Every time I see the moon in the sky, it makes me happy. Our natural satellite is so familiar it’s easy to take it for granted, but in fact it’s an amazing sight. Much of its history is written on its face, with huge dark spots marking where ancient gigantic impacts scarred its surface, still visible after billions of years.
But what of its fate; what will happen to the moon in eons hence? That future is tied up in how it formed, how it’s gravitationally linked to Earth, and, in the end, what the sun has to say about all this.
The big picture of the moon’s origin is fairly well established. Several tens of millions of years after Earth itself formed, a large planet-sized object slammed into our nascent world, delivering a glancing blow that blasted vast amounts of material into space. This ejecta—a mix of Earth and the giant impactor—coalesced to form the moon.
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Back then, the young moon was extremely close to Earth, around 20,000 kilometers away—much nearer than its current average distance of 380,000 km. The moon would have been enormous in Earth’s sky, about 10 degrees across—roughly the size of your fist held at arm’s length.
However, that soon changed. Earth’s gravity had a huge effect on the nearby moon, creating immense tidal forces. I describe how this works in the April 12, 2024 The Universe article, but in a nutshell, the force of gravity weakens rapidly with distance. This means the side of the moon closer to Earth feels a stronger gravitational force from the planet than the side farther away. This change in force across the moon’s 3,400-km diameter stretches our satellite from a sphere to a subtle egg shape, with Earth-aligned bulges on the near and far side.
When the moon first formed it was also rotating rapidly. And, because of inertia, this spin swept the tidal bulges slightly ahead of the imaginary straight line between it and Earth. Our planet’s gravity, however, yanked on the bulges, trying to align them once again. This slowed the moon’s rotation, lengthening its “day”—and some of that energy lost as the moon’s spin slowed was transferred to its orbital motion, moving the moon away from Earth. That recession was initially quite rapid, but ebbed as the moon moved farther out and the tidal force diminished. Eventually the moon’s spin slowed so much it matched how long the moon takes to orbit Earth once. This is called tidal locking, and is why we see essentially just one face of the moon today.
But this also works both ways; the moon also exerts tides on Earth, raising two bulges on the planet. Earth’s rotation sweeps them out of alignment as well, with the moon yanking back on them, gradually slowing Earth’s spin. Our day is getting longer…but only by about two milliseconds per century, so no need to reset your clock. Still, this effect also adds to the enlargement of the moon’s orbit, so that even today, billions of years after it formed, the moon recedes from Earth at a rate of about four centimeters per year.
Put all this together and extrapolating the moon’s fate seems simple: it will keep receding, more and more slowly, until eventually Earth’s rotation matches the moon’s orbital period. At that point Earth will always show the same face to the moon, and, for an observer on the planet, the moon will never rise nor set, always appearing in the same place in the sky.
If it ever happens, that is. This process would take billions of years to play out, and we may not have that long.
Earth’s oceans play the dominant role in this grand gravitational process. They absorb most of the energy from the moon’s tidal force, sloshing around and creating friction that slows Earth’s rotation. But those oceans won’t last forever.
The sun is heating up. This is a natural outcome of how our star generates energy by fusing hydrogen into helium. That helium “ash” is building up in the sun’s heart, compressing and getting ever hotter as it accumulates. This makes the sun shine brighter, which in turn heats up Earth. In about a billion years, give or take, this will make the Earth so hot that the oceans literally boil away, leaving behind a parched, barren world.
Besides the obvious problems this poses for life on Earth, the lack of oceans would dramatically slow any subsequent tidal evolution. The moon’s recession would wane, as would the change in Earth’s spin rate, adding many billions of years to the timescale—time the sun does not have.
Six to seven billion years from now, the sun will run out of hydrogen fuel in its core and swell into a red giant. Scientists argue whether it will expand enough to engulf Earth or not, but that’s fairly irrelevant to the point here; the moon and Earth will be cooked, and the ultimate tidal fate of our constant lunar companion will be the least of our problems.
Eventually the sun will shed its outer layers, losing about half its mass, and the exposed core will become a white dwarf, slowly cooling over billions of years. That’s not the end of the tidal story, though: the sun raises tides on Earth as well, and the current effect is about half the strength of the lunar tides. However, the physics of how this solar tide will work on the Earth-moon system in the distant future is quite complicated and not fully understood. The loss of solar mass means the effects will be smaller, too.
On the other hand, by then we’ll have nothing but time; over the eons the white dwarf sun’s tides may yet destabilize the moon’s orbit, yanking it free from Earth or possibly causing it to collide with our planet. Either way, at that point it will hardly matter to our broiled world.
Which is it? We may just have to wait and see. This won’t occur for tens of billions of years, which in the potentially infinite future of the universe is like a single tick of the cosmic clock. Better enjoy your view of the moon now while you can.
