The Beauty and Science of a Total Solar Eclipse

Aug 18, 2017

This story originally appeared in the TIME special edition Beautiful Phenomena available now at retail outlets and through the TIME shop and through Amazon

The moon was not placed in space for our entertainment. In fact, it was placed there by accident, most astronomers believe, as the product of a nearly mortal blow the Earth sustained more than 4 billion years ago, when our planet was sideswiped by a Mars-size planetesimal speeding through local space. That collision produced a massive debris cloud that eventually coalesced into our moon. The sun didn’t pop into being for our enjoyment either; it spun down out of a cloud of primordial dust and gas, just as Earth itself did. Not much glamour or drama in all of that.

Yet now and then, the debris ball that is the moon passes in front of the dust ball that is the sun and produces the glorious phenomenon we know as a solar eclipse. Even for scientists, there can be a temptation to see the eclipse as something intended to thrill, a sky show put on for the only species in the solar system able to appreciate it.

Consider that the sun is about 400 times the diameter of the moon, which would make it awfully hard for the lunar disk to fit so perfectly over the solar one—except that the sun is also about 400 times more distant, meaning that the two bodies appear to be the same size in the earthly sky. Consider the way the moon’s ragged mountains, which are impossible to see from as far away as Earth, form a sawtooth pattern at the lunar edges through which the last of the sun’s light streams in the moments before a total eclipse is complete, creating the brilliant burst of light astronomers call the diamond ring effect.

And consider too the rarity of the eclipse. At some point on the planet a total solar eclipse will occur every 18 months, but at any particular spot—the spot where you live, say—it will happen just once every 350 years or so. When an eclipse does occur, totality never exceeds a mere seven minutes, 30 seconds—and it’s usually much shorter. Then the show’s over for another three and a half centuries. Frequency cheapens the currency of any spectacle; by that measure, a total solar eclipse is priceless.

Even to modern humans, long since past the fear that the disappearance of the sun in the middle of the day is a curse or a blight or the work of a winged dragon eating the solar fires to replenish its own, there is something deeply unsettling about the sight of an eclipse. The sky darkens, which it does every day, but to a shade of blue and then black and blue that occurs at no other time. The dimming of the light means a cooling of temperatures, and a portentous lick of wind may come up as the eclipse reaches totality. Crickets and night birds, knowing light and dark far better than they know fear or superstition, begin to chirp and sing at the wrong time of day.

The sense that all of this is off, all of this is wrong, is something that our rational brains, which are still operating on neurological software that was written when we humans were on the savannas, can’t shake. There is a reason the Lydians and the Medes ended their war in 585 B.C., when a total eclipse darkened the sky and convinced the combatants that it was a sign of disapproval over the ongoing fighting. There is a reason the English saw an unhappy cause and effect between the eclipse of Aug. 2, 1133, and the death of King Henry I, even though Henry died more than two years later. That the loss of a king could be foretold by the loss of the very light in the sky made a certain kind of 12th century sense.

Eclipse Equations

For all of that, though—for all of the loveliness and spookiness and historical impact of a solar eclipse—there is a cold, reductionist science behind it. The moon’s brief passage between the Earth and the sun is simply the inevitable result of the wheels-within-wheels design of our solar system. The brevity of the phenomenon derives from the fact that the wheel on which the moon rides circles the Earth at a zippy 2,288 miles per hour. No sooner has the moon approached and obscured the sun than it is gliding on past it.

Of course, if the moon circles the Earth once every 27.32 days, an eclipse ought to occur on that same near-monthly schedule too. But the celestial mechanics are more complicated than that. The moon’s orbital plane is inclined relative to the Earth’s equator by anywhere from 18.28 to 28.58 degrees, meaning that some of the times it crosses the path of the sun, it actually appears to sail above it, while at other times it crosses below it. It is only when the moon passes the sun at the same time it crosses the Earth’s own orbital plane that an eclipse occurs.

Even when the alignment is correct and the planes intersect, the eclipse that occurs may not be total. That’s because the moon’s orbit around the Earth is an ellipse, not a circle. The moon’s average distance is almost 239,000 miles, but its perigee, or closest approach to Earth, is about 225,000 miles, and its apogee is about 252,000. A moon at apogee appears smaller than a moon at perigee, and if an eclipse takes place during one of those high-flying periods, the 400-400 balance between the size of the sun and moon and the distance separating them is thrown off. That results in what’s known as an annular eclipse—with the moon gliding in front of the sun but never completely obscuring it, permitting a lot of solar glare that spoils the effect. As many as 73% of the moon’s transits of the sun occur when the moon is too distant from Earth to allow a true total eclipse to occur.

If everything does line up perfectly—if a total solar eclipse is going to make landfall—it’s best to arrive at wherever it will be visible as early as you can, because seating will be limited. An astronaut in orbit looking down on Earth during a total eclipse would see the entire event as nothing more than a circular shadow on the ground, measuring from 70 to 155 miles across, cruising from west to east at high speed. To see the phenomenon, you have to be within that footprint as it passes by. A total eclipse that crosses the continental U.S. makes the entire coast-to-coast transit in just an hour and 32 minutes. The partial eclipse that precedes it and follows it adds more viewing time overall, but the best portion of the experience is short-lived.

Privileged Glimpses

Still, a lot can happen in those brief intervals. During the total eclipse of Aug. 18, 1868, French astronomer Jules Janssen studied the prominences—the flames and flares that dance around the edges of the sun’s blacked-out disk. Looking through a spectroscopic prism, he saw the signature of helium, thus discovering the second-lightest element in the universe before it had been found on Earth.

Much more significantly, on May 29, 1919, British astronomer Arthur Eddington used a total eclipse to prove one of the premises of Einstein’s general theory of relativity—that gravity will bend light by a specific, predictable amount. Months before the event, Eddington measured the precise position of the Hyades star cluster. Then, on May 29, when the sun was blacked out and the stars popped into view, he measured it again—and it was different by a factor perfectly consistent with the bending Einstein had predicted in his famed 1915 theory.

“REVOLUTION IN SCIENCE. New Theory of the Universe: Newtonian Ideas Overthrown,” wrote the Times of London, breathlessly but mostly accurately, the morning after the discovery was announced. Newton did survive—but his work was forever altered by a later scientist who saw much farther, much deeper into the universe.

That, in some ways, illustrates one more gift of the solar eclipse: that it allows for two different kinds of vision—the aesthetic and the insightful, the glimpse of beauty and the glimpse of the workings of the cosmos itself.

Part of the terror and charm of eclipses used to be that they came utterly unexpectedly. “On the day of the new moon, in the month of Hiyar, the Sun was put to shame and went down in the daytime, with Mars in attendance,” wrote a surely surprised observer in a Mesopotamian account of the eclipse of May 3, 1374 B.C. Now, however, our ability to reverse-engineer the turning of the cosmic wheels means we can pinpoint the precise date of past eclipses, and it also means we can run the wheels forward and predict all of the ones that are still to come.

There is a loss in that—in the elimination of some of the wonder at the universe’s seeming caprice. But there’s a great gain too: an eclipse we know is coming is an eclipse we can be ready to watch. And to watch it is to be changed forever—and for better.

This story originally appeared in the TIME special edition Beautiful Phenomena available now at retail outlets and through the TIME shop and through Amazon

All products and services featured are based solely on editorial selection. TIME may receive compensation for some links to products and services on this website.