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Every year the moon’s orbit gets a little bigger and it moves just a little farther away. Should we worry about the Moon breaking free?

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Sources:
http://www.bbc.com/news/science-environment-12311119
http://curious.astro.cornell.edu/our-solar-system/the-moon/37-our-solar-system/the-moon/the-moon-and-the-earth/146-what-happens-to-the-earth-as-the-moon-gets-farther-away-and-will-it-ever-go-away-entirely-intermediate
http://curious.astro.cornell.edu/our-solar-system/the-moon/37-our-solar-system/the-moon/the-moon-and-the-earth/111-is-the-moon-moving-away-from-the-earth-when-was-this-discovered-intermediate
https://eclipse.gsfc.nasa.gov/SEcat5/secular.html
https://bowie.gsfc.nasa.gov/ggfc/tides/intro.html
https://www.universetoday.com/128350/will-earth-lock-moon/
http://www.haydenplanetarium.org/tyson/read/1995/11/01/the-tidal-force
https://www.scientificamerican.com/article/earth-rotation-summer-solstice/
https://www.space.com/3373-earth-moon-destined-disintegrate.html
http://large.stanford.edu/courses/2007/ph210/mccourt1/
https://www.researchgate.net/figure/284985952_fig8_Fig-9-Tidal-bulge-retardation-of-Phobos-a-and-Deimos-b-The-retardation-of-the
http://onlinelibrary.wiley.com/doi/10.1029/1999RG900016/abstract
https://www.nist.gov/nist-time-capsule/any-object-any-need-call-nist/moon-and-back-25-seconds
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Images:
https://www.nasa.gov/image-feature/galileo-earth-and-moon
As kids, we learn that the Moon stably orbits the Earth at an average distance of 384,000 kilometers.

That’s not completely true, though—every year, the moon’s orbit gets a little bigger, and our favorite satellite moves just a little farther away. But luckily for us, we don’t have to worry about the Moon ever getting so far away it breaks free.

On average, the Moon’s orbit is currently growing about 3.8 centimeters per year—at around the same rate as your fingernails. That rate has varied a lot since the Moon’s formation about 4.5 billion years ago. Back then, models suggest it was as close as 22,500 kilometers from Earth, which would have made it look 17 times bigger in the sky than it does now.

Talk about a moonlit night. You would think that if the moon keeps slipping away, it would eventually drift beyond the influence of Earth’s gravity. But the reason it’s moving away also explains why it will never leave us—a principle known as the conservation of angular momentum.

In physics, angular momentum describes something’s tendency to stay rotating once it’s started. That something can be one object, like a figure skater, or it can be a system of multiple objects moving together, like the Earth and Moon. Angular momentum depends on two things: the way the system’s mass is distributed, and how fast it’s rotating.

When the momentum is conserved, that means it has to stay constant. So if the rotation speed changes, the way the mass is distributed also has to change to compensate, and vice versa. You know how a spinning figure skater slows down when they extend their arms?

That’s conservation of angular momentum. Extending their arms moves some mass farther away from their body, which would increase their angular momentum. So their rotation speed slows down to compensate and keep their angular momentum the same.

Something similar happens with the Earth and the Moon. The way things are now, the Moon’s gravity tugs on the Earth, creating tidal bulges. But because our planet rotates faster than the Moon orbits, the bulge spins ahead of the Moon.

As the Moon pulls on it, Earth’s rotation slows down. Forces always come in equal and opposite pairs—thanks, Mr. Newton—so the Earth is simultaneously tugging on the Moon to try and make it catch up.

The Earth loses a lot of energy due to friction as its insides and the oceans slosh around during this whole process, which also slows its spin. Since the Earth’s rotation is slowing down, some mass in the Earth-Moon system moves farther from the center to compensate— and that mass, in this case, is the Moon. Theoretically, tens of billions of years from now, this tug-of-war between the Earth and the Moon would cause the Earth’s rotation to slow to exactly matched the Moon’s orbit.

The length of one day would be the same as one lunar month with that extended orbit, around 6 weeks or so. And just like how we see only one side of the moon, only one side of the Earth would face the moon. They’d both be tidally locked.

At that point, the Earth’s tidal bulges would always be directly in line with the Moon, so it would finally stop getting farther away. Unfortunately for theory, after only a couple of billion years, our Sun will have grown into a red giant, likely consuming both the Earth and the Moon way before they ever get to that point. So I guess you could say that the Moon will leave Earth’s orbit... but only because it’ll come crashing back into Earth when the Sun reaches its midlife crisis.

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