Online Astronomy eText: The Sky
The Eddington-Einstein Solar Eclipse Link for sharing this page on Facebook
(also see Eclipses, Lunar Eclipses, Solar Eclipses, and NASA Eclipse Home Page )
Page created Mar 26, 2021

A Matter Of Some Gravity: First Proof Of The Theory Of General Relativity
 According to Einstein's General Theory of Relativity, gravity is a warping of space-time by mass. If that is correct, then the most massive object in the Solar System (the Sun) should be capable of bending the path of light of stars nearly behind it by a measurable amount (Einstein first proposed that in 1907, and published an estimate of its numerical value in 1911); but because the Sun is so bright, stars close to the Sun cannot be seen under normal circumstances, so the only way to tell if that prediction is correct is during a total solar eclipse, when the light of the Sun is blocked by the Moon. The first actual test of Einstein's proposal was provided by the total solar eclipse of May 29, 1919. Sir Arthur Eddington was in charge of expeditions sent to record the eclipse on glass plates, so that the positions of stars visible on those plates could be compared to plates of the same part of the sky taken when the Sun was elsewhere and the stars could be observed at night, with their light (and its path) undisturbed by the effect predicted by Einstein.
 To celebrate the 100th anniversary of the eclipse, the ESO (European Southern Observatory) restored a plate taken in Sobral, Brazil, by astronomers Andrew Crommelin and Charles Rundle Davidson, who had traveled to Sobral from the Greenwich Observatory in London specifically for the purpose of taking the image shown below. The original plate provided (as is still true for all professional images) a black-and-white image of the Sun during totality, and as a result, the ESO restoration was also black-and-white (as shown below). However, as reported on on March 17, 2021 (see the Archive link just above the first color image below), seeing the black-and-white image inspired Petr Horálek (ESO Photo Ambassador, Institute of Physics in Opava) to create a color restoration (all the details of how he and his colleage, Miloslav Druckmüller (Brno University of Technology), created the color version are on the page). Using the latest technology for turning CCD images (which are also almost always black-and-white images) into detailed color images resulted in a stunning view of what a modern, state-of-the-art "true-color" image of the eclipse would have looked like.
 What most people immediately notice is the fine detail in the coona and the spectacular coronal loop (which was easily visible with the naked eye to the original observers); but what is most important about the image are the few faint dots that are stars. Measurements of their positions, when compared with night-sky measurements of their positions, showed that within the limit of accuracy of those measurements, the light from the stars had been bent from its usual straight-line path to the Earth by the Sun's warping of space-time, in exactly the way predicted by Einstein. The sensation this created led to Einstein's Nobel Prize for Physics in 1921, though since those voting for the award didn't fully understand general relativity and were wary of awarding the Prize for that, in case it turned out that the agreement between the eclipse observations and the theory's prediction was merely a coincidence, the Prize was awarded "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect," Einstein's contribution to the photoelectric effect being the prediction that light does not consist of waves, but of particles (photons).
 Note: Although the Prize was for 1921, uncertainties about whether it fully qualified for being awarded led, per a clause in Nobel's will that allowed a prize for one year to be presented the following year, Einstein didn't actually receive it until 1922.
ESO restoration of Crommelin and Davidson's photograph of the solar eclipse of May 19, 1919
Above, the ESO restoration of the May 29, 1919 eclipse photo, with labels for stars
(Image Credit ESO/Landessternwarte Heidelberg-Königstuhl/F. W. Dyson, A. S. Eddington, & C. Davidson)
Below, the color restoration discussed in detail in's Archive
(Image Credit as above, with colorization by P. Horálek (Institute of Physics in Opava) and M. Druckmüller
Color restoration by P. Horálek and M. Druckmüller of Crommelin and Davidson's photograph of the solar eclipse of May 19, 1919
Below, a cropped version of the image above, to show 65 and 67 Tauri (at lower right) (Image Credit as above)
Color restoration by P. Horálek of Crommelin and Davidson's photograph of the solar eclipse of May 19, 1919, cropped to better show stars near the Sun
Below, a news headline and illustration of the principles involved from 1919
(Image Credit, (left) New York Times, Nov 10, 1919; (right) Illustrated London News, Nov 22, 1919)
1919 news headlines and illustration of the results of the May, 1919 solar eclipse's confirmation of Einstein's theory of the bending of starlight by the Sun's mass
Below, the illustration shown above right, enlarged and sharpened to make the text (mostly) legible
The distance shown for the stellar background is less than 17 light-years, and very few stars are that close to us
Also, though the displacement direction (away from the Sun) is correct, its size is (as stated) 500 times smaller than shown
Note that the displacement is larger for stars apparently near the Sun's disc, and smaller for ones further away

Nov 22, 1919 illustration of the results of the May, 1919 solar eclipse's confirmation of Einstein's theory of the bending of starlight by the Sun's mass