“We live in a moment of history where change is so speeded up that we begin to see the present only when it is already disappearing.” -R. D. Laing

Throughout all of human history, when the new Moon passed directly between the Earth and the Sun, one of three things have happened. Either we’ve gotten a total solar eclipse, where the Moon is close enough to Earth to have its shadow fall on it; an annular eclipse, where the Moon is too far from Earth and its shadow ends before it reaches our planet; or a hybrid eclipse, where sunrise/sunset observers see an annular eclipse and midday observers see a total eclipse, with the ~4,000 mile (~6,000 km) disparity in the Earth-Moon distance making all the difference. Only about 40% of solar eclipses are total eclipses these days, but that ratio is much smaller than it used to be. Moreover, the Moon continues to migrate away from the Earth, meaning our planet’s final moment of totality will come in just 650 million years.

The Moon and Sun each take up approximately half a degree on the sky as viewed from Earth. When the Moon is slightly larger in angular size than the Sun is and all three bodies perfectly align, a total solar eclipse is the result. Image credit: Romeo Durscher / NASA / Goddard Space Flight Center.

About twice a year, the Moon appears to pass in front of the Sun, giving rise to a partial solar eclipse if the alignment is imperfect, but leading to either a total or annular solar eclipse if the Earth, Moon and Sun all form a straight line in space. It’s only by chance that the Moon and the Sun each take up approximately half-a-degree on the sky as seen from Earth’s surface, something that wasn’t true in the past and won’t be true in the future.

Right now, the largest (perigee) full Moon appears bigger than the Sun at all times of the year. However, over time, the Moon will migrate away, causing its angular diameter to shrink. When the perigee full Moon is smaller than the aphelion Sun, no total solar eclipses can occur anymore. Image credit: Ehsan Rostamizadeh of Astrobin.

Because both the Earth’s orbit around the Sun and the Moon’s orbit around the Earth are ellipses rather than circles, sometimes the Moon appears larger than the Sun, casting its shadow all the way down to Earth’s surface, while at other times the Sun appears bigger, with the Moon unable to completely cover the solar disk.

When the Earth, Moon, and Sun perfectly align during the new Moon, a solar eclipse will result. But whether that’s annular, total, or hybrid depends on the Moon’s distance from Earth. Image credit: NASA’s Scientific Visualization Studio.

Back when the Moon first formed, it was much closer to the Earth, while our planet spun much more rapidly. Just like a spinning tire slows down slightly when you touch your finger to it, the Moon — thanks to the tidal forces it exerts on the spinning Earth — has caused our day to lengthen considerably over the history of the Solar System. Modern measurements teach us that with each year that goes by, it takes an extra 14 microseconds for the Earth to complete its daily rotation. This is why we add a “leap second” to catch up every 18 months. It’s a very slow process, but one that incrementally builds upon itself.

A massive collision of large objects in space can cause the larger one to kick up large amounts of debris, which can then coalesce into multiple large objects, such as moons, that remain close to the parent body. An early collision like this likely created the Moon, which has been slowing Earth’s rotation and migrating away from our world ever since. Image credit: NASA/JPL-Caltech/T. Pyle (SSC).

Over geological times, this really adds up! If we go back to the daily patterns left in the soil from the tides — known as tidal rhythmites — we can calculate what the period of Earth’s rotation was from it. If we look at the most ancient one we know of on Earth, from 620 million years ago, we find that a day back then was a little under 22 hours long!

The Touchet formation showcases the rhythmic deposits of material occurring over very long timescales, telling us information about the lengths and durations of the tides throughout Earth’s history, enabling us to reconstruct how long a day was in the past. Image credit: Wikimedia Commons user williamborg.

If you extrapolate this tidal braking back to when the Earth was first formed, 4.5 billion years ago, you’ll find that a day was originally only around 23,000 seconds, or six-and-a-half hours! Some four billion years ago, a “day” on Earth lasted barely 25% as long as the 24 hour day we know at present. So over time, the Earth has lost angular momentum due to the tidal friction of the Moon.

The asymmetrical nature of Earth, compounded by the effects of the Moon’s gravitational pull, causes the length of a day on Earth to lengthen over time. To compensate and conserve angular momentum, the Moon must spiral outwards. Image credit: Wikimedia Commons user AndrewBuck, modified by E. Siegel.

But there’s a law of nature — a quantity that’s conserved — that tells us if Earth’s rotation is slowing down, something else needs to happen to compensate for it. That law is the conservation of angular momentum, and the thing that compensates is that as the Earth’s spin slows down, the Moon spirals farther and farther out from Earth! And the farther away the Moon gets, the smaller its angular size, and thus the smaller it appears in the sky. As time goes on, more and more of the solar eclipses will be annular rather than total, as the Moon’s size will appear insufficient to block out the Sun.

While approximately half of all eclipses today are annular in nature, the increasing Earth-Moon distance means that in approximately 600–700 million years, all solar eclipses will be annular in nature. Image credit: Wikimedia Commons user Kevin Baird.

On the timescale of a year, you can’t even notice the increased distance with sophisticated laser lunar ranging: the difference in the Moon’s orbit is mere centimeters-per-year. But over long periods of time, this adds up significantly. Approximately 570 million years from now, the very last total solar eclipse will occur, and after another ~80 million years, the last hybrid eclipse will occur. That will be the last time any portion of Earth finds itself bathed in the shadow of the Moon. Beyond that point, the Moon will no longer be close enough to Earth at any point in its orbit to have its shadow fall on our surface. From that moment onward, the only way to see a total solar eclipse will be to take to the skies, or to soar in space itself, where we can find ourselves in the Moon’s shadow once again.

We may have had roughly three billion total solar eclipses on Earth so far, but that’s more than 90% of all the darkness-bringing eclipses our planet will ever see. After another 650 million years, the Sun will always appear larger in the sky, even at aphelion, than the closest, largest new Moon will ever be. Appreciate the unique natural sights that the world has to offer today, because all things, in time, will pass away. Solar eclipses are slowly disappearing, and there’s nothing we can do to stop it.