History & Culture
Our Island Universe:
History’s Most Profound Total Solar Eclipses
Through the millennia, scientists have used eclipses to make crucial discoveries about our universe.
By Shanil Virani
NOIRLab
Aristarchus's 3rd century BCE calculations (from a 10th-century CE Greek copy) regarding the relative sizes of, from left: the Sun, Earth and Moon.
[Image courtesy Library of Congress]
Solar eclipses have frightened, bewildered, and surely fascinated our ancestors! To explain what they saw, they created elaborate and creative stories. It has only been in the past few millennia that we have understood the mathematical patterns of what was happening. As our scientific understanding of these events matured, it was also realized that eclipses are a wonderful opportunity to do some clever scientific research that can yield profound consequences about our understanding of the cosmos.
Using an eclipse to undertake fundamental science is actually not very new. The earliest example is from classical Greece. Aristarchus of Samos — the same Greek who hypothesized that the Sun is at the center of the solar system long before Copernicus — in the 3rd century BCE. Aristarchus (310 – 230 BCE) was an ancient Greek astronomer and mathematician born on the Greek island of Samos. He correctly reasoned that because the angular sizes on the sky of the Moon and Sun are roughly the same, the time it takes for the Moon to move through Earth’s shadow during a lunar eclipse can tell us directly the distance and sizes of the two objects.
His first idea was beautifully simple! He realized that when the Moon is at its quarter phase (either first or third), there is a 90-degree angle triangle created between Earth, the Moon, and the Sun. If he could measure the Moon-Earth-Sun angle, then he could determine the ratio of distances to the Sun and Moon from trigonometry (who said you’ll never use SOH-CAH-TOA?!). From his work, Aristarchus reasoned that this ratio was somewhere between 18 and 20. In other words, the distance to the Sun was somewhere between 18 and 20 times greater than the distance to the Moon. He also reasoned that as the angular size of the Sun and the Moon were the same, and that the distance to the Sun was between 18 and 20 times farther away than the Moon, the Sun must therefore be 18 to 20 times larger. Now, it turns out that his Earth-Sun distance estimate, and hence his size of the Sun estimate, is about 25 times too small. Carl Sagan once said “that’s pretty good figuring for 2,200 years ago!” with regard to Eratosthenes’s calculation of Earth’s diameter; that clearly applies here, too.
His second idea was recognizing that during a lunar eclipse, you can set up two similar triangles involving the diameters and distances of the Sun, Earth, and Moon. This website guides the reader through the set up and the algebra of his ingenious idea. The result is that Aristarchus derived an equation that gives the radii of the Moon and Sun entirely in terms of observable quantities, as well as an equation that derives the distances to both the Sun and Moon. His estimates significantly underestimated both the distances and sizes of Earth and the Sun (Hipparchus would later refine this method of using a lunar eclipse to determine sizes and distances) but it nevertheless remains a remarkable achievement that undoubtedly influenced his belief that the Sun rather than the Earth must be at the center of the solar system — an idea that wouldn’t emerge again until the 16th century.
Using modern imaging techniques, the Heidelberg Digitized Astronomical Plates (HDAP) project a few years ago scanned a copy of a photographic plate captured during Eddington and colleagues' 1919 solar eclipse expedition. That scan is shown here. The labeled stars show which ones the scientists studied during that eclipse.
[ESO/Landessternwarte Heidelberg-Königstuhl/F. W. Dyson, A. S. Eddington, & C. Davidson]
The November 10, 1919, front page of the New York Times featured the results from Sir Arthur Eddington and colleagues.
[The New York Times Company]
Perhaps the most famous example of an eclipse that yielded a profound conclusion about the nature of our cosmos occurred just over 100 years ago. It involved testing the ideas of a then unknown theoretical German physicist who had a completely different view of how gravity works than had been accepted at the time. The long-standing theory of gravity originated with Sir Isaac Newton, a giant in the pantheon of science. He had viewed gravity as an imaginary force (a push or pull) that exists between any two bodies. This unknown German physicist working at the federal patent office in Switzerland, Albert Einstein, had a fundamentally different view of how gravity works. He published his hypothesis in 1915.
Einstein viewed gravity as a consequence of the curvature of space-time. The total solar eclipse on May 29, 1919, was the ideal laboratory to see if Einstein's view of gravity is right. According to his new general theory of relativity, instead of gravity being the pull and push between two objects, an object with mass warps space-time and other objects follow those curves. If his idea was correct, then the light of a nearby star should be deflected by an amount larger than what Newton’s theory would predict. It’s important to point out that even Newtonian gravity predicts starlight will be deflected when it passes near a massive object; however, it’s the size of the deflection that is different between these two competing ideas. Einstein’s prediction for deflection of starlight near the limb of the Sun is about twice as large compared to what you would expect from Newton’s formulation of gravity. However, because these angles are very small, and because our Sun is very bright, the only way to test this idea would be during a total solar eclipse. During such an eclipse, stars with an apparent position near the Sun become visible as the Moon shields the Sun’s glare. A difference in the observed position of the stars during the solar eclipse, compared to their normal position, which researchers would have measured months earlier at night when the Sun is not in the field of view, would indicate that the light from these stars had been deflected as it passed close to the Sun. Therefore, a solar eclipse provided the perfect laboratory to see if a giant of science was wrong.
Two research expeditions took advantage of the 1919 solar eclipse to measure the positions of stars during totality. Sir Arthur Eddington and Sir Frank Watson Dyson led an expedition to the island of Príncipe off the west coast of Africa, and Andrew Claude de la Cherois Crommelin and Charles Rundle Davidson journeyed to Sobral in Brazil for the other one.
The results of their experiments agreed perfectly with Einstein's prediction, and as a result, a new giant of science was crowned. The media considered the confirmation spectacular news, and in fact, it made the front page of most major newspapers around the world. In particular, it made Einstein and his general theory of relativity world-famous. Because of a few minutes of daylight turning to night, Einstein catapulted from a relatively unknown physicist to a “rockstar” scientist.
Eddington, at a dinner held by the Royal Astronomical Society after the return and publication of his team’s results, composed the following verse that parodied the style of the Rubaiyat of poet Omar Khayyam:
Oh leave the Wise our measures to collate One thing at least is certain, light has weight One thing is certain and the rest debate Light rays, when near the Sun, do not go straight.
SHANIL VIRANI is an astronomer, educator, and a science communicator with the Astronomical Society of the Pacific. He is the co-author of Daughter of the Stars, a coffee-table astrophotography book about light pollution and what we lose when we lose the night, and also the host of the “Our Island Universe” podcast available via SoundCloud.
More than 100 years later, and decades of interrogating the universe for deeper clues as to its true nature, experiments have not yet shown a departure to general relativity (or how it may be reconciled with quantum mechanics — the other pillar of 20th-century physics). We’re forced to accept that Einstein’s theory is still right. Indeed to date, there is no experiment we’ve done that Einstein’s general theory of relativity has not passed.
So as April 8 approaches, and you take the once-in-a-lifetime opportunity to witness a solar eclipse, think about how our species has viewed, interpreted, and used these events to yield profound truths about the universe that we inhabit. ✰
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