This article is part of a special report on the total solar eclipse that will be visible from parts of the U.S., Mexico and Canada on April 8, 2024.
Total solar eclipses, such as the one set to sweep across a swath of North America this April, are among the most sublime and transcendental natural phenomena that one can experience. The spectacle of totality—when the moon completely covers the sun to cast a dark shadow on Earth below—is almost unreal, as if the natural rhythm and regular order of the cosmos has come undone. It’s no wonder, then, that throughout history, these events have incited fear, wonder and reverence. They’ve also served as the perfect opportunity for astronomers to test advanced theories of physics and discover new aspects of our natural world. Here are just three of the many times that a total solar eclipse has transformed our views of the heavens, Earth and everything in between.
Halley’s Eclipse
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If it weren’t for Edmond Halley, we might have never had Isaac Newton’s revolutionary theory of gravity. In 1684 one of Halley’s contemporaries, Robert Hooke, claimed to be able to derive Kepler’s laws of planetary motion from simpler principles. When he was challenged on his assertion, however, he couldn’t back it up. Halley had also agreed to take a crack at the problem, only to come up short himself, so he turned to his old friend Newton. Newton surprised Hooke by saying he had already found a solution but had “lost” the notes. To satisfy Halley’s persistent encouragement, Newton produced perhaps the greatest work of physical insight ever, his Principia Mathematica.
To say that Halley was Newton’s superfan would be an understatement. Halley personally financed the first publication of Newton’s work and played a key role in communicating its importance and significance to the public. In doing so, he became the first person known to history who accurately predicted an upcoming solar eclipse.
Cultures throughout history had successfully made rough guesses at the timing of eclipses. By wielding Newton’s freshly minted gravitational laws, however, Halley was able to predict the timing and path of a total solar eclipse that passed over London on May 3, 1715 with decent precision. The timing and path were accurate to within about four minutes and 20 miles, respectively. (Halley may have been thrown off not from any failing of Newton’s laws but because of inaccuracies in records of the moon’s motion).
Naturally the event made news, with scientists and laypeople around the world recognizing Newton’s genius. And the way that Halley chose to map the eclipse’s geographic path (with dark bands showing totality and partiality) was so good that we still use that style today.
Janssen’s Eclipse
By the mid-1800s chemists, physicists and astronomers alike were aflutter over the newfound technique of spectroscopy, in which splitting light into a rainbowlike spectrum of its constituent colors could reveal a source’s elemental composition. (Exactly what those elements were was still up for debate because atoms had not yet been proven to exist!)
Using spectroscopy, astronomers could, for the first time ever, peer through their telescopes and identify the substance of what they saw as easily as if they could reach out and touch those far-off planets and stars. Today spectroscopy is the bedrock of modern astronomy. For every alluring astronomical image of a celestial object you may encounter, there are probably a dozen papers published about its spectrum.
Seeing as the sun is the brightest thing in the sky, it was a natural target for spectroscopy. With this technique, astronomers found hydrogen, iron, oxygen, carbon, and more lurking in the sun’s incandescent atmosphere—as well as hints of one element that defied easy understanding. The earliest observations suggested that it might be some strange sort of iron, but no explanation fully matched the data.
A critical advance occurred on August 18, 1868, when international teams of astronomers observed a total solar eclipse in southern India and Southeast Asia. Among them were Norman Lockyer and Jules Janssen, who together studied the spectra of solar prominences that were suddenly visible around the moon’s occulting silhouette. Those spectra allowed them to dispel the murk, clearly revealing the presence of a new element on the sun that was previously unknown on Earth.
It would take decades for Earth-bound chemists to isolate the element, which they named helium, after the Greek word helios, meaning “sun.” Helium was the first—and to date only—element that was discovered in the heavens before it was found on Earth.
Eddington’s Eclipse
As beautiful and accurate as Newton’s account of gravity was, it was incomplete and could not adequately explain certain phenomena, such as the precession of Mercury’s orbit around the sun. Such incompleteness was a key motivator for Albert Einstein’s efforts to forge a new concept of gravity all his own—his general theory of relativity, which treats gravity as the curvature of spacetime induced by massive objects. With general relativity, Einstein was able to explain the mysteries of Mercury’s orbit. That was technically a postdiction, however—the concoction of a theory to explain already known results. What he needed was a prediction—something new to demonstrate just how powerful his theory really was.
Einstein quickly hit on the idea of using general relativity to predict the degree to which light should be deflected by the gravitational field—that is, the curvature of spacetime—around a massive object such as the sun. The sun’s gravity should slightly deflect any passing light rays. Normally we can’t see this effect, because it’s incredibly tiny, and most light rays from distant stars don’t pass sufficiently close to the sun. But during a total solar eclipse, someone could potentially measure the precise position of a star right on the sun’s apparent edge and then compare its position at any other time to discern this deflection.
Newton’s theory also predicted this sort of deflection, and Einstein’s first relativity-derived forays found identical results. In a 1911 paper Einstein urged astronomers to go looking for this effect. Although they tried at several subsequent eclipses, their attempts were spoiled because of bad weather.
That turned out to be a good thing for Einstein: once he fully fleshed out his theory, he realized that his calculations gave a stronger deflection than was predicted by Newtonian gravity. After Einstein once again sought the aid of his astronomical colleagues, Frank Watson Dyson and Arthur Eddington, took up his challenge. Leading two expeditions—one to the island of Principe and the other to Brazil—these astronomers measured the apparent positions of stars near the sun during the total solar eclipse of May 29, 1919, and found them out of alignment in exact accordance with Einstein’s predictions.
The next year, during a dinner at the Royal Astronomical Society, Eddington recited the following poem, which he had written as a parody of TheRubáiyát of Omar Khayyám.
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.
Tomorrow’s Eclipse
Nowadays Earth-bound astronomers usually don't need to wait for the next fateful alignment of the moon to study the sun because they can make their own “eclipse” on demand with a clever instrument called a coronagraph. Such devices can be as simple as a disk affixed to a telescope that precisely blocks out the sun. Astronomers often use coronagraphs to study the sun’s outer atmosphere, where there are many mysteries still to be found: no one yet knows exactly why this region is so scorching hot, compared with the sun’s visual surface, why it has such strong and tangled magnetic fields or why it’s able to launch the never-ending stream of charged particles known as solar wind.
Natural eclipses of the past have helped us revolutionize our perspectives of the universe, and these human-made ones of the present will surely propel us into the future of astronomy. Who knows what new secrets the sun will reveal to us next?