Why Future Earthlings Won't See Total Solar Eclipses

Aug 14, 2017
Originally published on August 16, 2017 7:40 am

Anyone who gets to see the total solar eclipse on August 21 will be lucky — and humanity is lucky to live on a planet that even has this kind of celestial event.

Mercury and Venus, after all, don't even have moons. Mars has a couple, but they're too small to completely blot out the sun. Gas giants like Jupiter do have big moons, but they don't have solid surfaces where you could stand and enjoy an eclipse.

And, even with solid land and a moon, Earth only gets its gorgeous total solar eclipses because of a cosmic coincidence.

"They appear to be the same size because of their distance away from us," explains Amber Porter, an astronomer at Clemson University, which is in the path of the upcoming eclipse. The diameter of Earth's moon is about 400 times smaller than the diameter of the sun, but "even though the moon is about 400 times smaller than the sun, it's about 400 times closer to us here on Earth, which is how that perfect kind of magic happens."

Because of this quirk, the tiny moon can obscure the entire face of the sun and reveal its eerie corona, at least right now. In the past, Earth's eclipses did not look like this.

"The size of the sun hasn't really changed over the age of Earth, but the moon has been moving away from Earth over eons. So in the past it looked bigger," says Matija Cuk, a planetary scientist at the SETI Institute.

The moon is still moving away from Earth, he says. Every year, it shifts outward about an inch-and-half.

"So actually for billions of years you can have a total eclipse, but this very evenly matched eclipse, where it is barely total, that happens for a relatively short amount of time," says Cuk.

In only about 600 million years, the moon will look small enough that it no longer completely covers the sun, and whoever is left on Earth won't see any more total solar eclipses. So, get them while you can.

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RACHEL MARTIN, HOST:

So do you have your solar glasses yet? Maybe you're already on your way to a total eclipse destination. Or maybe, like those of us in Washington, D.C., you'll have to just look for weird shadows and try not to burn out your corneas. Either way, in just five days, across a broad stretch of the United States, the moon will appear to perfectly cover up the glowing disk that is the sun. A total solar eclipse is a rare occurrence. But NPR's Nell Greenfieldboyce reports that it is a celestial stroke of luck that it can happen at all.

NELL GREENFIELDBOYCE, BYLINE: Earth is lucky to have total solar eclipses. Mercury and Venus don't even have moons. Mars has a couple, but they're too small to cover up the sun. Gas giants like Jupiter do have big moons, but it's not like these planets have a solid surface where you can stand and watch an eclipse. Here on our planet, it all comes down to cosmic coincidence.

AMBER PORTER: Even though the moon is about 400 times smaller than the sun, it's about 400 times closer to us here on Earth, which is how that perfect kind of magic happens.

GREENFIELDBOYCE: Amber Porter is an astronomer at Clemson University, which is in the path of the upcoming eclipse. From the Earth the moon and the sun seem to take up about the same amount of space in the sky.

PORTER: They appear to be the same size because of their distance away from us.

GREENFIELDBOYCE: And here's another way we're lucky. It's just chance that humans exist at a time when we get to see this perfect alignment, which reveals the eerie, glowing crown of light around our sun. In the past, Earth's eclipses did not look like this. Matija Cuk is a planetary scientist at the SETI Institute.

MATIJA CUK: The size of the sun hasn't really changed over the age of Earth, but the moon has been moving away from Earth over eons. So in the past, it looked bigger.

GREENFIELDBOYCE: The moon is still moving away from Earth. He says every year, it shifts outward about an inch and a half.

CUK: So actually, for billions of years, you can have a total eclipse. But this very evenly matched eclipse, where it's barely total, that happens for a relatively short amount of time.

GREENFIELDBOYCE: In only about 600 million years, the moon will look small enough that it no longer completely covers the sun. That means no more total solar eclipses, so get it while you can. Nell Greenfieldboyce, NPR News.

(SOUNDBITE OF SONG, "BLUE MOON")

THE MARCELS: (Singing) Blue moon, you saw me standing alone without a dream in my heart... Transcript provided by NPR, Copyright NPR.