The Moon is slowly distancing itself from Earth, which means total solar eclipses have an expiration date.
Concept art of an eclipse. Image: Pixabay/Vancarlosfr
The space rock we call home has a lot of great things going for it. Not only is Earth the only world in the universe known to support life, it's also generous about producing gorgeous spectacles for these lifeforms to enjoy, with total solar eclipses being a particularly topical example. (The United States will be treated to such an eclipse on August 21.)
No other planet has lucked out with such a sensational glimpse into our solar system's orbital mechanics. Neighboring terrestrial worlds are either bereft of moons, or in the case of Mars, which has two, they are not large enough to block the Sun. Jupiter has dozens of moons, but you couldn't watch an eclipse from there, either: There's no terra firma from which to experience the shadows these satellites cast on the gas giants. Way out in the solar boonies, there's Pluto and Charon, its largest moon, which eclipse each other. But the Sun is so distant from this system's vantagepoint that the results are far less dramatic than the well-matched apparent sizes of the Moon and Sun from Earth.
In other words, Earth has found yet another way to be the MVP (most valuable planet). But Earthlings won't have it this good forever. Right now, total solar eclipses occur once every 18 months on average, a frequency that makes it easy to take them somewhat for granted, even though they're often only visible from places on our planet that are hard to reach. One day in the distant future, however, the Moon will fully block out the Sun for the last time.
The tidal forces that Earth and the Moon exert on each other are causing our natural satellite to gradually distance itself from us, while also slowing down Earth's rotation ever so slightly. In about five billion years, when the Sun enters old age as a red giant star, the Moon's orbit will be about 40 percent wider than it is now, and Earth's day may last over 40 hours. Long before that, the Moon will have wandered so far from our planet that it will no longer be able to blot out the full shape of the Sun when it passes in front of it.
According to Richard Vondrak, emeritus scientist for lunar exploration at NASA Goddard Space Flight Center, the last total solar eclipses visible from our planet's surface will occur some 600 million years from now. After that, the orbital show we've enjoyed for millennia on Earth will be over forever, assuming life on our planet is still around, that is.
"We do know that the Moon is now moving away from the Earth at the rate of nearly four centimeters each year," Vondrak told me. That may seem like a small amount, but on cosmic timescales, it adds up. After 600 million years, give or take, the Moon will have shifted over 22,000 kilometers from Earth, making it appear too small in the sky to fully cover the Sun's disk, even at the most optimal orbital positions.
For context, it was around 600 million years ago that complex multicellular organisms began to proliferate in Earth's seas during the Ediacaran period. Given how drastically different our planet looked back then—with one super-continent and life almost entirely restricted to oceans—it follows that future denizens of Earth, if there are any, will inhabit an unrecognizable world by the time the last total eclipse adorns the sky.
The Moon's umbra cast on Earth during the 2016 solar eclipse. GIF: NASA Earth Observatory/DSCOVR/EPIC
"Of course, it is impossible to predict where the lunar shadow will fall on the Earth [in 600 million years], or even where the Earth's continents might be at that time," Vondrak noted. Eclipse prediction is sophisticated, but it has its limits.
It is possible to accurately predict total solar eclipses on shorter time scales, though. For instance, Fred Espenak, a prominent eclipse expert who goes by the nickname Mr. Eclipse, created NASA's "Five Millennium Canon of Solar Eclipses," which charts out thousands of solar eclipses from 2,000 BCE to the year 3,000. The last total solar eclipse predicted in the canon occurs on April 26, 3,000, with the path of totality falling over South America, the Atlantic Ocean, and North Africa.
Naturally, the universe could always throw a wrench even in these meticulous predictions. A rogue planet could come crashing into our neighborhood to disrupt the orbital dance between Earth and the Moon, or a runaway greenhouse effect could veil our world in clouds that permanently obscure our view of these alignments from the surface. But if not—and let's hope not—Espenak's catalog has eclipse lovers covered, at least for the next 10 centuries.
"Determining the geographic location of eclipses beyond these dates would require a greatly improved estimate of the Earth's rotation rate with time," Vondrak said.
Because of the deep social significance of total solar eclipses as scientific catalysts, historical time-stamps, and ominous religious signs, we tend to imagine that they are as much a part of life on Earth as stars in the night sky or salt in the sea. But we are extremely fortunate to inhabit this planet at a time when the Moon is positioned at precisely the right distance to obstruct the Sun during totality, exposing the surreal corona (the outer solar atmosphere) for all to see. (PSA: If you plan to watch the eclipse on Monday, protect your sight with ISO-certified eyewear.)
Our blue planet today is not only a Goldilocks world in terms of habitability, it's also the best time and place in the entire solar system to stand in awe of total eclipses. Keep that in mind if you are watching Monday's eclipse, and stay safe in the Moon's shadow. And for anyone unlucky enough to have their view of this momentous event blocked by clouds, remember that you have 600 million years left to take in another one.
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