The Eclipse That Changed Our View of Gravity

Introduction

The 1919 solar eclipse on Príncipe was one of the rare moments when a remote island in the Gulf of Guinea became the centre of world science. On 29 May 1919, British astronomer Arthur Eddington and his team travelled to Príncipe, then part of Portuguese colonial territory, to photograph stars appearing close to the eclipsed Sun. Their aim was to test Albert Einstein’s new theory of gravity: general relativity. The strange prediction was that the Sun’s gravity should bend starlight, making distant stars appear slightly displaced. The results helped transform Einstein from a little-known theoretical physicist into a global scientific figure and made Príncipe part of the history of modern physics.[European Space Agency]esa.intEuropean Space Agency ESAEuropean Space AgencyESA - Relativity and the 1919 eclipse…

Although the event is sometimes remembered as a dramatic “proof” of a revolutionary idea, the reality is more interesting. The Príncipe expedition was a difficult scientific gamble involving tropical weather, fragile photographic equipment and extremely small measurements. Its mystery was not supernatural, but intellectual: could an invisible distortion of space itself be detected by watching tiny changes in the positions of stars?[ESA Science]sci.esa.intScience ESA Science & TechnologyESA ScienceESA Science & Technology - Relativity and the 1919 eclipseSeptember 1, 2019…Published: September 1, 2019

The 1919 expedition to Príncipe

Príncipe was chosen because it lay along the path of a total solar eclipse, when the Moon would briefly cover the Sun. This created a natural experiment. Normally, the Sun’s glare makes nearby stars impossible to photograph during daylight, but during totality the darkened sky allows astronomers to record stars close to the Sun’s apparent position. The eclipse was especially valuable because the Sun would appear against the Hyades star cluster, providing a useful group of bright reference stars.[Astronomy at Durham University]astro.dur.ac.uknomy at Durham UniversityCelebrating the 1919 eclipse at Principe…

The expedition was organised by British astronomers including Frank Watson Dyson, Astronomer Royal, and Arthur Eddington of the University of Cambridge. Two teams were sent to different locations to reduce the risk of failure from clouds or equipment problems: Eddington travelled to Príncipe, while another group observed from Sobral in Brazil. The joint approach reflected the practical difficulties of early twentieth-century astronomy, where a single storm could destroy months of preparation.[Royal Society Catalogues]catalogues.royalsociety.orgRoyal Society Catalogues Search ResultsRoyal Society CataloguesSearch ResultsNovember 14, 1919…Published: November 14, 1919

Eddington and his assistant Edwin Cottingham set up their instruments at Roça Sundy, a plantation estate on Príncipe. The setting added an unusual human dimension to the story: a tropical plantation landscape became the stage for an experiment that would alter humanity’s understanding of gravity. The work was carried out in challenging conditions, with humidity, technical failures and uncertain weather threatening the observations.[Royal Observatory Greenwich]royalobservatorygreenwich.orgRoyal Observatory GreenwichGeneral Relativity and the 1919 Solar Eclipse…

On eclipse day, heavy rain and cloud created serious doubts about success. At Príncipe, a thunderstorm continued into the morning before conditions improved enough for observations. Eddington managed to obtain a small number of useful photographic images, while the Brazilian team achieved clearer results. Those images later became among the most famous astronomical photographs ever taken.[Royal Observatory Greenwich]royalobservatorygreenwich.orgRoyal Observatory GreenwichGeneral Relativity and the 1919 Solar Eclipse…

Stars, sunlight and Einstein’s theory

The scientific puzzle behind the expedition was simple to describe but extremely difficult to measure. Isaac Newton’s older theory treated gravity as a force acting between objects. Einstein’s general theory of relativity, published in its final form in 1915, proposed something far stranger: gravity is linked to the way mass curves space and time. If this was correct, light itself should follow a curved path when passing near a massive object such as the Sun.[European Space Agency]esa.intEuropean Space Agency ESAEuropean Space AgencyESA - Relativity and the 1919 eclipse…

The eclipse offered a way to test this invisible effect. Astronomers photographed the positions of stars near the Sun during the eclipse, then compared them with photographs taken when the same stars were visible away from the Sun’s glare. If the stars appeared shifted, the explanation could be that the Sun’s gravity had bent their light on its journey to Earth.[Imagine the Universe]imagine.gsfc.nasa.govImagine the Universe Cosmic TimesImagine the UniverseCosmic TimesDecember 8, 2017…Published: December 8, 2017

The predicted movement was tiny. Einstein’s theory predicted a deflection of about 1.75 arcseconds for light passing close to the Sun’s edge — an angle so small that it required careful photography and measurement. For comparison, an arcsecond is a very small fraction of a degree, far beyond what the human eye can notice.[OpenAIRE - Explore]explore.openaire.euOpen AIREExploreIX. A determination of the deflection of light by the sun's gravitational field, from observations made at the total ec…

The results from Príncipe and Sobral were presented in London in November 1919 at a joint meeting of the Royal Society and Royal Astronomical Society. The announcement suggested that the observations supported Einstein’s prediction rather than the smaller deflection expected from Newtonian gravity. Newspapers quickly presented the result as a scientific revolution, and Einstein became internationally famous.[European Space Agency]esa.intEuropean Space Agency ESAEuropean Space AgencyESA - Relativity and the 1919 eclipse…

Science solving the impossible

The Príncipe eclipse became famous because it appeared to answer a question that had seemed almost beyond human reach: could scientists measure the shape of space itself? The answer helped establish general relativity as the leading description of gravity and opened the way for later discoveries involving black holes, gravitational waves and gravitational lensing, where gravity bends light from much more distant objects.[arXiv]arxiv.orgEinstein and Eddington and the consequences of general relativity: Black holes and gravitational wavesNovember 5, 2019…Published: November 5, 2019

However, the story is more complicated than the popular version suggests. Later historians and philosophers of science questioned aspects of the 1919 analysis, particularly the way some measurements were selected and interpreted. Critics argued that the original observations were not as decisive as later retellings implied. Modern reassessments have generally concluded that the famous experiment was an important early confirmation, but not the only reason scientists accepted relativity. The theory was strengthened over decades by many additional tests.[arXiv]arxiv.orgThe 1919 eclipse results which verified General Relativity and their later detractors: a story re-toldOctober 26, 2020…Published: October 26, 2020

This debate does not make Príncipe’s role less significant. Instead, it shows how science actually works: extraordinary ideas are tested with imperfect instruments, analysed, challenged and improved. The eclipse was not a magical instant when a camera “proved” a theory beyond doubt; it was a landmark step in a continuing process of measurement and verification.[arXiv]arxiv.orgThe 1919 eclipse results which verified General Relativity and their later detractors: a story re-toldOctober 26, 2020…Published: October 26, 2020

For São Tomé and Príncipe, the eclipse remains a unique piece of scientific heritage. The islands are often associated with rainforest, colonial history and cultural traditions, but Príncipe also holds a place in the history of humanity’s attempts to understand the universe. A quiet tropical morning in 1919 produced one of the most famous scientific mysteries of the modern age: the discovery that even light itself can be moved by gravity.[Eclipse 1919]eclipse1919.orgEclipse 1919Eclipse 1919 - Home…

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Endnotes

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Title: European Space Agency ESA
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Link:https://eclipse1919.org/

3. Source: sci.esa.int
Title: Science ESA Science & Technology
Link:https://sci.esa.int/web/observational-astronomy/-/13851-relativity-and-the-1919-eclipse

Published: September 1, 2019

4. Source: explore.openaire.eu
Title: Open AIRE
Link:https://explore.openaire.eu/search/result?pid=10.1098%2Frsta.1920.0009

5. Source: arxiv.org
Link:https://arxiv.org/abs/1911.01959

Published: November 5, 2019

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Published: October 26, 2020

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Link:https://astro.dur.ac.uk/~rjm/Principe/

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Published: May 1919

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Title: Royal Society Catalogues Search Results
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Published: November 14, 1919

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Title: Imagine the Universe Cosmic Times
Link:https://imagine.gsfc.nasa.gov/educators/programs/cosmictimes/online_edition/1919/eclipse.html

Published: December 8, 2017

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Link:https://www.astro.dur.ac.uk/~rjm/Principe/exhibition.php

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Title: hubble gravitational lenses
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Published: August 8, 2025

14. Source: royalsociety.org
Title: VIDA MILOVANOVIC TELL
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Published: August 6, 2024

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Published: December 8, 2017

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Published: December 8, 2017

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Title: Image: Comme
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Published: May 29, 2009

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Published: January 1, 1979

22. Source: astro.dur.ac.uk
Link:https://astro.dur.ac.uk/~rjm/Principe/photos/

Published: May 2009

23. Source: astro.dur.ac.uk
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24. Source: degruyterbrill.com
Link:https://www.degruyterbrill.com/document/doi/10.4159/harvard.9780674366688.c132/html

25. Source: eclipse2017.nasa.gov
Title: what’s relative eclipse
Link:https://eclipse2017.nasa.gov/what%27s-relative-eclipse

26. Source: eclipse2017.nasa.gov
Title: By measuring how the images of st
Link:https://eclipse2017.nasa.gov/testing-general-relativity

Published: May 29, 1919

27. Source: lonelyplanet.com
Title: Roça Sundy | São Tomé & Príncipe, Africa | Attractions
Link:https://www.lonelyplanet.com/sao-tome-principe/principe-1340546/attractions/roca-sundy/a/poi-sig/1558356/1340546

Additional References

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31. Source: mpg.de
Title: Arthur Eddington, the famous scientist and secretary of the Royal Astronomical
Link:https://www.mpg.de/9244824/solar-eclipse-1919

Published: May 29, 2019

32. Source: theguardian.com
Title: Eddington worked feverishly and managed to take 16 photographic plates. Late
Link:https://www.theguardian.com/science/2019/may/12/100-years-on-eclipse-1919-picture-that-changed-universe-arthur-eddington-einstein-theory-gravity

Published: May 12, 2019

33. Source: smithsonianmag.com
Title: Niels Bohr Institute, Copenhagen University Though the two-pronged exper
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Published: May 24, 2019

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35. Source: science.esundy.tecnico.ulisboa.pt
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36. Source: scribd.com
Title: Light Deflection by the Sun’s Gravity | PDF | Gravity | Science
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Published: January 1, 1920

37. Source: salon.com
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