Celebrating the 20th Century's most important experiment


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 Antonio, 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.


Dr Gisa Weszkalnys

University of Oxford

Mob: +44 (0)7800 554845

E-mail: g.weszkalnys.97@cantab.net


Dr Richard Massey

Institute for Astronomy, University of Edinburgh

Royal Observatory Edinburgh

Mob: +44 (0)7740 648080

E-mail: rm@roe.ac.uk


Professor Pedro Ferreira

University of Oxford

Mob: +44 (0)7796 695036

E-mail: p.ferreira1@physics.ox.ac.uk


Eleanor Gilchrist, PR Officer

Royal Observatory Edinburgh

Tel: +44 (0)131 668 8397

E-mail: eleanor.gilchrist@stfc.ac.uk


Príncipe eclipse expedition: http://www.1919eclipse.org/

São Tomé and Príncipe tourist board: http://www.turismo-stp.org/pages/en/


Diagram of experiment from Illustrated London News in 1919 Headline from Yew York Times 1919
Illustrative (and amusing) newspaper headlines from the time.






Picture of Plaque
The Plaque.


A map showing the location of Príncipe http://www.1919eclipse.org/principe.php

looking up

A selection of images of the trip is available from the expedition website.



The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.


The International Year of Astronomy 2009 (IYA 2009) will be a global celebration of astronomy and its contributions to society and culture. It is intended to stimulate worldwide interest not only in astronomy, but in science in general, with a particular slant towards young people. IYA 2009 will mark the 400th anniversary of the monumental leap forward that followed Galileo Galilei’s first use of the telescope for astronomical observations. In the UK the chair of IYA2009 is Professor Ian Robson, director of the UK Astronomy Technology Centre in Edinburgh, and the co-ordinator for IYA 2009 activities is Steve Owens, also a UKATC employee. UK IYA 2009 activities are jointly funded by the Royal Astronomical Society (www.ras.org.uk), the Institute of Physics (www.iop.org) and the Science and Technology Facilities Council (www.stfc.ac.uk).

IYA 2009: UK home page