© Robin Catchpole, Institute of Astronomy, CambridgeThe application of Einstein’s theories of special and general relativity are central to the lives of all of us who rely on satellite navigation (GPS). Without application of these theories, positional accuracies would be measured in kilometres not centimetres. Very simply, Special Relativity tells us that the moving clocks on satellites seem to us to run slow and General Relativity tells us that the clocks situated ever farther from the surface of the Earth will appear to us to run fast. It is by measuring the difference in time given by different satellites, that we can triangulate our position on Earth.
Einstein published his theory of Special Relativity in 1905 and his General Theory in 1915. One of the predictions of the General theory, is that the Sun will displace the images of nearby stars by an amount;
displacementradian = 4GM⊙/c2r ≡ 1.75′′ r⊙/r
Where r⊙ is the radius of the Sun and r the radial distance from the centre of the Sun. On the other hand, assuming that the energy of light is subject to gravitation in the same way as ordinary mass, Newton’s theory, predicts half the displacement. While a third option was that there would be no displacement at all.
Einstein’s theory had successfully explained a small discrepancy in Mercury’s orbit around the Sun, but his prediction of the gravitational redshift of absorption lines in the Sun had not been confirmed.
Kennefick (2012) suggests that the few physicists and theoreticians, such as Eddington who understood Einstein’s theory, needed no observational proof, but many astronomers, including Dyson and the wider world were highly sceptical of the theory, so the results of the 1919 eclipse would be crucial to its acceptance. Kennefick also puts to rest any suggestion that Eddington selected his results to support Einstein’s prediction.
An unsuccessful attempt was made to measure the deflection, by a group from the Lick Observatory at the total eclipse of 8 June 1918.
The line of the 1919 total eclipse crossed the equator, stretching from South America to Uganda, giving almost 6 minutes of darkness, during which the Sun and surrounding stars could be photographed.
A British expedition was organised under the direction of Dyson. It was decided that two groups of two observers would be sent. One group, consisting of staff from the Royal Observatory, Greenwich, went to Sobral in Brazil. The second group, from the Cambridge Observatory, went to the Island of Principe, a small volcanic island 220 km off the West African coast. The Greenwich group consisted of Andrew Crommelin and his more junior colleague Charles Davidson. The Cambridge group consisted of the Observatory’s Director, Arthur Eddington, and Edwin Cottingham who looked after the Observatory’s clocks. The expeditions were supported and partially funded by the joint standing eclipse committee of the Royal Society and Royal Astronomical Society.
Both parties convened at Greenwich and then left Liverpool together on 8 March 1919 on the SS Anselm . At Madeira, Eddington and Cottingham embarked on SS Portugal for Principe, while Crommelin, and Davidson continued on the Anselm arriving at Para (now called Belem), on the 23 March. Since they had time to spare they decided to continue the trip in the Anselm, another thousand miles up the Amazon to Manaus. Crommelin (1919) comments on ‘the luxurious forests of the Amazon, with their denizens of gorgeous plumage’ returning to Belem on the 8 April. From there they took a ship 580 miles east along the coast to Camocim and then the train, for the final 70 miles to Sobral, arriving on 30 April. At Sobral they were joined by an eclipse party from Washington and another from Brazil.
Both parties took identical 13-inch astrographic lenses of 11 feet 3 inches focal length. That taken to Sobral came from the Astrographic Telescope at Greenwich. That taken to Principe came from the Astrographic Telescope at Oxford. The object glasses were attached on arrival to hollow steel tubes (made by Harvey & Co.), with the lens at one end and the photographic plate holder at the other and then mounted horizontally so that the sunlight could be reflected into them by a 16 inch diameter flat mirror.
The flat mirror or coelostat, was slowly rotated to follow the Sun, using a clock driven by a heavy weight, that ran for about half an hour before the weight had to be rewound. Both astrographic lenses were stopped down to 8 inches diameter to improve their image quality.
The Greenwich Sobral group, also took a 4 inch diameter lens with a focal length of 19 feet, loaned to them by Father Cortie. This was fed by an 8 inch diameter coelostat loaned by the Royal Irish Academy. The Greenwich group also took a 3½ inch Detached Telescope (R.O. No. 3) and the Eros Micrometer.
Mounting the telescope horizontally avoids the need to transport and align a heavy equatorial mount and makes it easier to keep the apparatus cool and free of wind shake, but the flat coelostat mirror introduces an extra optical element, capable of spoiling the image quality.
Having arrived in Sobral, Crommelin and Davidson selected a convenient site for their observing shed just in front of the house where they were staying. Located on the race-course of the Jockey Club, their chosen site had the advantage of a nearby covered grand stand that they were able to use for unpacking, storage and preparatory work.
Although the coordinates of the Greenwich observing site do not appear to have been recorded and the race-course no longer exists, an examination of surviving photographs showing its position relative to surviving buildings (Figs 2 & 10, in Crispino (2018)), suggests that the Greenwich observing site was located at the point indicated on the map below (3.692164S, 40.352800W).
Three days before the eclipse, it rained heavily settling the dust and clearing the sky. Totality started at about 9 in the morning. The day started cloudy and then cleared during totality except for a minute in mid eclipse, when there was thin cloud. The thin cloud reduced the brightness of the Sun, enough to show a large prominence on the limb on one of the their plates.Seven out of the eight images taken with the 4 inch Cortie lens showed star images. When measured back at Greenwich, they provided excellent confirmation of Einsteins prediction as well as it’s expected variation with distance. One of these plates with some of the stars marked is shown here.
The 19 plates taken with the astrographic lens were very disappointing, the star images appearing diffused and out of focus. This was put down to the distortion of the coelostat mirror caused by the heat of the Sun. Assuming the blurring was due to a change of focal length, gave a result close to Newton’s, while assuming the focal length had not changed, gave a result closer to Einstein, both with such a large uncertainty that the results were discounted.
After the eclipse, the Sobral group packed away their equipment and went back down the coast to Fortaleza, returning on 9 July, when the stars they had observed around the Sun would be at the same altitude but in the dark pre-dawn sky. They set up their equipment and took more plates, to compare with the eclipse plates. One plate was exposed with the plate back to front, meaning that the star light passed through the glass before forming an image in the emulsion. This plate was then placed emulsion to emulsion, first with the eclipse plates and then the star field plates and the differences in star positions were measured. Images obtained before dawn with the astrographic lens were found to be in perfect focus, suggesting that the problem had indeed been with the coelostat.
The Sobral team finally returned to Greenwich on 25 August.
At Principe on 29 May, the day of the eclipse started with a very heavy thunderstorm which finished at 11:30. The sky began to clear just before totality at 14:13 and was briefly completely clear. Stars appeared on 7 of the plates and the remainder showed good images of the prominence. Only two plates were off sufficient quality and gave results consistent with Einstein’s prediction.
Because of an impending strike by the steamship company, Eddington and Cottingham left Principe on 12 June and got back to Liverpool via Lisbon on 14 July.
They would have had to wait almost 6 months in Principe for the Hyades to appear in the night sky at the same position as they had during the eclipse, so the calibration plates of the eclipse field were taken with the astrographic, earlier in January and February that year, at Oxford, where the telescope was normally sited.
After the eclipse, Eddington was also able to enjoy some of the pleasures of Principe. He wrote to his mother (Eddington papers A4/9); ‘lunch on the beach off fish which we watched being caught’. ‘All the beaches are very pretty - a strip of golden sand between the cocoanut palms and the blue sea. I had a good bathe at Lapa – the only time in Principe & a black man went with me to see that I did not go too near the sharks.’ Other outings included, a Monkey Hunt at which no monkeys were seen and a trip to the ruined palace and church of a famous female slave dealer (Maria Correia) on the beach near Bom Bom.
Returning home on the SS Zaire, Eddington wrote to his mother; ‘ But the one good plate that I measured gave a result agreeing with Einstein and I think I have got a little confirmation from a second plate’ (Eddington papers).
One 12 September Eddington attended the British Association for the Advancement of Science meeting in Bournemouth, where he showed the eclipse plates with the spectacular prominence and mentioned that measurements made to date indicated a deflection somewhere between the predictions of Newton and Einstein (Observatory Magazine, 1919, 42, 361).
On 27 September Einstein (collected papers) wrote to his mother; ‘Today some happy news. H. A. Lorentz telegraphed me that the English expeditions have really verified the deflection of light by the Sun’. Before going on to ask after his mother’s health.
The results of combined observations from Cambridge and Greenwich were finally announced to the world at a joint meeting of the Royal Society and Royal Astronomical Society on 6 November 1919 (Observatory Magazine. 1919, 42, 389). They were subsequently published in Dyson, Eddington & Davidson (1920). In this paper Diagram 2 (below), based on the Sobral results with the 4 inch Cortie lens, convincingly shows that the variation of displacement with distance from the Solar limb closely follows Einstein’s predictions and differs significantly from Netwon’s prediction.
Contemporary barriers to a ready acceptance of the theory seem to have been twofold. Firstly, the reported non detection of the gravitational redshift in the Sun by St John (1917) and the feeling that a fundamentally new theory should allow itself a simple explanation, not requiring sophisticated mathematics. The gravitational redshift in the Sun (0.6 km s-1 ) is made difficult to detect by the convective motion of gasses at the surface of the Sun. Gravitational redshift has since been measured in white dwarf stars and in the laboratory.
Although Eddington is the name most associated with this eclipse and its success in confirming Einstein’s prediction, his results would have been of little value, without the observations made by Crommelin’s party from the Royal Observatory, Greenwich.
The following day, he went to London, where he was the guest of the former Labour Lord Chancellor and fellow of the Royal Society, Richard (Viscount) Haldane, who accompanied him to a meeting of the Royal Astronomical Society (chaired by Eddington) and then co-hosted a dinner for the Einsteins at his house in Queen Anne’s Gate with his sister. According to The Times, the following guests also attended: Randall Thomas Davidson, Archbishop of Canterbury; Charles Sherrington, president of the Royal Society; Frank Dyson, the Astronomer Royal; Joseph J Thompson, Eddington, Professor Whitehead; Ernest Barker, the Principal of King’s College; Lady Horner; Dr Freudlich; Edmund Gosse; Professor JS Haldane, Haldane’s brother, and Graeme Haldane, Haldane’s nephew. The Prime Minister was unavoidably prevented from attending. After the dinner, a number of other guests arrived for a reception, including George Bernard Shaw.
Over the following days, Einstein attended many other events (some with Haldane and some without), before returning to Berlin on 17 June. They included laying a wreath on Newton’s grave in Westminster Abbey on 13 June.
In 1979, Harvey (1979) reported the re-measurement of five of the original seven, 4 inch, and all 16 original astrographic plates from Sobral, using modern measuring machines. He obtained essentially the same result, although in the case of the astrographic camera plates with much improved accuracy.
Today, the Hubble Space Telescope has shown many beautiful examples of the gravitational lensing of distant galaxies by foreground clusters of galaxies, which provide one of the lines of evidence for dark matter. General Relativity has also be tested by the timing of Quasar occultations by the Sun, the decrease in orbital period of binary pulsars (PSR B 1913+16) and most recently the direct detection of gravitational radiation from merging neutron stars and black holes.
The GAIA satellite, which measures the position of stars a million times more accurately than the deflection of star light at the edge of the Sun, must allow for the gravitational effect of the Sun over the whole sky, as well as for the effect of the gravitational deflection of the Earth Moon and remaining planets. Its observations will provide a stringent test of the predictions of General Relativity.
A more through discussion of the 1919 eclipse results has been given by Longair (2015).
A determination of the Deflection of light by the Suns gravitational from observations made at the total eclipse of May 29, 1919. Dyson, F. W., Eddington, A. S. & Davidson, C., 1920, Phil. Trans. R. Soc., 220, 291.
The eclipse expedition to Sobral. Crommelin, A. C. D. 1919. Obs. Magazine 42, No 544, p.368
The principle of generalized relativity and the displacement of Fraunhofer Lines toward the red. St John, C. E., 1917, ApJ, 46, 249
Collected Papers of Albert Einstein, Vol 9, The Berlin Years, Correspondence, Jan 1919 – April 1920, page 98, letter 113. Princeton. Eds. Buchwald, D. K., Schulmann, R., Illy, J., Kennefick, D. J., Sauer, T.
Ninth Astronomer Royal – A life of Frank Watson Dyson, pp.191–94. Margaret Wilson (1951)
Expeditions for the observation in Sobral, Brazil, of the May 29, 1919 total solar eclipse. Crispino, L. C. B., 2018, Int. J. Mod. Phys. D, 27 No 11, 1843004. (paywall)
Gravitational deflection of light – a re-examination of the observations of the solar eclipse of 1919. Harvey, G. M., 1979, Obs. Magazine, 99, 195
Not Only Because of Theory: Dyson, Eddington, and the Competing Myths of the 1919 Eclipse Expedition. Kennefick D. (2012). In: Lehner C., Renn J., Schemmel M. (eds) Einstein and the Changing Worldviews of Physics. Einstein Studies, vol 12. Birkhäuser Boston
Bending space-time: a commentary on Dyson, Eddington and Davidson (1920) ‘A determination of the deflection of light by the Sun's gravitational field’. Longair, M., 2015, Phil. Trans. R., Soc., A373:20140287
Collected papers based on the presentations at the 7th international conference on the history of general relativity, La Orotava, Spain, March 6–13, 2005. Beckman and Eric Stengler.
The following image is © National Portrait Gallery, London. Reproduced under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0) licence.
Portrait of Frank Dyson. Bromide print by Walter Stoneman, 1919. National Portrait Gallery Object ID: NPG x88116.