I’ve always been amazed that total solar eclipses are possible. The sun, an 870,000-mile-wide ball of gas over 90 million miles away from us gets completely blocked by the moon, a 2,100-mile-wide ball of rock 240,000 miles away. If the sun were a bit bigger or closer, or if the moon were a bit smaller or farther, totality would not occur. There’s no scientific reason for this; it’s a wondrous coincidence.
And for millennia, this coincidence has fostered scientific discoveries — about the sun and moon, of course, but also about the Earth itself. Eclipses occur like clockwork, but not perfect clockwork, and this has helped humans understand how the size and movement of the earth shifts over time.
As millions of people move into position Monday to experience the precious moments of totality — the next total solar eclipse in the contiguous United States comes in 2044 — they might keep in mind what this spectacle has meant to our understanding of Earth down to its core.
Thousands of years ago, early scientists used eclipses to refine their calendars by pinning down the regular characteristics of the Earth’s and the moon’s orbits. As the moon passed in front of the sun, an ancient astronomer could also figure out the diameters of both bodies. Today’s astronomers take advantage of the moments of totality to study the sun’s fainter atmosphere, observing important phenomena such as solar flares, which spew hazardous high-energy particles at Earth.
Solar eclipses have also taught us how Earth operates. By looking at the historical record of them, we have learned about changes in our planet’s spin or, put another way, how long a day lasts. Ancient astronomers dutifully recorded the times and locations of the eclipses they observed, and their data now suggest how the Earth’s spin rate has changed. Consider that, if we rewind the Earth and moon back in time using the current spin rate and lunar orbit, the eclipses don’t line up in the same places where the records say they happened. Locations are off in some cases by thousands of miles.
But this can be explained by the way Earth’s spin rate has altered over time. It’s not a huge amount — we’re talking about days that shorten by a couple of milliseconds every hundred years — but such tiny changes can add up to big differences in the alignment of eclipses that occurred centuries ago.
Changes in spin, in turn, say something about Earth’s shape, because shape determines spin rate. This is due to a law of physics known as conservation of angular momentum. Just as figure skaters can spin faster by pulling in their arms, the Earth spins faster when more of its mass is closer to the axis.
The Earth is constantly changing shape, albeit in tiny ways. One example is the rebound of the crust in places such as Hudson Bay and northern Eurasia that has continued since the end of the last ice age (10,000 years ago), when the heavy glaciers melted away. Another example is the change in Earth’s equatorial bulge due to its slightly elliptical orbit around the sun.
Earth’s surface also can get pushed on and dragged by wind currents and ocean flows. Tides can slow the Earth’s turning. Essentially, ocean waves impede the spin as they break on the continental shelves.
By taking into account these shape changes and braking effects, we can get a good picture of how the spin rate has changed over time — which in turn gets the eclipses to line up in the right places when we rewind the Earth and moon. But they’re still not perfectly located — unless we take into account how the Earth’s spin has also oscillated. Every decade or so, the day gets a bit shorter for a while, then a bit longer, oscillating back and forth. What causes this?
The surprising answer is (drum roll) the Earth’s core. Just as breaking ocean waves can push on Earth’s surface, motions in the liquid core can push on the core-mantle boundary. Scientists thus use eclipse timings to illuminate the Earth’s dark core.
Finally, the Earth’s spin rate is tied to the moon’s very slow movement away from us — about 1.5 inches per year. Because of the tidal interactions between the Earth and its satellite, as the moon recedes, the Earth spins a bit more slowly. And this will dramatically affect future solar eclipses: Eventually, the moon will be too small in our sky to block the entire sun.
This means that total eclipses are going to be around only for another 600 million years, give or take. So catch the experience while you still can.
About the author: Sabine Stanley, a Bloomberg distinguished professor in Earth and planetary sciences at Johns Hopkins University, is the author of “What’s Hidden Inside Planets?”
