Image 1 Caption: The Solar eclipse of May 29, 1919 (a total solar eclipse) from a reprocessed image by 1919 Solar Eclipse Expedition. The corona and a large prominence are seen. The background stars are in the constellation Taurus. The background stars, in fact, belong (at least mostly) to the open star cluster Hyades which is close in angle to the bright red naked-eye star Aldebaran (which is NOT a member of the Hyades and is much closer to Sun). Neither Aldebaran nor brighter members of the Hyades are in the image.

Image 2 Caption: "Negative photograph of the 1919 solar eclipse taken from the report of Sir Arthur Eddington (1882--1944) on the 1919 Solar Eclipse Expedition to verify Albert Einstein's (1879--1955) prediction of the bending of light around the Sun: i.e., of gravitational lensing by the Sun." (Somewhat edited.)

Because it is a negative photograph, bright is dark and dark is bright. We are seeing near side of the Moon in its nighttime (it's the white circle) during a total solar eclipse. The sky is whitish and the solar corona is black.

Features:

1. Albert Einstein's (1879--1955) theory of general relativity presented in 1915 (Wikipedia: History of general relativity) predicted that light rays should be bent by gravity: an effect we now call gravitational lensing.

2. In years immediately after 1915, there was high interest in quantitatively testing gravitational lensing. The only light rays which would be sufficiently bent for such a test were those that pass very close to the Sun coming from background stars. These stars could only be observed during a total solar eclipse. The 1919 solar eclipse was highly favorable for the test since it had very long period of totality 6 minutes 50.75 seconds (which permitted long exposures to photographically image faint stars) and a large number of faint background stars to image in the Hyades. Observing a large number of faint background stars permitted multiple tests which in turn allow for good statistical inference in making the test. The 1919 solar eclipse was also after the end of World War I (1914--1918) which was convenient for everyone.

3. Observations for the test were carried out by what we now call the 1919 Solar Eclipse Expedition led by Sir Arthur Eddington (1882--1944) (a cosmologist among other things) and Sir Frank Watson Dyson (1868--1939) (an expert on solar astronomy and solar eclipse observations). Dyson was Astronomer Royal which meant director of the Royal Greenwich Observatory before 1972.

   By the by, Sir Frank Watson Dyson has NO known relation to physicist Freeman Dyson 1923--2020, but their families both originated in West Yorkshire where the surname Dyson is common, and so they probably are related way back.

   

4. Image 3 Caption: Clockwise: Albert Einstein's (1879--1955), Paul Ehrenfest (1880--1934), Willem de Sitter (1872--1934), Hendrik Lorentz (1853--1928), and Sir Arthur Eddington (1882--1944) in the office of de Sitter, Leiden University, Leiden, the Netherlands, 1923.

5. Two teams from 1919 Solar Eclipse Expedition were sent to make the observations on the eclipse path: one to Sobral, Creara in Brazil and the other (which was led by Eddington personally) to Principe, an island of the west coast of Africa. Useful observations were obtained at both locations.

6. To within observational error, the observations of the 1919 Solar Eclipse Expedition were consistent with gravitational lensing (as predicted by general relativity) and inconsistent with a Newtonian physics prediction of light bending by gravity and the prediction of NO light bending by gravity. Of course, the results did NOT prove general relativity absolutely, but they were strong evidence for it and showed that is passed a falsification test that the other hypotheses did NOT. The reasonable leading physicist of that time would conclude that general relativity was the best available theory of gravity and most did so. Circa 2020, general relativity is still the best available theory of gravity and it has passed many more falsification tests and failed none. It may well be the true emergent theory of gravity at the macroscopic scale: i.e., the macroscopic limit of quantum gravity---for which there is NO verified theory as of now.

7. The results of the 1919 Solar Eclipse Expedition were announced at a meeting of the Royal Society (the United Kingdom's national academy of sciences) on 1919 Nov06. The announcement made Einstein world famous. Before he his fame was mostly only among physicists.

8. Later of observations of gravitational lensing to circa 1960 generally confirmed the predictions of general relativity, but did NOT improve the accuracy/precision of the 1919 Solar Eclipse Expedition (see Gilmore & Tausch-Pebody 2020, p. 10--11). This was probably largely because total solar eclipses used were less favorable than the highly favorable 1919 solar eclipse (see Gilmore & Tausch-Pebody 2020, p. 10--11). Since 1960, the predictions of general relativity for gravitational lensing have been verified to very high accuracy/precision.

9. There is a sorry story to add. In 1980, an article in the history of science genre (also relevant to the philosophy of science) alleged that the results of the 1919 Solar Eclipse Expedition suffered from confirmation bias and did NOT really verify general relativity. Many people, including yours truly, were vaguely aware of the allegation and vaguely thought it must be somewhat true if for NO other reason than later analysis should be better than earlier analysis. By 1980, of course, the allegation had NO affect on people's confidence in general relativity which had been been verified to very high accuracy/precision by many other tests. Vaguely believing the allegation simply meant one vaguely believed that people in 1919 believed in something that was true for inadequate reasons. It is always possible to believe true things for inadequate/wrong reasons.

   In fact, the 1980 article was wrong (see Gilmore & Tausch-Pebody 2020, and references therein). There was NO confirmation bias by Eddington and Dyson and their results were as good as they thought they were. Of course, they and everyone else knew that they could have had systematic errors of which they were unaware. But, in fact, all the verifications of general relativity since then show there were NO systematic errors.

   The 1980 article's allegation of confirmation bias can be relegated to a footnote in the history of the history of science.

Images:

1. Credit/Permission: © ESO, Heidelberg-Koenigstuhl State Observatory, Sir Frank Watson Dyson (1868--1939), Arthur Eddington (1882--1944), Charles Rundle Davidson (1875--1970), 2019 (uploaded to Wikimedia Commons by User:Jmencisom, 2019) / Creative Commons CC BY-SA 4.0.
   Image link: Wikimedia Commons: File:Highest resolution image of the 1919 solar eclipse.tif.
   
2. Credit/Permission: Sir Frank Watson Dyson (1868--1939), Sir Arthur Eddington (1882--1944), Charles Rundle Davidson (1875--1970), 1919 (uploaded to Wikimedia Commons by User:Fastfission, 2005) / Public domain.
   Image link: Wikimedia Commons: File:1919 eclipse negative.jpg.
   
3. Credit/Permission: © H. van Batenburg, 1923 (uploaded to Wikimedia Commons by User:Vysotsky, 2014) / CC BY-SA 3.0.
   Image link: Wikimedia Commons: File:PhysicistsinLeiden.jpg.