Sabine Stanley, Bloomberg Distinguished Professor of Earth and Planetary Sciences at Johns Hopkins University, is the author of “What's Hidden Inside Planets?”
I have always been amazed that total solar eclipses can occur. The Sun, an 870,000 mile wide ball of gas more than 90 million miles from us, is completely blocked by the Moon, a 2,100 mile wide ball of rock 240,000 miles away. If the Sun were a little larger or closer, and the Moon a little smaller or further away, totality would not occur. There is no scientific reason for this. That's a wonderful coincidence.
And for thousands of years, this coincidence has fostered scientific discoveries about the sun and moon, as well as the earth itself. Solar eclipses happen like clockwork, but they're not perfect clockwork. This helped humans understand how the Earth's size and movement change over time.
On Monday, as millions of people move into position to experience the precious moment of a total solar eclipse, they will ponder what this spectacle means for our understanding of the Earth's core. It may be kept at .
Thousands of years ago, early scientists used solar eclipses to improve calendars by identifying regular features in the Earth and Moon's orbits. When the moon passed in front of the sun, ancient astronomers could also determine the diameters of both objects. Astronomers today use totality moments to study the sun's dark atmosphere and observe important phenomena such as solar flares, which spew dangerous high-energy particles toward Earth.
Solar eclipses also teach us about how the Earth works. By looking at those historical records, we learned about changes in the Earth's rotation, or in other words, how long a day lasts. Ancient astronomers diligently recorded the times and locations of solar eclipses they observed, and that data now suggests how the Earth's rotation rate changed. Consider that if you rewind the Earth and Moon back in time using their current rotational speed and the Moon's orbit, the eclipse would not line up in the same place as it did in the record. In some cases, locations can be thousands of miles apart.
However, this can be explained by how the Earth's rotation speed has changed over time. This is not a huge amount. We're talking about days getting shorter by a few milliseconds every 100 years. But these small changes can add up to make big differences in the alignment of solar eclipses that occurred centuries ago.
The shape determines the spin rate, so changes in spin tell us something about the shape of the Earth. This is due to a physical law known as the law of conservation of angular momentum. Just as a figure skater can spin faster by pulling his arm, the closer the Earth's mass is to its axis, the faster the Earth spins.
The Earth is constantly changing its shape, even if only slightly. One example is the rebound of the Earth's crust in places like Hudson Bay and northern Eurasia, which has been going on since the end of the last ice age (10,000 years ago) when heavy glaciers melted. Another example is the change in the Earth's equatorial bulge due to its slightly elliptical orbit around the sun.
The Earth's surface can be pushed and dragged by wind currents and ocean currents. Tides can slow the Earth's rotation. Basically, ocean waves disrupt their rotation as they break on continental shelves.
By considering these shape changes and braking effects, we can better understand how the rotational speed has changed over time. This will ensure that the eclipse will line up in the correct position as it unwinds the Earth and Moon. But they still aren't quite in place, unless you take into account how the Earth's rotation is vibrating. Every 10 years or so, the days get a little shorter for a while, then get a little longer, and then they go back and forth. What could be causing this?
The surprising answer is (drum roll) the Earth's core. Motion within the liquid core can push on the core-mantle boundary, just as crashing waves in the ocean push on the Earth's surface. Therefore, scientists use the timing of solar eclipses to illuminate the Earth's dark core.
Finally, the Earth's rotation rate is responsible for the very slow movement of the Moon away from us (about 1.5 inches per year). Because of tidal interactions between the Earth and its satellites, the Earth's rotation slows down a bit as the Moon moves away. And this will have a dramatic impact on future solar eclipses. Eventually, the moon becomes too small in the sky to block the entire sun.
This means that total solar eclipses will only occur for another 600 million years, more or less. So, grab that experience while you can.
