate measure- another in June 1918 in Washington State due to from Oxford at night time in January and Feb- ments of Mercury’s orbit did not quite fit the thick cloud. In hindsight, it was quite fortunate ruary 1919. In May, heavy clouds on Príncipe Newtonian paradigm. Mercury was observed for Einstein that no-one between 1911 and 1918 cleared just in time for the eclipse and, during to precess around the Sun slightly too quickly, found cloudless skies; an incorrect calculation the six-and-a-half minutes of totality, Edding- by an extra degree for every 8400 years. By the in his early work meant that his theory would ton took comparison photographs: the stars end of the 19th century, attempts to explain the have been disproved. Einstein refined his calcu- were shifted by the full Einsteinian deflection. anomaly with classical solutions, such as unseen lations during the war and predicted that stars The result was announced to the world in news­ moons or interplanetary dust, had failed. at the limb of the Sun would appear to move papers and then in the Philosophic Transac- 1.7 arcseconds from their true positions. tions of the Royal Society. It helped reunite a Einstein’s new idea It was Sir Frank Dyson, the Astronomer Royal, world torn apart by war. As J P McEvoy wrote New theories were suggested in which gravity, who recognized the unique opportunity of the in Eclipse: “A new theory of the universe, the rather than acting instantaneously, is transmit- 1919 eclipse. The Sun would be in front of the brainchild of a German Jew working in Ber- ted only at the . The most elegant rich Hyades star cluster, the densest population lin, was confirmed by an English Quaker on a of these was Einstein’s General Theory of Rela- of bright stars on the ecliptic plane. These would small African island.” Eddington considered his tivity. Einstein proposed that the fabric of both provide plenty of background sources in which verification of the deflection, which he glimpsed space and time is warped, and that the speed at to measure a deflection. However, Eddington via an early analysis of his plates while still on which these warps can propagate is limited. The became the driving force behind the expedition Príncipe, the greatest moment of his life. precession of Mercury can then be explained: because he was personally inspired by the signif- it is always being attracted towards the place icance of Einstein’s theory. The RAS organized Poor weather where the Sun was three minutes earlier. Warped two parallel teams: Eddington and Edwin Cot- While some historians suggest that Edding- space-time around massive bodies implies that tingham to the cocoa plantation at Roça Sundy, ton’s faith in the General passing rays of light should be deflected. Argu- Príncipe; and Andrew Crommelin and Charles may have blinded him to contrary evidence, a ments can be made for the gravitational deflec- Davidson to Sobral, in northeastern Brazil. recent analysis by D Kennefick has dispelled tion of light by treating it as corpuscular within At the time of Eddington’s expedition, Prínc- this notion. The basis of the controversy was Newtonian theory; however, if space and time ipe formed part of Portuguese colonial terri- that remaining partial cloud at Príncipe permit- are distorted, Einstein calculated that the deflec- tory, and correspondence recently unearthed ted only two successful astrographic plates to tion should be exactly twice that predicted in at the Geographical Society of Lisbon reveals be taken, with five stars on each; the resulting Newtonian theory. It was quickly realized that that the RAS had already begun asking for maps deflection was 1.61±0.30 arcseconds, while Ein- the Sun was the only local body massive enough and permission to visit the island during the stein predicted 1.75 arcseconds. By contrast, sev- to produce a measurable deflection and, while war. The eclipse would be visible across South eral plates taken with an astrograph at Sobral,

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2: Unveiling of the plaque, with President Cassandra on the right. 3: Close-up of the plaque explaining why Eddington came to Príncipe.

with 12 stars on each, yielded a deflection of sparked a brief British–Portuguese controversy to track through calculation the evolution of only 0.93 arcseconds. a few years before Eddington’s visit. the universe as a whole. It has been claimed that Eddington dispensed By the early 20th century, São Tomé and has proved hugely successful in predicting the with some of the Sobral data without good Príncipe had become world leaders in cocoa overall structure of the universe and many finer reason. However, the Sobral plates were out of production. Around 1905, the young British details. For example, it predicts that the universe focus, leading to great uncertainty in the plate journalist Henry Nevinson called attention to is expanding and that distant galaxies are reced- solution. An independent Sobral telescope with the appalling conditions in the Portuguese colo- ing from us at a speed that is approximately pro- a 4 inch lens gave a deflection of 1.90±0.11 nial plantations, alleging infringement of 19th- portional to their distance. This has been amply arcseconds, consistent with the deflection century international treaties on the abolition confirmed by measurements through- observed at Príncipe. And it was in fact Dyson of slavery. These allegations indicted chocolate out the 20th century. The Big Bang universe who initially rejected the results from the Sobral manufacturer Cadbury Brothers, which drew a was hotter and denser in the past, and we have astrograph, not Eddington. In 1979, a reanaly- large part of its supply from Príncipe, forcing the now seen exquisite evidence for the primordial sis of the Sobral plates using modern equipment company to justify its use of Portuguese cocoa fireball in the cosmic microwave background. It measured a deflection of 1.55±0.32 arcseconds. before an English court. Cadburys boycotted is in the cosmic furnace of the initially hot and The expedition was the first to satisfactorily Santomean cocoa until the death rate of labour- dense state that we believe that the first light confirm Einstein’s prediction, and Eddington’s ers declined, but even after the boycott had been elements were forged, in abundances again con- observations from Príncipe were a key compo- lifted continued to import cocoa primarily from sistent with modern measurements. nent of that process. the Gold Coast, where production on locally Although even Einstein saw no practical use owned land had proved more successful. for gravitational lensing, as the bending of light Cocoa controversy The incident remains controversial to this day, by mass has come to be known, the phenomenon The Portuguese discovered the uninhabited with some commentators suggesting that the has emerged as one of the most powerful probes islands of São Tomé and Príncipe in the 1470s. slavery allegations were engineered in order to of the cosmos. Gravitational lenses work effec- They introduced vast plantations of sugar give the British colonies an advantage in the tively like glass lenses, focusing and magnifying cane, followed later by coffee and cocoa. The increasingly overcrowded global cocoa market. light – but on a cosmic scale. They enable us to intensive plantation economy was initially Although we can presume that Eddington was see objects otherwise too far away or too faint made possible by slave labour and, from the aware of the controversy – Lord Cadbury was for even the largest telescopes on Earth. The 19th century, by the so-called service and con- a fellow Quaker, and the trial had been widely lensing signature can also be observed around tract labourers who were brought in primarily publicized – he does not mention it in the few any nearby concentration of mass, regardless of from Angola and Cape Verde. These labourers letters he sent back home from his trip. whether that material is shining. Dark matter is now known to be the most “Although Einstein saw no ’s legacy abundant gravitating material in the universe. practical use for gravitational Einstein’s theory and Eddington’s experiment It is thought to be mysteriously distinct from transformed the landscape of and the baryonic , neutrons and . lensing, the phenomenon has astronomy. The General Theory of Relativity Crucially, dark matter does not interact via the emerged as one of the most established, for the first time, a mathemati- electromagnetic force, so is completely invisible: powerful probes of the cosmos.” cal framework within which it was possible it does not emit, reflect, nor even absorb light.

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4: The team from Britain, São Tomé e Príncipe, Portugal and Brazil. 5: Music and dancing in Sundy: the dêxa .

But it does create a gravitational field and can revenue management law, public information with a parallel team organized by the Lisbon thus be revealed by gravitational lensing. In the campaigns and similar measures are being Geographical Society, under the stewardship of same way that lines of text can become bent designed to curb Santomeans’ expectations of Portuguese social anthropologist Sónia Frias. when read though a magnifying glass, images quick and easy wealth. Whether or not STP’s oil of distant galaxies can appear grossly distorted is explored in the future, of prime importance Celebrations and magnified when behind a concentration of will be the introduction and diversification The official celebrations kicked off on 22 May dark matter. It is possible to work out the shape of sustainable economic measures to help the in Lisbon, with Pedro giving a talk on the legacy and material properties of a magnifying glass country out of poverty. of Eddington’s measurements. The talk was from distorted images of text, even though the It was in this context that the idea of celebrat- attended by Santomean and Portuguese digni- glass is transparent. Using the same technique ing the eclipse observation was first raised by taries and citizens, astronomers, anthropolo- to investigate dark matter, astronomy is cur- one of us, Gisa Weszkalnys, who has worked gists and journalists. The Portuguese Minister rently leading particle physics in what may turn as a social anthropologist in STP, in a conver- of Science closed the day’s ceremonies, discuss- out to be the most important scientific investi- sation with the regional president in 2007. We ing the contemporary relevance of Einstein’s gation of the 21st century. discussed what could be done to improve the General Theory of Relativity. That evening, situation of Príncipe’s barely 6000 inhabitants, Gisa, Richard M and Pedro, along with repre- Our expedition who find themselves considerably disadvantaged. sentatives from the Lisbon Geographical Soci- Príncipe today is an autonomous region of the Everything but the most basic food stuffs has to ety, flew to São Tomé for a week of celebrations. micro-state of São Tomé e Príncipe (STP), which be imported, yet there is no port. Only one small Throughout our stay, we were accommodated with its 170 000 or so inhabitants is the second plane and some irregular ships connect Prínc- in Rombout Swanborn’s Omali Lodge (in São smallest African country. Since independence ipe to its larger twin island. There is a small Tomé) and the Bom Bom Resort (in Príncipe). in 1975, STP has undergone a phase of state hospital, a handful of schools, and very few The week in STP was peppered with a series of socialism, followed, from the early 1990s, by opportunities for young people to find employ- popular talks about Eddington and astronomy. a process of political opening, democratiza- ment. In collaboration with Richard Ellis, a plan A lecture at the Portuguese Cultural Centre in tion and the privatization of previously state- was developed to install an informative plaque downtown São Tomé was lively, and the audi- owned land and property. Its prime export is benefiting both locals and tourists at the Roça ence – comprised mainly of academics, jour- still cocoa, but current output is only a small Sundy plantation, where Eddington made his nalists and members of the government – were fraction of what was once extracted from the observations. The 90th anniversary of the expe- engaged and inquisitive. This atmosphere was islands by Portuguese colonizers. Experiments dition coincided with the International Year of echoed at the Instituto Superior Politécnico, are being made into the production of organic Astronomy 2009, and Richard and Gisa secured the island’s public institute of higher educa- cocoa and spices to revitalize an agricultural “Special Project” status from the International tion, where a sizeable crowd of students turned sector currently geared largely towards subsist- Astronomical Union. During a scouting visit to out to attend a talk on amateur astronomy. ence rather than export. Príncipe in September 2008, they were received We designed a series of posters on Eddington’s Given the absence of any larger industries and by the President of the Republic, Fradique de measurements, gravitational lensing and astron- STP’s continuing isolation, tourism is the most Menezes, who offered the project full support. omy in general, which will remain on display in promising route for economic and social devel- Logistical help was provided by the Dutch eco- the institute’s physics laboratory. opment. The islands are famed for their lush tourism entrepreneur Rombout Swanborn, who The public lectures were supplemented by rain forests, secluded beaches and biodiversity; operates resorts on both islands. receptions at various embassies and interviews efforts are being made to develop both conven- In the UK, Richard and Gisa teamed up with with the local radio, newspapers, the African tional and eco-tourism. Pedro G Ferreira and Richard Massey and branch of Portuguese public television, and Interest in the small island state has also secured the necessary funds from the RAS and the BBC Online and World Service. It became grown in recent years because of the announce- IAU to proceed with the May 2009 events. apparent in our conversations with Santomeans ment of potential offshore oil resources, which Richard Massey designed and oversaw con- during the week of the trip, that Eddington and as yet await exploration. A sophisticated oil struction of the plaque; Pedro coordinated plans Einstein had quickly become a popular topic of

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6: A new issue of stamps celebrate Príncipe’s place in astronomical history. 7: Trying out a telescope at Roça Sundy.

conversation in São Tomé. within an hour. The centrepiece of the celebra- in their observational programmes and, in the An educational campaign organized by the tions then took place in the forecourt of the same way that Eddington used lensing in his test Santomean Ministry of Education, which Sundy Plantation, where President José Cassan- of Newtonian gravity, we will do so again, on toured local schools in the run-up to the , dra unveiled the plaque. The local community larger, more distant scales. had been indispensable to preparing the ground turned out en masse, and a number of flights Our visit to Príncipe was met with great enthu- and raising additional awareness. had been chartered to fly in more diplomats, siasm and interest. Throughout our stay, our The expedition benefited tremendously from government ministers, civil servants, academics hosts repeatedly suggested that similar events such multiple collaborations. During our stay in and journalists. Almost 100 people attended should happen regularly and more often. The São Tomé, three members of the NGO Scientists the subsequent speeches and talks by, among regional and national governments are keen on in the World flew to Príncipe and throughout the others, the Santomean Minister of Education, establishing annual workshops for teachers and week ran workshops with the teachers and chil- President Cassandra, and Pedro representing students, or a permanent centre on Príncipe to dren in the island’s only secondary school. The the RAS and IAU. The formal events were fol- celebrate the experiment. Some of STP’s inter- children engaged enthusiastically in the various lowed by a lunch with open-air entertainment, national diplomatic partners have also shown sessions, learning about modern physics and a session of dance and dêxa, a traditional form interest in financially supporting the endeav- astronomy for the first time. The teachers saw of music from the island. That afternoon, we ours. It is clear that Eddington’s trip to Príncipe the visit as an opportunity to access new and flew back to São Tomé and the following morn- is recognized as an important part of Santomean interesting material with which to engage their ing we returned to Europe. cultural heritage and can be an anchor for ini- students. The unique connection that Príncipe tiatives through which the population of these has with this major scientific event proved to be The future remote islands may, in the future, take part in

a true source of excitement. Eddington’s 1919 measurement of the bending scientific knowledge and discovery. ● On the morning of 28 May we were flown over of light was used to determine the nature of to Príncipe on a small Dornier aeroplane to meet gravity, and this research continues to this day. Richard Ellis, California Institute of Technology, our hosts, Regional President José Cassandra For the past decade, scientists have been con- [email protected]; Pedro G Ferreira, University and members of the Príncipe government. That fronted with the peculiar observation that the of Oxford, [email protected]; Richard afternoon we gave two public talks after which expansion of the universe seems to be accelerat- Massey, Royal Observatory Edinburgh, rm@roe. we were invited for dinner at the Sundy plan- ing away from the Big Bang, rather than slowing ac.uk; and Gisa Weszkalnys, University of Oxford, tation where the local villagers celebrated our down as expected under the force of gravity. The [email protected]. arrival. In the middle of drumming, singing and acceleration can be explained by postulating a dancing, we brought out some telescopes and strange new form of energy, dark energy, which More information observed the clear night sky with enthusiastic is repulsive under gravity and drives space-time http://www.1919eclipse.org

children, young people and adults, and later ate apart. An alternative possibility, preferred by Further reading a delicious meal of traditional dishes in the din- some theorists, is that the apparent acceleration Grant K 2005 A Civilised Savagery: Britain and the New ing hall of the central building of the former is due to deviations from the General Theory of Slaveries in Africa, 1884–1926 (Routledge, London). plantation, hosted by President Cassandra. Relativity on the largest scales. Kennefick D 2009 Physics Today 62 37–42. The formal celebration began on 29 May at Whether Einstein’s theory requires slight McEvoy J P 1999 Eclipse (Fourth Estate, London). Mendes Ferrão J E 2008 in Instituto Marquês de Valle the island’s post office: a new issue of stamps adjustments is now the overriding question in Flôr (ed.) São Tomé, Ponto de Partida (Chaves Ferreira, commemorating Eddington’s visit sold out cosmology, and is driving international research Lisbon). in the field. The geometric nature of gravita- Satre L J 2005 Chocolate on Trial: Slavery, Politics and “It is clear that Eddington’s trip tional light deflection makes it ideally suited the Ethics of Business (Ohio University Press, Athens). to trace the geometry and size of the universe Seibert G 2006 Comrades, Clients and Cousins: Colo- to Príncipe is recognized as an nialism, Socialism and Democratization in São Tomé e at different epochs, and to test small inconsist- Príncipe (Brill, Leiden and Boston). important part of Santomean encies in . Up and coming Waller J 2002 Fabulous Science: Fact and Fiction in the cultural heritage.” experiments incorporate gravitational lensing History of Scientific Discovery(Oxford University Press).

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