When Will Earth Lock to the Moon?

Article written: 11 Apr , 2016
Updated: 23 Feb , 2017
by

We always see the same side of the Moon. It’s always up there, staring down at us with its terrifying visage. Or maybe it’s a creepy rabbit? Anyway, it’s always showing us the same face, and never any other part.

This is because the Moon is tidally locked to the Earth; the same fate that affects every single large moon orbiting a planet. The Moon is locked to the Earth, the Jovian moons are locked to Jupiter, Titan is locked to Saturn, etc.

As the Moon orbits the Earth, it slowly rotates to keep the same hemisphere facing us. Its day is as long as its year. And standing on the surface of the Moon, you’d see the Earth in roughly the same spot in the sky. Forever and ever.

Forever and ever and ever... unless we finally manage to destroy the Moon. Credit: NASA/Goddard/Arizona State University

Because of tidal locking, you’d see Earth in roughly the same spot from the Moon forever. For-eh-ver. For-EH-VER. Credit: NASA / Goddard / Arizona State University

We see this all across the Solar System.

But there’s one place where this tidal locking goes to the next level: the dwarf planet Pluto and its large moon Charon are tidally locked to each other. In other words, the same hemisphere of Pluto always faces Charon and vice versa.

It take Pluto about 6 and a half days for the Sun to return to the same point in the sky, which is the same time it takes Charon to complete an orbit, which is the same time it takes the Sun to pass through the sky on Charon.

Since Pluto eventually locked to its moon, can the same thing happen here on Earth. Will we eventually lock with the Moon?

Before we answer this question, let’s explain what’s going on here. Although the Earth and the Moon are spheres, they actually have a little variation. The gravity pulling on each world creates love handle tidal bulges on each world.

And these bulges act like a brake, slowing down the rotation of the world. Because the Earth has 81 times the mass of the Moon, it was the dominant force in this interaction.

In the early Solar System, both the Earth and the Moon rotated independently. But the Earth’s gravity grabbed onto those love handles and slowed down the rotation of the Moon. To compensate for the loss of momentum in the system, the Moon drifted away from the Earth to its current position, about 370,000 kilometers away.

But Moon has the same impact on the Earth. The same tidal forces that cause the tides on Earth are slowing down the Earth’s rotation bit by bit. And the Moon is continuing to drift away a few centimeters a year to compensate.

It’s hard to estimate exactly when, but over the course of tens of billions of years, the Earth will become locked to the Moon, just like Pluto and Charon.

Pluto and Charon are tidally locked to each other. Credit: NASA/JHUAPL/SwRI

Pluto and Charon are tidally locked to each other. Credit: NASA / JHUAPL / SwRI

Of course, this will be long after the Sun has died as a red giant. And there’s no way to know what kind of mayhem that’ll cause to the Earth-Moon system. Other planets in the Solar System may shift around, and maybe even eject the Earth into space, taking the Moon with it.

What about the Sun? Is it possible for the Earth to eventually lock gravitationally to the Sun?

Astronomers have found extrasolar planets orbiting other stars which are tidally locked. But they’re extremely close, well within the orbit of Mercury.

Here in our Solar System, we’re just too far away from the Sun for the Earth to lock to it. The gravitational influence of the other planets like Venus, Mars and Jupiter perturb our orbit and keep us from ever locking. Without any other planets in the Solar System, though, and with a Sun that would last forever, it would be an inevitability.

It is theoretically possible that the Earth will tidally lock to the Moon in about 50 billion years or so. Assuming the Earth and Moon weren’t consumed during the Sun’s red giant phase. I guess we’ll have to wait and see.

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4 Responses

  1. bazbsg says

    “Its day is as long as its year.”, referring to the moon. This one requires some thought. Is it a mistake and should be “Its day is as long as its month.”? What is a year on the moon (or any moon), the amount of time it takes to go around the earth or around the sun? What is a month; it’s based on the amount of time the moon takes to go around the earth, but not as precisely as a year is based on the amount of time it takes the earth to go around the sun. I couldn’t find anything online except in Yahoo answers that equates a year of a moon to its orbital period and that was also disputed within the same set up answers.

    • Dashdragon says

      The language used is more akin to: Year=how long a body takes to orbit its parent.

      For us and other planets in the solar system, that’s how long we take to orbit the sun. For any moon, it would be how long they take to orbit their planet. It’s meaningful since their solar year would be the exact same as the planet’s anyway.

  2. Pasander says

    Assuming the Sun would never go red giant (and no other big changes in the Solar System), would the Moon actually stay in orbit around the Earth long enough for the Earth to tidally lock with the Moon?

    • BlackWolfStanding says

      By definition, no. The moon has a retrograde motion to the Earth’s rotation. Both the Earth and the moon would have to stop. The moon is slowly drifting away from the Earth; Unless the moon can find a Lagrange point to rest in. Why? As it’s rotation slowed down, it was forced away from Earth to conserve angular momentum. As the Earth slows down to lock with the Moon, the moon will slow down further in it’s orbit. It will have to come to a geostationary orbit with respect to the Earth and the Sun. which happens at L1 and L2. If it can’t stop at one of those points, then it’s orbit will become unstable and it will leave Earth for good.
      What will happen with the moon? It could be flung out of our solar system. It could fall into the Sun. It could fall into the Earth. Or my favorite, is it could start it’s own unstable highly elliptical orbit around the Sun to crash into another planet or asteroid. Or maybe even be captured by another planet in our solar system.
      Anyway you slice it, we in for some fun times a head.

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