Announcing the results

Enhanced version of diagram 2 from Dyson et al. 1920, showing the observed shifts (blue dots) of stars measured on the 4 inch Cortie telescope plates, compared to the predictions of Einstein and Newton as a function of the distance of the stars from the centre of the Sun.

On 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 (above), 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 Newton’s prediction. The enhanced version of diagram 2 from Dyson et al. 1920 shows the observed shifts (blue dots) of stars measured on the 4 inch Cortie telescope plates, compared to the predictions of Einstein and Newton as a function of the distance of the stars from the centre of the Sun.

 

An acetate copy of one of the 4 inch Cortie telescope plates, showing the magnitude and direction of the deflections predicted by Einstein's theory magnified 320 times, for various stars. Image courtesy of Robin Catchpole.
An acetate copy of one of the 4 inch Cortie telescope plates, showing the magnitude and direction of the deflections predicted by Einstein's theory magnified 320 times, for various stars. Image courtesy of Robin Catchpole.

 

Enlarged image of star 4 from one of the 4 inch Cortie telescope plates. The red arrow shows the shift of 0.75 seconds of arc predicted by Einstein for this star. Image courtesy of Robin Catchpole.
Enlarged image of star 4 from one of the 4 inch Cortie telescope plates. The red arrow shows the shift of 0.75 seconds of arc predicted by Einstein for this star. Image courtesy of Robin Catchpole.

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.