---
title: "Eddington experiment"
chunk: 2/5
source: "https://en.wikipedia.org/wiki/Eddington_experiment"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:39:05.943562+00:00"
instance: "kb-cron"
---

== Theory ==

The theory behind the experiment concerns the predicted deflection of light by the Sun. The first observation of light deflection was performed by noting the change in position of stars as they passed near the Sun on the celestial sphere. The approximate angular deflection δφ for a massless particle coming in from infinity and going back out to infinity is given by the following formula:

  
    
      
        δ
        φ
        ≈
        
          
            
              2
              
                r
                
                  s
                
              
            
            b
          
        
        =
        
          
            
              4
              G
              M
            
            
              
                c
                
                  2
                
              
              b
            
          
        
        .
      
    
    {\displaystyle \delta \varphi \approx {\frac {2r_{s}}{b}}={\frac {4GM}{c^{2}b}}.}
  

Here, b can be interpreted as the distance of closest approach. Although this formula is approximate, it is accurate for most measurements of gravitational lensing, due to the smallness of the ratio rs/b. For light grazing the surface of the Sun, the approximate angular deflection is roughly 1.75 arcseconds. This is twice the value predicted by calculations using the Newtonian theory of gravity. It was this difference in the deflection between the two theories that Eddington's expedition and other later eclipse observers would attempt to observe.

== Expeditions and observations ==

The aim of the expeditions was to take advantage of the shielding effect of the Moon during a total solar eclipse, and to use astrometry to measure the positions of the stars in the sky around the Sun during the eclipse. These stars, not normally visible in the daytime due to the brightness of the Sun, would become visible during the moment of totality when the Moon covered the solar disc. A difference in the observed position of the stars during the eclipse, compared to their normal position (measured some months earlier at night, when the Sun is not in the field of view), would indicate that the light from these stars had bent as it passed close to the Sun. Dyson, when planning the expedition in 1916, had chosen the 1919 eclipse because it would take place with the Sun in front of a bright group of stars called the Hyades. The brightness of these stars would make it easier to measure any changes in position.
Two teams of two people were to be sent to make observations of the eclipse at two locations: the West African island of Príncipe and the Brazilian town of Sobral.
The Príncipe expedition members were Eddington and Edwin Turner Cottingham, from the Cambridge Observatory, while the Sobral expedition members were Andrew Crommelin and Charles Rundle Davidson, from the Greenwich Observatory in London. Eddington was Director of the Cambridge Observatory, and Cottingham was a clockmaker who worked on the observatory's instruments. Similarly, Crommelin was an assistant at the Greenwich Observatory, while Davidson was one of the observatory's computers.
The expeditions were organised by the Joint Permanent Eclipse Committee, a joint committee between the Royal Society and the Royal Astronomical Society, chaired by Dyson, the Astronomer Royal. The funding application for the expedition was made to the Government Grant Committee, asking for £100 for instruments and £1000 for travel and other costs.

=== Sobral ===

In mid-1918, researchers from the Brazilian National Observatory, determined that the city of Sobral, Ceará, was the best geographical position to observe the Solar Eclipse. Its director, Henrique Charles Morize, sent a report to worldwide scientific institutions on the subject, including the Royal Astronomical Society, London.
The Greenwich Observatory team sent to Brazil consisted of Charles Davidson and Andrew Crommelin, with Frank Dyson coordinating everything from Europe and, later, being responsible for analyzing the team's data. The team arrived in Brazil on March 23, 1919, and its gear was waived without inspection as a courtesy from the Brazilian government. While Eddington took part in the Príncipe expedition, it is unknown why Dyson did not travel to Brazil.
The gear was made by two astrographic telescopes coupled to mirror systems known as coelostats; a main telescope from the Royal Greenwich Observatory with a 13-inch aperture and mounted to a 16-inch coelostat and a small backup telescope with a 4-inch aperture borrowed from Aloysius Cortie.
On April 30 the team arrived at Sobral. The eclipse day started cloudy, but the sky cleared and the Moon's disk began to obscure the Sun shortly before 8:56 am; the eclipse lasted 5 minutes 13 seconds. The team remained at Sobral until July to photograph the same star field at night.
The main telescope recorded twelve stars, while the backup one recorded seven. The main telescope had blurred images, which were discarded from the final conclusion though its estimated deflections were closer to the Newtonian-based prediction, while the smaller one had the clearest images and was deemed the most trustworthy and had a estimated deflection slightly above the Einsteinian prediction. Daniel Kennefick defends that without the Sobral photographs, the results of the 1919 eclipse would have been inconclusive and that the expeditions during future eclipses failed to improve the data.
The British team was joined by the Brazilian team led by Henrique Charles Morize and the astronomers Lélio Gama, Domingos Fernandes da Costa, Allyrio Hugueney de Mattos and Teófilo Lee with the objective of producing spectroscopic observations of the Sun's corona.
The team set its gear at a plaza in front of the church of Patrocínio, where the Eclipse Museum is today. The team took several 24-by-18 and 9-by-12 cm plates capturing the Sun and the stars' positions near its edge, but unfortunately, no meaningful conclusions were drawn from the data produced by the Brazilian team, and its contribution was defined as just logistical support for the British team and climate observations. Its plates were restored by the National Observatory in 2015, while the British team plates were lost after 1979.
The third expedition from that day was formed by Daniel Maynard Wise and Andrew Thomson, from the Carnegie Institution. Their goal was to study the eclipse effects on the magnetic field and atmospheric electricity.
In 1925, Einstein stated to the Brazilian press about the results, "The problem conceived by my brain was solved by the bright Brazilian sky".