Solar Eclipse and Scientific Discoveries

Rintu Nath

Solar eclipses have offered opportunities for groundbreaking observations that have helped science progress. In this article, among many scientific advances that were made during past solar eclipses, two notable observations are mentioned, namely experimental support for the general theory of relativity and the discovery of the element helium. The general theory of relativity In 1915, Elbert Einstein proposed the general theory of relativity. He explained gravity based on the way space could curve in presence of mass and associated the force of gravity with the changing geometry of space-time. Newton had explained gravity as a force, pulling objects together. Einstein described gravity as a warping of time and space - a distortion in the fabric of the universe. According to the general theory of relativity, light changes its path while travelling through the warped space-time. The more massive an object, the bigger the distortion, and the more its gravity can bend light. Einstein predicted that light rays from any distant star would bend while passing near any massive object like the Sun. As a result, the star would appear to be in a slightly different position in the sky during the day, compared to its position we see at night, when the Sun is in another part of the sky. Although according to Newton's gravitational theory too, light would experience gravitational attraction towards the Sun, the degree of the deviation was computed to be significantly higher according to Einstein's theory. It was evident that general relativity theory could be validated experimentally if the bending of light is established through observation. The only problem was that the Sun needed to be blocked from the view; otherwise, stars would not be visible during the day in the dazzling light of the Sun. A total solar eclipse was the only option to establish a positional shift. If the same star were observed in the night sky and also during the day during totality, comparison of the two observations could bring out the positional shift, if any. Of course, during an eclipse, the measurement needs to be taken from a spot where the totality is visible and at a time when the disc of the Sun is completely blocked and other stars are visible.

Eddington's work:

Sir Arthur Stanley Eddington was an English astronomer. In 1915, Einstein presented the general theory of relativity to the Prussian Academy of Sciences. It was wartime, and direct communication of scientific results between England and Germany was not possible. But Eddington was lucky to receive a copy of Einstein's papers from his Dutch friend Willem De Sitter. Eddington could understand the importance of the theory and wanted to validate it experimentally. In 1917, he presented a report to the Royal Astronomical Society and explained how observational validation could be carried out using measurements of star position during Eddington realised that the eclipse of 29 May 1919 could be the golden opportunity to test Einstein's theory. They also noted that at the time of eclipse, the Sun would 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, totality. Along with Eddington, another astronomer, Sir Frank Watson Dyson, also became interested in observational validation. Dyson and 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.

Discovery of helium

In mid-1800, many scientists were thrilled with a new instrument called a spectroscope. Similar in design to a telescope, the spectroscope worked like a prism, dispersing visible light into measurable wavelengths. It was observed that heating any element produced bright light; for example, sodium burns with a yellow flame. When one looks at the hot glowing gas through a spectroscope, a few discrete bright 'lines', called line emission spectrum, are observed. The same gas, when cooled and passed through background white light, absorbs light of precisely those colours that it would have itself radiated when hot. This time, one would see dark lines, called line absorption spectrum in a rainbow-coloured background. Every chemical element produces a unique spectrum, a sort of 'fingerprint', that confirms the presence of an element. Researchers started analysing the emission and absorption spectra of specific elements in the lab. Then they observed stars through spectroscopes and tried matching the spectra with known elements. Thus, it became possible to make out the chemical composition of stars across the galaxy. French astronomer Pierre Janssen was very interested in spectroscopic analysis of visible light. He travelled across Europe and Asia to observe the night sky. He chased after eclipses, visiting Italy in February 1867 and then to Guntur, India, for the total solar eclipse of 18 August 1868.

            He camped in Guntur to watch the solar corona visible during a total solar eclipse. From the spectroscopic analysis, Janssen observed that the prominences were mostly made of superhot hydrogen gas. But he also noticed that a yellow line in the spectrum did not match the wavelength of any known element. Around the same time, one English amateur astronomer, Norman Lockyer, made a similar observation. His observation of Sun was, however, without the eclipse, using a special instrument called coronagraph that blocked out the light of the Sun so that researchers can glimpse the burning star's hot, thin, corona. Lockyer and English chemist Edward Frankland named the unknown element helios, after the Greek word for the Sun. Helium is the first and only element to be discovered and identified outside Earth. For some time, helium was believed to exist only on the Sun and other stars. However, in 1882, Italian physicist Luigi Palmieri recorded helium's yellow spectral line in his data while analysing lava from Mount Vesuvius. Helium is probably best known today as the gas that fills birthday balloons. Helium is used in medical equipment (Magnetic Resonance Imaging) as well as in spacecraft and radiation monitors. It is also used in microscopes, airbags in cars, and in many physics experiments.

Courtesy: Vigyan Prasar, Dream 2047) (The author is Scientist 'F' in Vigyan Prasar, email:  rnath@vigyanprasar.gov.in    

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