Page 20 - Dream June 2020

ANNULAR S LAR ECLIPSE
REVELATION
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
Pierre Janssen
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
Spectroscope used in the nineteenth century
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