The 1919 Eclipse Expeditions

Albert Einstein published the General Theory of Relativity in 1915. One of the experiments he proposed to test the theory was to measure the deflection of light by the Sun. The Astronomer Royal, Frank Dyson, devised an experiment to measure deflection by looking at the position of stars visible in the background of a total solar eclipse. Two groups of astronomers went on expeditions to Sobral, Brazil, and the island of Principe, to observe the eclipse on 29th May 1919.

In 1917, in response to the publication of Einstein’s theory, Frank Dyson, the Astronomer Royal and director of the Royal Observatory Greenwich, published a paper reporting his failure to measure a significant displacement for the few stars visible on eclipse plates taken in 1905. He predicted that the total eclipse in 1919 would provide a much better chance of success, as the Sun would be situated in the richer star field of the Hyades.

RMS Anselm

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.

 Side view of the Eros Micrometer (which was designed to measure the photographic plates taken with which Astrographic Telescope). Plate 15 from Monthly Notices of the Royal Astronomical Society, Vol. 64 (1904)

Both parties took similar 13-inch astrographic lenses of 11 feet 3 inches focal length.

Royal Observatory Greenwich 1919 eclipse Sobral observing site

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.

Arthur Eddington

Arthur Eddington, Plumian Professor of Astronomy and Experimental Philosophy at Cambridge, Director of the Cambridge Observatory

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).

Einstein (left) and Haldane. From a photo by Walter Benington. Originally published in The Sphere on 18 June 1921, it was later reproduced in Hutchinson’s Splendour of the Heavens (1923)

Einstein (left) and Haldane. From a photo by Walter Benington. Originally published in The Sphere on 18 June 1921, it was later reproduced in Hutchinson’s Splendour of the Heavens (1923).

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.

The information about the 1919 eclipse expeditions and general relativity is based on Robin Catchpole's article on Graham Dolan's website about Royal Observatory Greenwich. The accompanying images have been kindly provided by Robin Catchpole and Graham Dolan, unless otherwise stated.