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If there truly is some extraterrestrial Hitchhiker’s Guide to the Galaxy it would undoubtedly list a total solar eclipse as one of the sights to see while taking a break from probing Earth’s natives. Total solar eclipses, called totality for short, are pretty rare here on Earth; a casual observer might see only one or two during their lifetime.

Since the Earth is the only planet we have ever known, we can’t really appreciate how truly lucky we are. The odds of the size of a planet’s moon exactly matching the apparent size of its sun are pretty low. If the moon is too large, it blocks the majestic solar corona visible during totality. If it is too small, then all solar eclipses would be annular, allowing a ring of the sun’s light to pass . The “Goldilocks” combination of Moon and Sun sizes on Earth makes totality possible, and unique in our solar system.

But alas, this beautiful phenomenon is ephemeral, at least in the geologic sense. The lunar disc shrinks slightly every year as the Moon recedes from the Earth; the chance of a total solar eclipse decreases correspondingly. Somewhere near 1 billion years from now, the last total solar eclipse will grace whatever residents of Earth there may be.

Unless you’re planning on living forever, 1 billion years probably seems safely tucked away into the future. So much so, perhaps that it may be entirely irrelevant. But, as the Moon moves further away, the length of a day here on Earth increases by about half a second each year. So every few years, the official arbiters of time at the Greenwich Royal Observatory add a leap second to our day.

This effect, while small during a human lifespan, has dramatically increased the length of the day over geologic time. When the moon was first formed, an Earth day was approximately 6 hours. By the time dinosaurs roamed Pangea, a day had reached 21 hours. The ultimate fate of the length of a day is that it will match that of a month at about 47 days. At that point, the Moon will hang suspended over a single point on the Earth for all time. But again, don’t panic! This is predicted to occur sometime long after the Earth and Moon have been utterly destroyed by the red-giant phase of our sun.

Strangely enough, the Moon is the cause of its own diminution in our sky. The pull of the Moon’s gravity causes tides here on Earth; that much we all know. But, because the Earth rotates more rapidly than the Moon travels in its orbit, that tidal bulge pulls the Moon forward ever so slightly. This constant extra force accelerates the Moon, which forces it further away from the Earth.

So how fast is the Moon pulling way? About 3.8 cm/year right now. One of the great successes of Apollo lunar science was to place a series of corner-cubic laser reflectors on the surface of the Moon. Scientists back on Earth then fire laser beams at those reflectors through carefully aimed telescopes. The travel time between firing and receiving the return signal gives the distance of the Moon accurate to about 1 cm. Scientists are working on improving this accuracy to around 1 mm.

Historically, the Moon did not recede so quickly. We know this because certain species of coral have daily growth patterns and are buried by annual loads of sediment called rhythmites. So, by counting the growth rings of the coral buried in one year of sediment, geologists have calculated the length of the day as far back as 600 million years or so. Based on these findings, we know that the Moon has been receding at about 2 cm/year on average. It just so happens that the current orientation of the Earth’s continents is almost optimal for accelerating the Moon.

If you missed the opportunity to catch that last total eclipse, don’t worry, there will be plenty more for the next few hundred million years. And when you do witness an eclipse (hopefully not while staring directly at it) perhaps you can ponder the staggering odds which have brought us all this amazing, and temporary, event.

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