The 1919 Solar Eclipse in Brazil, Einstein and Ireland

Over 100 years ago, Albert Einstein proposed his now famous General Theory of Relativity. Not only did Einstein’s ideas overturn those of Isaac Newton but they led on to such exotic physics as black holes and gravitational waves. Einstein’s “Big Idea” was that bodies like the Sun wrap space and time around them and that objects in their vicinity, and even photons of light, follow the natural curvature of this so-called spacetime. Einstein in fact predicted that light from a distant star, passing close to the Sun, would not travel in a straight line, as most physicists thought, but instead would be bent. The expected deflection is tiny: light from the star is bent by an angle equivalent to that subtended by a beach ball in Holyhead as seen from Dublin! Nevertheless in May 1919 an expedition was mounted to Brazil to measure this tiny effect during a solar eclipse.

Three astronomers — Arthur Eddington, Frank Watson Dyson, and Andrew Crommelin â€” played key roles in this 1919 expeditions. Eddington and Crommelin travelled to locations at which the eclipse would be total — Eddington to the West African island of Príncipe, Crommelin to the Brazilian town of Sobral — while Dyson coordinated the attempt from England. 

Eddington and Crommelin imaged the eclipse using the technology of the time: photographic plates made of glass. Sadly, the original plates from the 1919 expedition (one of which was reproduced in Dyson’s original paper) have been lost — but, luckily, copies of one of the plates were made and sent to observatories around the world to allow scientists everywhere to see the evidence in support of relativity with their own eyes. One copy of a plate from Sobral went to Landessternwarte Heidelberg-Königstuhl, who recently scanned theirs as part of the Heidelberg Digitized Astronomical Plates (HDAP) project.

The image shown above is arguably the highest resolution image of the 1919 eclipse, and is the result of applying modern image processing techniques — including image restoration, noise reduction, and removal of artifacts — to that plate copy (un-annotated version here). It unveils stunning details in the solar corona, a giant prominence emerging from the upper right part of the Sun, and stars in the constellation of Taurus (The Bull) that were used to confirm general relativity’s predictions.

The Irish coelostat that took this high resolution image was first used in 1900 when the Royal Irish Academy (RIA), in collaboration with the Royal Dublin Society, mounted an expedition to observe an eclipse in Spain. The lens and the coelostat were both manufactured by Howard Grubb at his optical works in Observatory Lane, Rathmines, Dublin. Grubb was one of the most famous telescope makers in the world and a prominent member of the RIA. The Grubb Coelostat reflects light from the sky onto a fixed telescope, and a lens (known as the Einstein Lens) is used to focus the light onto a photographic plate.

The Grubb Coelostat is usually displayed at Dunsink Observatory, although it temporarily returned to Sobral in 2019 for a centenary celebration of the famous eclipse expedition.