The total solar eclipse that stops traffic, brings strangers together in fields and parking lots and sends people driving hundreds of miles to stand in the exact right spot for exactly two minutes of darkness in the middle of the day, that specific experience may be one of the most fortunate accidents in the history of the universe.
According to physics that has been confirmed by lasers bounced off mirrors left on the moon by Apollo 11 astronauts in 1969, it is not going to last forever.
The moon is moving away from Earth. It has been moving away since it formed.
It is currently moving away at a rate of 3.8 centimeters per year, less than an inch and a half, and that number sounds so small that it is easy to dismiss as irrelevant.
Space Daily, which covered the research behind this finding, was specific about why that reaction is wrong:
“That number is small enough to sound trivial. It is not. The same measurement, extended forward, eliminates an astronomical phenomenon that human beings happen to be alive at exactly the right moment in geological time to witness.”
In approximately 600 million years, the moon will be too far away to completely cover the sun from Earth’s perspective.
Total solar eclipses, the kind where the sky goes dark at noon and the sun’s corona blazes around a black disk and every animal in a field stops what it is doing, will cease to be possible.
What will remain are annular eclipses, the moon passing in front of the sun but leaving a ring of fire around the edges because it no longer appears large enough to cover the full disk.
The people alive today are living in the window. The window is 600 million years wide. It is also closing.
How We Know The Moon Is Moving Away
In July 1969, Apollo 11 astronauts Neil Armstrong and Buzz Aldrin walked on the surface of the moon and left behind, among other things, a suitcase-sized array of corner-cube retroreflectors on the Sea of Tranquility.
Corner-cube retroreflectors are optical devices that reflect light directly back toward its source regardless of the angle of incidence, they work for the same reason that highway signs appear to glow in your headlights.
The ones on the moon are designed to reflect laser beams fired from Earth directly back to the point of origin.
Observatories have been firing lasers at that mirror, and at three others placed on the lunar surface by later Apollo missions and by Soviet Lunokhod robotic missions, for more than fifty years.
The specific facilities involved include the Apache Point Observatory in New Mexico and the Côte d’Azur station in France.
They measure the round-trip travel time of laser pulses with precision down to a few picoseconds, a picosecond being one trillionth of a second.
From that round-trip travel time, measured over decades and across thousands of individual laser firing events, scientists have produced the single most precise measurement in all of lunar science. The moon is moving away from Earth at exactly 3.8 centimeters per year.
The measurement is not an estimate based on modeling. It is a direct physical measurement, conducted for more than fifty years, by some of the most precise timing instruments humans have ever built.
Neil deGrasse Tyson stated the reality plainly on StarTalk: “Ever since the moon was formed, it has been spiraling away from us, continuously.”
Why Do Total Solar Eclipses Happen?
The reason a total solar eclipse is possible at all is one of the most improbable coincidences in planetary science. The sun is approximately 400 times wider than the moon.
It is also approximately 400 times farther from Earth than the moon is. Those two facts together mean that from Earth’s surface, the sun and the moon appear to be almost exactly the same size in the sky, the sun’s enormous physical diameter compensated precisely by its enormous distance, the moon’s modest physical diameter compensated precisely by its modest distance.
That coincidence is not required by any law of physics. There is no reason the sun should appear the same size as the moon from Earth’s surface. It just does.
It is only because of that coincidence that the moon can, at certain orbital alignments, move directly between Earth and the sun and cover the solar disk completely, blocking the photosphere while leaving the corona, the sun’s outer atmosphere, visible as a ring of light around a black circle in a darkened sky.
If the moon appeared slightly smaller in the sky than the sun, as it will in 600 million years, the best we would get during a solar eclipse is an annular eclipse. The moon covering most of the sun’s face but leaving a bright ring around the edges.
You can see the ring. You cannot see the corona. The sky does not go dark. The phenomenon is interesting but it is not total.
The window in which total solar eclipses are possible, the window in which the moon appears exactly large enough to cover the sun’s disk from Earth’s surface, is not permanent.
It is a function of the moon’s current distance from Earth. As the moon moves away, its apparent size shrinks. Eventually the geometry no longer works and total solar eclipses become impossible.
The Physics Behind The Moon Moving Away
The moon’s outward drift is not random. It is a direct consequence of tides, the same gravitational interaction that pulls the oceans toward the moon and causes high and low tides twice a day.
David Waltham, a professor of geophysics at Royal Holloway, University of London, who studies the relationship between the moon and Earth, explained the mechanism directly:
“It’s all about tides. The tidal drag on the Earth slows its rotation down, and the Moon gains that energy as angular momentum.”
The tidal bulge that the moon raises in Earth’s oceans is not perfectly aligned between Earth and the moon, because Earth rotates faster than the moon orbits, the tidal bulge sits slightly ahead of the moon’s position.
That offset creates a small but persistent gravitational tug that pulls the moon forward in its orbit.
Being pulled forward in an orbit is counterintuitive, it actually causes the orbiting object to move to a higher, larger, slower orbit rather than speeding up in place.
The moon gains orbital energy and drifts farther out. Earth loses rotational energy and slows its spin.
The slowing of Earth’s rotation is not currently noticeable on a human timescale. But across geological time, the effect is significant. Geophysicist Tom Eulenfeld, who led a study on Earth’s historical rotation at Friedrich Schiller University in Jena, Germany, described what deep time analysis of this process reveals:
“The faster-rotating Earth shortened the length of the day so that within 24 hours there were two sunrises and two sunsets, not just one each as today. This may have reduced the temperature difference between day and night, and may have affected the biochemistry of photosynthetic organisms.”
When the Earth rotated faster, days were shorter. The difference in temperature between the daytime maximum and the nighttime minimum was smaller.
The specific biochemistry of organisms that rely on photosynthesis, every plant, every algae, every organism that converts sunlight to chemical energy, was operating in a different temporal environment than it operates in today.
The moon’s distance from Earth, and the rate at which it is changing, has been quietly shaping life on this planet for billions of years.
Six Hundred Million Years In Cosmic Terms
The number 600 million sounds enormous. In human terms it is incomprehensible, it is roughly three times as long as complex animal life has existed on Earth, roughly 120,000 times as long as recorded human history.
The statement that total solar eclipses have approximately 600 million years remaining is functionally equivalent to saying they will be available essentially forever by any scale humans can meaningfully contemplate.
But cosmic scales are not human scales. The Earth is approximately 4.5 billion years old. Total solar eclipses, the precise alignment of apparent sizes that makes them possible, exist within a window of perhaps a billion years out of that 4.5-billion-year history.
The window is not centered on the present, it has been open for hundreds of millions of years already and it has hundreds of millions more to go. In geological terms, we are in the middle of it.
In cosmic terms, as Space Daily noted, it is fleeting. The specific accident of the apparent size matching, the coincidence that makes the corona visible and the sky dark and people drive hundreds of miles to stand in a field and look up, is not a permanent feature of the solar system. It is a temporary condition that happens to describe the current era.
The people who watched the April 2024 total solar eclipse from the path of totality across North America were not watching a reliable annual event. They were watching one of the remaining occurrences of a phenomenon that will eventually stop happening.
The next total solar eclipse visible from the continental United States is in 2033. Every one of those events is one of a finite number still available to human observers on this planet.
The lasers bouncing off corner-cube retroreflectors on the Sea of Tranquility measure 3.8 centimeters per year. That number, extended forward, ends something extraordinary.
