Although ancient sky watchers viewed eclipses as portends of ill-omen, modern scientists have found many uses for eclipses that could scarcely be imagined even 100 years ago.
This Hubble Space Telescope photo shows the actual lensing of light by the gravity of an entire cluster of galaxies. The images of several galaxies located behind the cluster, Abell 2218, have been distorted into arcs. (NASA: HST)
For centuries, astronomers had studied total solar eclipses with an eye towards understanding the delicate details in its corona and curious prominences of gas that occasionally leapt from its darkened limb. By the turn of the 20th Century, however, astronomers had mastered the trick of studying the corona without the need of solar eclipses, and so the thrill of eclipse-watching abated… but only for a short while. On May 29, 1919 a total solar eclipse offered scientists a revolution in thinking about gravity and the very nature of space itself.
A young German physicist, Albert Einstein, had worked tirelessly on his new theories of relativity beginning with his first published papers in 1905, and culminating with his magnum opus ‘General Relativity’ in 1915. Only a handful of physicists had apparently paid Einstein’s latest creation much mind. One of these was Sir Arthur Eddington in England.
One of the predictions made by Einstein was that, in the presence of a gravitating body, space would be bent causing light rays to take a different path than predicted by Newton’s physics. Eddington soon realized that this light deflection could be measured by looking at the positions of stars before and after a total solar eclipse. The May 29, 1919 solar eclipse was perfect for this task because of its unusual length (6 minutes) and a track that passed over countries not involved in World War I. It was at that time that Einstein’s new, but entirely obscure, theory of gravity would be pitted against that of the physics Grand Master, Sir Isaac Newton. Through the action of warped space surrounding the sun, the sun would act like a colossal lens, deflecting the images of stars near its occulted limb.
Calculations made with the help of Newton’s theory of gravity and motion also predicted such deflections of starlight, but by exactly one-half as much. The deflection was invisible to the human eye, but the fact that it occurred at all, and by a specific amount, was the key that led to a major revolution in physics. Some say that, were it not for this spectacular discovery, all of Einstein’s relativity theories would have languished for many more years.
This is a photograph of one of the stars measured during the May 29, 1919 solar eclipse to confirm Einstein’s light deflection. The image has been magnified 281 times. The red dot shows where the star’s position should have been had the sun not been present. (Courtesy: Royal Observatory, Greenwich)
According to Newton, space is a fixed stage upon which the actions of matter and forces take place. Space is entirely inert and was not a player in the drama of matter and energy. A beam of light would be deflected by the gravity of the sun, and in a way that could be easily calculated. Star images nearest the limb of the sun would be deflected more than at greater distances because the light rays travel through a more intense gravity field when they travel close to the sun’s occulted limb. With photographic images and careful measurements, the star shifts can be determined at various distances from the limb. The size of the shift can be calculated using Newtonian mechanics.
For Einstein, however, Newton’s space was only a crude approximation to a larger reality. Einstein’s theory proposed no less a magical thing than, what we call space, is really a piece of a larger ‘fabric’ called spacetime. The geometry of spacetime can be curved and warped in the presence of matter and even pure energy (like light itself). This ‘space warpage’ effect is completely unheard of in Newtonian Mechanics, and it causes a slight but measurable change in all calculations that involve gravity. For light passing near the sun, the warpage of space leads to a deflection that is exactly twice what Newton’s theory of gravity and motion would predict!
Believing that his ‘General Theory of Relativity’ was too beautiful to be wrong, Einstein offered one of his most frequently quoted opinions to a reported who had asked him what he would have done had the eclipse experiment not agreed with relativity, “Then I would feel sorry for the good Lord. The theory is correct!”.
Soon after the results had been carefully analyzed by Eddington, Einstein’s theory emerged victorious. Until the London Times published the much-awaited findings on November 7, 1919, no one had ever heard of Albert Einstein in the popular press. But a headline such has "Revolution in Science: New Theory of the Universe: Newton’s Ideas Overthrown" was more than enough to propel this then-obscure German physicist to international celebrity status. Of course, no fundamental discovery is without its controversies. Some physicists at this time were not convinced by the poor quality of the data taken during the eclipse, that Einstein’s bizarre theory had, indeed, been confirmed. The estate of the famous mathematician, David Hilbert, even accused Einstein of plagiarizing Hilbert’s own version of General Relativity.
Meanwhile, many more tests of Einstein’s theories have been completed since 1919. His basic theory remains our best and most reliable window onto the nature of gravity, spacetime and the very nature of our universe. Black holes would not be possible to describe without Einstein’s theory. Gravity waves are being strenuously sought as a ‘last’ test of his elegant theory. Even the ‘cosmical constant’ that he added to his equations to describe the evolution of the universe has now found its mirror in the vast sea of Dark Energy that suffuses our cosmos.
There are at least 2 solar eclipses per year somewhere on the Earth.