On Arthur Eddington and the 1919 Eclipse

Arthur Eddington is mostly remembered for the 1919 solar eclipse experiment, which helped make Einstein and his theory of general relativity famous. But he was also a rather important astrophysicist who worked on the internal structure of stars, radiation and a few other problems in physics. It is a little strange that most people remember him for an experiment that took only a few minutes considering how much else he worked on.¶

Eddington was born in England in 1882 and studied mathematics at Cambridge. He later became a professor of astronomy there. Around 1915, Einstein had just completed his general theory of relativity which suggested that gravity was not simply a force acting between objects, but could instead be understood through the curvature of spacetime. One consequence of this was that light passing close to a massive object, such as the Sun, should be deflected. Einstein predicted an angle of about $1.75$ arcseconds for light passing just beside the Sun. An arcsecond is $1/3600$ of a degree, so measuring this was not exactly much of an easy task.¶

The problem was that stars close to the Sun are normally impossible to photograph because of its brightness. Fortunately a total solar eclipse was expected on 29 May 1919. Two expeditions were organised, one to Sobral in Brazil and another to Príncipe which is an island off the coast of Africa, and that’s where where Eddington travelled. During the eclipse the Moon would block the Sun, allowing the astronomers to photograph stars appearing close to it. They could then compare their positions with photographs taken when the Sun was elsewhere.¶

Unfortunately for Eddington the weather at Príncipe was rather terrible. There were clouds and rain, and only a few photographs could be used. The expedition at Sobral had better conditions, although one of its telescopes had problems with its images. After analysing the photographs the accepted measurements gave approximately $1.61$ arcseconds at Príncipe and $1.98$ at Sobral, both reasonably close to Einstein’s prediction. The results were announced in November 1919, and newspapers soon began reporting that Einstein’s theory had been confirmed.¶

However there is a small controversy here. One of the Sobral instruments had produced a result closer to the older Newtonian prediction, but those photographs were excluded because of problems with the telescope. Some later argued that Eddington had deliberately selected the results that agreed with Einstein. However historical investigations suggest that the exclusion had reasonable instrumental grounds and the analysis was not Eddington’s decision alone. The experiment certainly had limitations, but later measurements have confirmed the deflection of light much more precisely.¶

What I find more interesting is that Eddington did not spend his entire career studying relativity. He also worked extensively on stellar physics, especially how stars maintain their structure and produce radiation. He studied the balance between gravitational attraction and internal pressure and helped develop models of stellar interiors. His name is still associated with the Eddington luminosity¹Check out my post about it here: https://aronnomirdha.com/expository/1238, which describes the point where outward radiation pressure can balance gravity under certain conditions. This is particularly useful in studying accretion around compact objects, including black holes.¶

He also suggested in 1920 that stars might obtain their energy from converting hydrogen into helium. The exact mechanism of nuclear fusion was not understood at the time, but the idea that a small loss of mass could release an enormous amount of energy was already possible through Einstein’s mass-energy relation. It seems quite obvious to us now because we learn about fusion so early but explaining why stars could shine for billions of years was a serious problem back then.¶

Eddington was not always correct though. In the 1930s, Subrahmanyan Chandrasekhar calculated that white dwarf stars should have a maximum mass beyond which electron degeneracy pressure could no longer support them. Eddington strongly opposed this result and publicly criticised Chandrasekhar’s work. Chandrasekhar’s conclusion was eventually accepted and became important in our understanding of compact stars. It is a rather interesting disagreement considering Eddington had previously supported Einstein’s theory when many physicists were still sceptical about it.¶

Eddington died in 1944 leaving behind contributions to both general relativity and astrophysics. I think it is worth remembering that his work was much broader than the famous eclipse. He helped develop our understanding of how stars work, introduced important ideas about radiation and stellar energy, and also made some rather significant mistakes. The 1919 experiment remains an important moment in the history of physics, although it was really just one part of his scientific career.¶

References and Footnotes

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  Check out my post about it here: https://aronnomirdha.com/expository/1238 ↩︎