The strange but true history of eclipse science — from 2137 BC into the future
Aug. 21’s total solar eclipse will be the first of its kind for American smartphone-toting citizen scientists. But people around the world have been studying eclipses for millennia. It’s sometimes difficult to draw the line between legends and science, but here are some interesting examples of scientists’ contributions to that long eclipse tradition.
China’s records of eclipses go way back — to Oct. 22, 2137 BC, when, the official records report, “the Sun and Moon could not live peacefully together in the sky.” And that was just the first total solar eclipse that made it into the history books — in fact, one scholar tracked down records of 916 solar eclipses between 2137 BC and 1785 AD in Chinese bureaucracy and literature.
In ancient China, people tied eclipses to politics. One astronomer’s failure to anticipate a solar eclipse that was dated to either 2137 BC or 2110 BC, reportedly resulted in his downfall. (The astronomer also happened to be a powerful tribe leader, so the eclipse may have just been a convenient cover story to kill him off.)
Observers interpreted different physical characteristics of a solar eclipse, like how visible the corona (the sun’s outermost layer, only visible during a solar eclipse) was, to draw different conclusions about the fate of political leaders. And tradition pressured leaders to acknowledge the celestial omen. According to one pair of scholars, “On the occasion of an eclipse, an emperor was supposed to think what wrong or evil he had done to the people and then correct it in an appropriate way. Of course this was no more than a gesture.”
Whether you realize it or not, you’re already familiar with the Babylonians’ celestial interests: You’re following in their footsteps every time you count seconds and minutes by 60s. They were observing eclipses by the seventh century BC and predicting them by the third century BC.
Those predictions relied on identifying what were later called Saros cycles — in which approximately every 18 years, the sun, moon and Earth line up in similar ways and create an eclipse over a new swath of Earth.
Some scholars argue that thousands of years ago, Aboriginal Australians were also studying eclipses and other relationships between the sun, moon, and Earth. But because of colonization and the cultural damage it brought, it’s difficult to piece together when that might have begun, what precisely Aboriginal Australians were watching and how they explained it.
Some evidence comes from traditional stories passed down over the generations — knowledge that scholars say could stretch back as far as 50,000 years — and rock carvings, which means it’s all essentially impossible to date. Originally, the carvings were interpreted as figures reaching up toward a boomerang, but some indigenous studies scholars argue the carvings are the wrong shape for a boomerang and the perfect shape for the crescent sun visible during a partial solar eclipse.
The same scholars also argue that the detailed stories some Aboriginal cultures tell about the sun and moon and their interactions indicate they were tracking the movements of both bodies and understood the mechanics behind phenomena like eclipses.
The cultures around the ancient Mediterranean, including the Greeks and Romans, turned their eyes to the eclipse too, including during one sixth century BC battle that an eclipse allegedly put an end to. The famous Antikythera Mechanism, a mechanical computing device built in the second century BC, included a gear for counting 223-month Saros cycles picked up from the Babylonians.
Greek literary sources confirm they were intrigued by eclipses. Plutarch reported what could be one of the first western descriptions of the sun’s corona, as seen during an eclipse, perhaps in 71 AD, reporting that an observer said “a kind of light is visible around the rim which keeps the shadow from being profound and absolute.” But for the most part, their work was observational.
The key exception was a prominent mathematician and geographer named Claudius Ptolemy. Writing around 150 AD and based on Babylonian predecessors and a Greek named Hipparchus, he discussed a range of astronomical phenomena, including both solar and lunar eclipses, and explained how to calculate the time of an eclipse within an hour.
The Enlightenment and beyond
Eclipse observations continued throughout the Arab and European worlds during the medieval period and into the Renaissance, but without much real advancement in technique or science to accompany them.
That began to change in the very late 1600s and into the 1700s, particularly thanks to Edmond Halley, the astronomer who also studied the comet that now shares his name. He’s responsible for naming Saros cycles, the 18-year realignments the Babylonians had noticed when studying eclipses more than 2000 years prior.
In 1715, he predicted a total solar eclipse’s path — the first person to do so across the whole totality belt — and drew a map to share that information with the public. The map was so popular he was sent a collection of observations from the ground, which he used to update and republish the map, writing “whereby it will appear that tho[ugh] our Numbers pretend not to be altogether perfect, yet the correction they need is very small.” This map also shows the path he predicted for an eclipse due May 11, 1724.
Modern eclipse science
But despite this long history, modern eclipse science only really emerged in the 1860s, facilitated by the rise of new technologies like spectroscopy and photography, which meant that scientists could gather more than just observational data. (The first known photograph of the sun’s corona was snapped during the eclipse of July 28, 1851 in what is now Kaliningrad, Russia.) And the new field of astrophysics meant it was popular to study the sun as the best star to observe from Earth.
Astronomer P.J.C. Janssen traveled to India to watch an eclipse in 1868, and he brought his spectography equipment as well. That let him break the corona’s light into individual wavelengths. The pattern of wavelengths that are present or absent is shaped by the chemical composition of the fuel source being burned in the star.
Janssen spotted a strange, bright yellow band, which had never been seen before. He named it helium, after the Greek sun god Helios — and identified an element that wasn’t identified here on Earth until 1895.
This same eclipse was the first time scientists named solar prominences, giant protrusions of gas rising off the sun. (Ancient Chinese writers may be referring to these features in some of their poetic descriptions of eclipses.)
Perhaps the eclipse with the most famous scientific results came on May 29, 1919, when British astronomer Arthur Eddington observed light bent by the sun’s gravity in a way that supported Einstein’s theory of general relativity and the spacetime-warping effects of gravity. The New York Times headline later that year read “Lights All Askew in the Heavens: Men of Science More or Less Agog Over Results of Eclipse Observations.”
Scientists have gone on to use eclipses to study the ionosphere, a level of Earth’s atmosphere bombarded by the sun’s radiation. Another popular quest has been to measure qualities of the light coming from the corona, like its polarization and intensity.
Modern scientific advances have also brought incredible new ways to study eclipses. During a three-day period in 1970, NASA launched 32 rockets armed with an array of scientific instruments, all to study a total solar eclipse along the East Coast.
In 1973, although commercial flights on the supersonic Concorde jet were still three years in the future, a special trip stretched totality into an incredible 74 minutes. In order to actually watch the eclipse, the plane had to be modified with windows on its roof, earning it an early retirement.
Into the future
Aug. 21 will update this science, complete with high-altitude balloon cameras livestreaming an incredible view of totality and a host of science projects. But it won’t be the end of this story, either: It’s been calculated that there will be 68 total solar eclipses over the course of the 21st century. So stay tuned for what comes next.