Page 20 - Dream_2047_June_2020

ANNULAR S LAR ECLIPSE
REVELATION
Eclipse instruments at Sobral
Image credit: C. Davidson - File: Eddington A. Space-Time and Gravitation. 1920.djvu, Public Domain: https://commons.wikimedia.org/w/index.php?curid=38550909
be in front of a prominent grouping of stars, known as the Hyades. At the time of totality, bright stars behind the Sun would be visible, whose positions could be measured and compared with the position of the same stars taken during the night at other times. They began to investigate possible observing sites. It was decided to send two expeditions, one led by Eddington and the other by Andrew Crommelin, who was an astronomer at the Royal Greenwich Observatory. Eddington and his team would travel to the island of Principe in West Africa, and Andrew Crommelin would travel to Sobral in northern Brazil.
Both expeditions had to overcome many hurdles. As the war was going on, it was not easy to get permission for such expeditions. After much persuasion, permission was obtained. However, both Eddington and Dyson encountered numerous technical problems. The expeditions needed specialised telescopes and photographic equipment. The availability of skilled human resources was difficult at wartime - they were either been conscripted or were engaged in war work. Both the expeditions were enormous – hence preparations were hectic. They had to start sailing at least three months before the eclipse date to ensure arrival in time and setting up camps and equipment.
On the day of the eclipse, heavy rain started in Principe, but near totality, the Sun began to appear dimly. Some photographic images could be taken through the cloud. Among several photographs, only two photos were considered useable. Sobral had better weather, but due to technical errors, photographs taken by the main telescope came out blurred. Fortunately, photographs taken by a backup 4-inch telescope came out well. These photographs became the most convincing proof in the final analysis.
It took over five months to analyse data collected during the eclipse and reach any conclusion. Dyson and Eddington presented their findings at a special joint meeting of the Royal Astronomical Society and the Royal Society of London convened on 6 November 1919. Results were consistent with Einstein’s prediction. Although several scientists criticised the findings and questioned the reliability of statistical evidence, J.J. Thomson, the Chair of the meeting, was convinced and commented: “This is the most important result obtained in connection with the theory of gravitation since Newton’s day.”
On the other hand, Einstein had no doubts. He expected consistent results
from the English expeditions before the formal announcement. On 27 September 1919, he wrote a postcard to his mother: “. . . joyous news today. H.A. Lorentz telegraphed that the English expeditions have actually measured the deflection of starlight from the Sun”. In 1922, another eclipse was viewed from Australia that yielded more convincing statistical data. Measurements of this kind during total solar eclipse using optical telescopes continued until 1950. With the advancements in astrophysics, these measurements are now possible to take in other frequencies of the electromagnetic spectrum, other than the visible range. However, the experiments of Dyson and Eddington during the total solar eclipse in 1919 are unique and very important in establishing the general theory of relativity.
Arthur S. Eddington
Frank W. Dyson
20 dream2047/june2020