ON MONDAY, AUGUST 21, 2017, the world will go under, or so it might well feel like if you position yourself along a 70-mile-wide swath of the US from South Carolina to Oregon. From here you can witness the moon move in front of the sun in the middle of the day and darken the skies above you. A total solar eclipse is a spectacular event that has struck fear into people throughout history, and at the same time has enlightened us in our quest to understand the cosmos. Our ability to predict this year’s event with such specificity is thanks to scientific inquiries dating back thousands of years.
The ancient Greek historian Herodotus tells us that as the Medes and Lydians were fighting by the Halys River, day suddenly turned to night. They took it as a sign that the gods were angry, ceased their fighting, and thus peace was restored to the troubled region. Strikingly, Herodotus goes on to say that the star-gazing thinker Thales of Miletus had foretold of the phenomena. This is the first known account of an accurate prediction of a solar eclipse, and the first historical event that we can date back to a specific day: May 28, 585 BC. More importantly, the story hints at how Thales broke with mythological thinking and began to recognize nature as a series of immutable constants that the human intellect can hope to uncover, as opposed to a consequence of the unpredictable mood swings of gods.
The Greeks believed that Thales had foretold of an eclipse, at least to within the year that it happened. They must have been inspired by the prediction when they built on his ideas about a rationally ordered cosmos and created a new tradition of philosophical and scientific thinking. However, it was not until more than 2,000 years later, in the early 18th century, that the first precise prediction of a solar eclipse was made by astronomer Edmund Halley. Using Isaac Newton’s revolutionary theory of universal gravitation, Halley estimated an eclipse to within four minutes. In what was likely the most widely distributed broadside in England up to its time, Halley announced his finding publicly in 1715, shortly before the eclipse on May 3. He proclaimed the successes of science, and advised that rather than seeing the eclipse as something ominous, portending evil, “hereby they will see that there is nothing in it more than natural, and no more than the necessary result of the motions of the sun and moon”.
The connection between the development of scientific thinking and solar eclipses was further strengthened when Albert Einstein expanded on Newtonian physics by stating that gravity should not be understood as the sun pulling objects toward it. Rather, he theorized, the sun literally bends the curvature of space like a heavy object on a trampoline, and this causes things to fall toward it. If correct, he reasoned, the light from distant stars that pass by our sun will be bent twice as much as Newtonian physics had earlier predicted. To test Einstein’s theory, astronomer Sir Edmund Ellington and his team put up cameras in advance of a total solar eclipse on May 29, 1919. That way, they could see the light from distant stars that passed by close to the sun without the interference of the sun’s own bright light.
After spending six months evaluating their data from the solar eclipse, Ellington’s findings were published in The New York Times. From there the news went around the world that Einstein’s strange theory of general relativity had been proven correct. Almost overnight, Einstein became a household name.
The forthcoming American eclipse is unlikely to stand out in our collective memory as a symbol of scientific evolution and ingenuity the way the eclipses of 585 BC, 1715, and 1919 have. But next week’s event represents more than mere spectacle and media hype. More data will be collected from this eclipse than from any previous one in history, enabling us to better analyze such things as the sun’s upper atmosphere, or corona, which is visible only during a solar eclipse, and perhaps find clues as to why the corona is 300 times hotter than the surface of the sun. Next week’s eclipse can also help us to better predict space weather in order to protect astronauts and satellites.
It is one of the most formidable testaments to the marvel and achievement of science that we can predict with great confidence, and with accuracy measured in seconds, that such an awe-inspiring phenomenon as a total solar eclipse will happen on August 21, 2017—whether the gods are angry on that particular day or not.