Will the Earth ever be tidally locked to the Moon?

Will the Earth ever be tidally locked to the Moon?

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From my basic understating,

Momentum is being transfered from the Earth's rotation to the Moon's orbit by tidal friction. The Earth's rotation slows down and the Moon receedes from the Earth as it moves into a higher orbit. This will continue until the Earth's rotational period is equal to the orbital period of the Moon, i.e the Earth is tidally locked to the Moon.

Assuming I have the above correct - and please correct me if I don't - will there, realistically, be enough time for tidal locking to occur before the sun expands and engulfs the Earth? Or is there another reason the Earth will never be locked towards the Moon?

As the moon orbits Earth, tidal forces slow down the Earth's rotation by 2 milliseconds per century. Eventually, in tens of billions of years, the Earth and Moon would achieve a double tidal lock, where both are stuck with one side facing the other as they orbit the Earth-Moon barycenter. In 7.5 billion years, the Sun will expand past the Earth's current orbit, but the Earth may drift out further, preventing it from being vaporized.

However, this is beside the point, because in about one billion years, all of Earth's water will have boiled away, meaning that there would be no more ocean tides, and thus the Earth-Moon system will likely never achieve a double tidal lock.


  • Will the Moon ever leave Earth's orbit? - Giles Sparrow, Space Answers

  • Harvesting Lunar Eccentricity? - Terry R. McConnell, Syracuse University

  • When Will Earth Lock to the Moon? - Fraser Cain, Universe Today

  • The Sun Will Eventually Engulf Earth--Maybe - David Appell, Scientific American

  • Distant future of the Sun and Earth revisited - K.-P. Schröder and Robert Connon Smith, MNRAS

  • When will Earth lose its oceans? - CNRS, Science Daily

A Rocky Relationship: Is the Moon Leaving the Earth?

It’s comforting to look up at the night sky. The bright stars seem like old familiar friends…the pockmarked Moon is like a steady and unchanging ally. From our perspective on Earth, the universe seems stable and eternal

Of course, this isn’t how things really are. In reality, our night sky is in a constant state of flux.

Most of you have probably heard that the relationship between the Earth and Moon is a little…unstable (for lack of a better term). Although the Moon seems nice and reliable, I’m afraid that the rumors are true — the Earth and Moon are drifting apart. Each day, the Moon is slowly getting farther and farther away. It is increasing its orbit and moving away from us at a rate of 1.5 inches a year (4 cm). True, it is a bit unsettling. However, I’ve heard a lot of people blowing this out of proportion. You see, there are a number of individuals who believe that the Moon is leaving for good (hopefully, you never believed these slanderous lies, but a lot of people do).

Individuals frequently talk about a permanent breakup. And this, I am glad to say, is utter nonsense.



As is true of many rocky relationships, the Earth and Moon only need a bit of time and space to work things out. Ultimately, we just need to be patient. In about 50 billion years, the Moon will stop moving away from us and settle into a nice, stable orbit. At this point, the Moon will take about 47 days to go around the Earth (currently, it takes a little over 27 days). When this new stability is achieved, the Earth and the Moon will be tidally locked to each other. As a result, it will look like the Moon is always in the same spot in the sky — so our relationship will seem more stable than ever!

…of course, the Sun is going to turn into a Red Giant in about 5 billion years, and many scientists believe that it will consume our planet in the process. So the Earth and Moon will be obliterated long before they achieve stability…and the human race will probably be extinct. But we all may be churning together beneath the searing surface of our Sun, together at last. It is kind of a beautiful thought…kind of.

As a Futurism reader, we invite you join the Singularity Global Community, our parent company’s forum to discuss futuristic science & technology with like-minded people from all over the world. It’s free to join, sign up now!

Will the moon ever become un tidal locked?

So currently the moon is tidal locked, but the moon is also very slowly moving away from earth. Does this mean it will ever become un tidal locked on the distant future ?

Tidal locking is generally an equilibrium that objects move towards, and they would need some external force to change that.

If the Moon ever left Earth's hill sphere, it would no longer be tidally locked, but IIRC the Sun is going to expand and eat us before that happens.

The primary mechanism which caused the Moon to become tidally locked to the Earth is exactly the same thing causing the Moon to slowly drift away. The Moon (and Earth) is slightly distorted by tidal forces resulting in a tidal bulge along the axis between the two bodies. If, for example, the Moon had a rotation faster than its orbital period the bulge would be rotated slightly ahead of this axis and the stronger gravitational force on the near-side of the Moon would apply a net torque back towards the axis, ultimately slowing down the Moon's rotation. In this way, the existence of tidal forces between the Earth and mMoon guarantees the moon would inevitably become tidally locked.

The Moon is moving away from the Earth for essentialy the same reason (although a bit more complicated as the Earth has a lot of liquid water). Roughly speaking, the Earth rotates too fast and hence the tidal forces from the Moon act like a brake, slowly lengthening our day. Conservation of angular momentum means that slowing the Earth's rotation increases the Moon's angular momentum and hence shifts its orbit further away from the Earth.

In principle, the Earth would eventually become tidally locked to the Moon and the Moon's orbit would stop shifting further away, assuming that the Moon remains within the Earth's hill sphere in which these sort of two-body considerations remain a good description. However, we can calculate that this would happen well after the Sun expands into a red giant.

Can the Earth ever become tidally locked to the Sun so one side is always facing inwards (like the Moon)?

The titles pretty much explains it all. I understand how the moon is locked to the Earth but wondering if this could ever happen on a scale as large as the Earth/Sun. Bonus question: What would happen to the moon if the Earth was locked to the sun?

The earth used to spin much faster millions of years ago and has been gradually losing rotational energy. Not sure of the exact details but it is caused by the interaction between the moon and the earth through ocean tides. A torque is set up which accelerates the moon and decelerates the earth. Here's the wiki

Anyway the earth will eventually be tidally locked to the moon so that 1 day = 1 month.

Because of the relative distance, the sun has a much smaller tidal effect on the earth so the process of becoming tidally locked to the sun will take a lot longer, but will occur by the same process as with the earth to the moon.

Edit: Someone has mentioned that the earth can't become tidally locked to the sun once it becomes tidally locked to the moon. I admit I don't understand the topic enough to understand if or why that would happen.

Anyway the earth will eventually be tidally locked to the moon so that 1 day = 1 month.

1 earth day = 1 lunar month?

one synodic month = 29.5 days (from full moon to full moon with reference to the Sun)

one sidereal month = 27.3 days (full orbit around Earth)

This is a good explanation, but it's actually tidal forces as a whole not ocean tides in particular. Any orbiting body will eventually become tide locked, even if there is no liquid (on the surface or otherwise).

The earth will be tidally locked to the sun at some point as well in theory, if the earth still exists at that point. The earth is slightly oblong so more of the mass is on one side. This makes the rotation of the earth get slower and slower until the heavy side will eternally point towards the sun. This is similar to the idea of a heavy boulder rolling between two hills and a valley, and ending at rest in the valley.

At this point there will be half of the earth with day, and half of the earth with night, which will wreak havoc on the environment. One side of the earth will constantly be heated, while the other side will always be cool, the only habitable zone with liquid water remaining will be a small band around the middle of the earth. Constant bombardment of electromagnetic waves from the sun, will mutate all life on one side of the planet, making it very difficult for any complex organism that relies on the finite network of inhibitory pathways, like us, to survive. This is due to mutations making the DNA translate to non-functioning proteins. Other effects of this will be higher cancer rates, faster aging, and much shorter life spans, assuming the temperature of the bright portion of earth even allows for any life at all.

EDIT: Mercury is not actually tidally locked to the sun, it is in a 3:2 spin-orbit resonance with the sun.

Tidal locking and Uranus

The formula they show for Time to tidal locking in the case of Uranus would look to be dominated by the a^6 term describing its orbital semi-major axis.

And this does not take into account the tidal locking of the other objects that are tidal locked to Uranus.

I'd say the sun expiry would intervene long before there would be a tidal locking of Uranus to the sun.

The formula they show for Time to tidal locking in the case of Uranus would look to be dominated by the a^6 term describing its orbital semi-major axis.

And this does not take into account the tidal locking of the other objects that are tidal locked to Uranus.

I'd say the sun expiry would intervene long before there would be a tidal locking of Uranus to the sun.

If it ever appears "locked" I'd have to wonder about the coincidence of it. The a^6 term - distance raised to the ^6 of its orbital semi-major axis [a rather large number] is offset only by Radius of it's mass in the denominator, raised to the ^5 - that is such a prodigious number to overcome that time to lock surely won't happen in the remaining Solar lifetime.

As I mentioned as an additional perturbating factor, Uranus has several moons (that happen as it turns out to be tidally locked to it) and I would have to think that any anomalies on Uranus would be subjected to greater forces from them than would be exerted by the Sun so far distant.

Are Moons Always Tidally Locked? (Not Really But…)

If you’re as curious as I am I’m sure you’ve fallen into quite the few rabbit holes looking into the nature of planets, moons in our solar system and the vast universe as a whole, occasionally asking yourself interesting questions like whether there plastic based planets in the universe, if planets can actually rain fire pellets or something a lot less fantastical like whether moons are always tidally locked.

To answer this very briefly, as far as we know all the major moons whether it be all 79 of Jupiter’s moons, Pluto’s moon Charon, our moon and the many other well known moons in our solar system are all in fact tidally locked. In theory all moons that we know of are tidally locked however, none of these moons were initially tidally locked therefore moons that have just started orbiting a planet will not be tidally locked.

So, in short not all moons within the universe would be tidally locked but all that we’ve observed are. Of course I’ll delve into this topic in a lot more depth throughout so stay tuned if you want a more thorough explanation.

What Is Tidal Locking?

If you’re unfamiliar with what tidal locking is, it’s when a body orbits around another object where its day and year length are equal to it’s orbit around said body, which results in that object only facing the other object in one locked position.

This is the reason why we only see specific side of the moon which has been the case since we first saw the entity. You may assume that the Moon is stationary however, this isn’t the case. In fact the moon spins around it’s axis once every full orbit it makes around our Earth, which roughly equates to 7 days for both.

As a result we only see one side of the moon even though it continues to spin on its axis. The same can be said for a large number of other moons that orbit their respective major planets too.

Sometime planets like Mercury can be confused as being tidally locked to the sun for example but, that’s a slightly different subject. (you can read about it by clicking here though)

Why Are Moons Tidally Locked To Their Respective Planets?

Moons are tidally locked to their respective planets due to the gravitational pull a planet has on them when they’re in the said planet’s region of orbit.

When a moon is within the gravitational pull of a planet the gravity of the planet forces a moon to orbit it, which results in a moon tidally locking to their respective planet (eventually).

How Do Moons Tidally Lock To A Planet?

The whole process is a little complicated to explain but, I’ll try to do so in a manner that is somewhat easy to understand.

First of all it’s worth noting that moons and particularly our moon weren’t always tidally locked to Earth. It only became as such over countless years and after physics had played its part in the equation.

According to many physicists it took roughly 100 million years for the moon to be tidally locked to Earth, which is also roughly the amount of time it took for the moons of the other planets within our solar system to achieve the same feat.

Moving onto the explanation, the reason for tidal locking is due to the gravity on a moon not being uniform throughout it’s entire body, meaning the side facing Earth, Jupiter, Saturn etcetera will have a larger gravitational force per unit mass over the opposite side.

As a result the weaker side will lag behind and distort the moon into an elliptical shape. In order to combat this distortion a moon has to adapt to the gravitational forces otherwise it will swing in a pendulum motion (which is theoretically what happens in the beginning stages).

Torque from the gravitational forces will try to maintain an equilibrium state for the moon that over time will be the tidally locked position for the moon.

In the earlier stages the use of gravitational torque will create a large amount of energy dissipation in the form of internal tidal heating that is created due to the squishing and squashing of the moon’s non ideal rotation on the axis. Eventually over many millions of years the moon of a planet will tidally lock.

What’s even more fascinating is that the respective planet of a moon will eventually tidally lock too the moon as well.

In fact that’s already the case with Pluto and Charon therefore, it’s very likely to occur with Earth too. At this current moment in time the Earth’s rotation around its axis is slowing down whilst the moon is orbiting even further out to compensate for this slow down.

Although the Earth or any of the other larger planets will eventually tidally lock to their moons, this isn’t something that should be expected for many billions of years.

Are Moons Always Tidally Locked To Thier Planets?

Technically speaking the answer to this question would be no as it does take a very very long time for an object orbiting around a planet to tidally lock. After all the tidal locking of a moon is the end process of this phenomenon

With that being said, at this current moment in time we don’t know of any moons that aren’t tidally locked.


Now you know that tidally locked planets aren’t just formed over night and that not all moons are tidally locked, at least not in the beginning.

Over time however the likelihood of a moon becoming locked tidally to a larger planet is almost guaranteed with both entities locking eachother like Charon and and Pluto in the end.

The moon orbits the earth in an elliptical orbit, and this means that the moon at some stages moves faster than in other stages. But at the same time, the moon is tidally locked to the earth. So how is it possible that the moon's rotation speed around its axis changes each week or something?

We see a slightly different face of the moon as it goes around its orbit.

It's called lunar libration

Official NASA video of the phenomena throughout 2011, at hourly intervals. This video better shows how much the side facing the Earth changes over time.

Yes, this. The moon's rotation is relatively constant. As it goes around the orbit, the rotation gets a little ahead in parts of the orbit, a little behind in other parts.

Or maybe the better way to say it is that the moon's location in its orbit gets a little ahead and behind its rotation.

It's also interesting that over time, that "tidal lock" has a wobble, such that we can see just a bit more than 50% if the lunar surface. I'm sure these two observations are related to your question.

I thought that lunar libration was because the Moon's axis of rotation wasn't at right angles to the orbital plane?

But anyways . I did a quick check with JPL Horizons. Can't get the inertial rotation rate, but I did get the rate relative to Earth. Over the range of the next 30 days, the instantaneous rotation period varies from 24.65 days up to 33.36 days. I presume that the long term average would be 29.53 days, the advertised synodic period.

Is Mercury Tidally Locked?

If you’ve looked even slightly into what tidal locking is you’d probably be well aware that our moon is tidally locked to Earth, which in simple terms means we only see one side of the Moon’s face at all times.

It’s often believed that due to Mercury’s relatively close proximity to the Sun, it would also be a tidally locked entity however, this isn’t quite the case.

Mercury is locked to the sun following a 3:2 orbital resonance, meaning it rotates 3 times around its axis every 2 times it orbits the Sun or 1.5 every full orbital cycle. Therefore, even though the Sun’s gravitational pull does affect Mercury (which I’ll explain in a little more detail below) it technically is not tidally locked to the Sun.

Can Planets Be Tidally Locked ?

Yes they can, in fact we know that the now dwarf planet Pluto and its moon Charon are tidally locked to each other whilst the Earth is likely to tidally lock itself to the Moon too considering every year its spinning slower around its axis, at a rate of about 1.4 milliseconds every century.

Which theoretically means our Earth will be tidally locked to the moon in a few billion years from now. Although it is more likely to be caught in the inflation of the Sun near the end of its life cycle over becoming locked.

What Is Orbital Resonance?

Orbital resonance is when two celestial bodies exert regular gravitational burst upon each other resulting in either a stable or unstable resonance between the 2 or more celestial bodies.

Some examples include Pluto and Neptunes resonance of 2:3 and the resonance of Jupiter’s moons Ganymede, Europa and Io of 1:2:4.

This how the tidal locking is for Io, Europa and Ganymede

For entities with an unstable resonance, one example being Saturn’s inner moons, they can cause objects within their vicinity to be ejected, which is why the inner most moons of Saturn have created gaps within the gas giants’ rings.

On the other hand, a 1:1 resonance between a body and moon is one element scientists have used to define a planet. When two celestial have a 1:1 resonance this means that debris won’t be able to fall within the orbit of either celestial object as the debris will either get flinged out or crash on to the surface of either entity.

Is Mercury Tidally Locked To The Sun?

No Mercury is not tidally locked to the sun as it follows a 3:2 orbital pattern as mentioned earlier on. However, it does make sense that some would assume that Mercury is tidally locked as older books (before 1965) from scientists did make these assumptions. Now of course we know it isn’t.

You can say the Mercury is under an elliptical tidal lock due to its 3:2 spin resonance with the sun. As for the reasons why, we don’t exactly know but, there are few theories that scientists have looked into.

Why Does Mercury Have A 3:2 Orbital Spin Resonance?

One theory that was tossed aside suggested that the friction from Mercury’s core mantle could be the reason however, due to the pinpoint accuracy of the viscosity values required of the planet, many deemed this theory to be highly unlikely.

Another theory suggests Mercury initially evolved from a much higher eccentricity then we observe today, which simply means the planet likely has started following a far more normal form of resonance in comparison to what it may have been many many years ago.

And the final theory that’s still somewhat plausible has to do with the planet evolving from a retrograde rotation which suggests Mercury used to orbit in the opposite direction to the sun spinning on its axis and has now potentially decelerated due to the tidal torques resulting in the planets current resonance.

Of course these are just theories as scientists still aren’t all too familiar with the dissipation process of planets and stars but, going by the consistent numbers that have been developed over the years, the above 3 theories are the most probable reasons for why Mercury follows an elliptical tidal orbit and not a 1:1 tidal lock.


In summary Mercury is not tidally locked to the sun as it’s currently spinning around its axis once every 1.5 times its orbit. On top of that there isn’t any concrete theory as to why Mercury is on a 3:2 spin resonance due to the lack of information scientist have about the energy dissipation of planets.

I guess you could say there is some potential in Mercury eventually tidally locking to the Sun however, in our current climate and lack of knowledge on where Mercury may have come from (in regards to its original orbit) just leaves us with more questions than answers.

Hopefully this article was helpful in figuring our if Mercury is genuinely locked tidally to our Sun. if you’ve made it this far I just want to say… thanks for reading.

Why Isn’t the Earth Tidally Locked to the Sun (or the Moon)?

Bernard – Looking out my window at the setting moon just now, and wondering: if the moon is tidally locked to the earth hence we always see only one face of the moon, why is the earth not similarly tidally locked to the sun? -Wayne

A good question, but I should say that we don’t see “exactly” the same face of the Moon. Because of details, the Moon shows us somewhat more than 60% of its face as it moves in its orbit. It wobbles a bit.

The Earth is not yet tidally locked to the Moon, which I know is not your question, yet. But it eventually will be, some billions of years from now. The Sun causes tides on the Earth, but they are about half the size of those of the Moon, so whatever the effects of tides are that lead to locking, the Sun’s will operate more slowly than the Moon’s on the Earth.

Here’s the big picture. I’m going to try to show you the Earth-Moon system to scale. The Earth is about 8,000 miles in diameter. The Moon about 2,000. The distance from the Earth to the Moon is about 240,000 miles, or 30 times the Earth’s diameter.

If the Earth is a circle a quarter inch in diameter, the Moon will be a circle a sixteenth of an inch about 7 ½ inches away.

As the volume of a sphere is proportional to the cube of the radius, the Earth’s volume is about 4 3 or 64 times larger than the Moon. Shall we say that the Earth’s mass is similarly about 60 or 70 times the Moon’s mass? I could look up more exact numbers, but this will do.

The force holding the two in orbit around each other is proportional to product of the masses and inversely proportional to the square of the distance: Newton’s famous law of Universal Gravitation. Consider for the moment, tides in the solid rock of the Moon. They arise because the nearer side of the Moon is closer by, say, 1000 miles than is the far side. The force on the nearer side is greater than the force on the farther side. While the Moon’s center of mass still follows its orbit, this tidal force pulls the Moon into a slightly elongated shape along the line between the Earth and the Moon, and into a slightly squashed shape perpendicular to that line. This tidal force is proportional to the Earth’s mass, and to the difference between the two sides of the Moon, (which is a derivative), so it turns out to be inversely proportional to the cube of distance between the Earth and Moon, 1/r 3 .

Suppose the Moon is not yet tidally locked to the Earth. It rotates with a different period than it revolves around the Earth. Depending upon whether the rotation period is shorter or longer than the revolution, the pulled and squashed shape of the Moon will not lie exactly along the line between the Earth and Moon. Perhaps the near side will lead a bit and the far side will lag. The Earth will exert a force on that leading bulge closer to it that tends to pull it back to the Earth-Moon line, but the force on the lagging bulge farther away will tend to pull the lagging bulge away from that line. But, the first force is a tiny amount greater than the second force, so pulling back to the Earth-Moon line wins.

Pulling on the Moon’s rocks, stretching them a bit, and then moving that stretch around as the Moon rotates relative to the Earth, involves a little friction. Mostly the rocks are elastic, which means they return to their shapes when you remove the force, but not 100% elastic. That means some of the rotational energy of the Moon gradually transfers from rotation energy into heat energy in the Moon. If you grab a metal strip and flex it rapidly, you will warm it. Thus, the Moon will gradually slow its rotation.

This will keep up, slowing, until the rotation rate and the revolution rate are about the same. Once the tidal bulge is no longer being pulled around the Moon, the frictional transfer of energy from rotation to heating will slow, and things will stay about the same. We see the same face of the Moon, mostly, and never see most of the far side. All this has to do with the Earth-Moon distance, and to the mass of the Earth.

Consider the reverse case. The Earth-Moon distance, of course, is the same as the Moon-Earth distance, but we guesstimated that the Moon is much less massive than the Earth. I looked it up, and a more correct figure is that the Moon is about 1.2% of the Earth’s mass. Or the Earth is about 80 times more massive than the Moon.

The tidal force pulling the solid Earth into an elongation along the Earth-Moon line and into a squashed shape perpendicular to that line is about 80 times smaller than the tidal force of the Earth on the Moon. The vertical displacement of the solid Earth’s tide is about a foot. Hardly noticeable, but scientists with sensitive instruments detect them.

The Earth has oceans and the tidal accelerations cause much larger displacements in the oceans than they do in the solid Earth. To bulge in one place and sink in another, water must flow from what will be lower to what will be higher. These currents experience frictional forces along the ocean floor, and again there is transfer of energy from rotation to heating.

Because, however, of that factor of 80 in the masses, these tidal forces and frictions are about 1% of the forces and frictions on the Moon. The Earth has more rotational energy too. The upshot is that it will take a long time before the Earth locks to the Moon.

What about your actual question, which was why isn’t the Earth tidally locked to the Sun? Now we know the factors to look at to estimate about the Sun-Earth system, as compared to the Earth-Moon system. The Sun is 93 million miles from the Earth, or about 400 times farther from the Earth than the Moon is from the Earth. We must cube this to get the relative tidal force: (4 X 10 2 ) 3 = 4 3 X 10 6 = 64 million. Oh yes. Don’t forget, inverse too. The Sun’s mass is about 3.3 X 10 5 times the mass of the Earth. So, the Sun is much more massive than either the Earth or the Moon, but it is much farther away.

That difference between r 3 and r 2 matters. The gravitational force of the Sun on the Earth is much greater than the force of the Moon on the Earth. After all, the Earth, we say, orbits the Sun. But the tidal force of the Sun on the Earth is about half or a third of the tidal force of the Moon on the Earth. We see these solar tides in the seas, and when the solar and lunar tides happen to line up, the tide range is highest, and when they are crosswise, the tides range is lowest.

The upshot of this, is that it will take much longer for the Earth’s day to become the same as an Earth year, and, indeed, much longer than it will take for an Earth’s day to become the same as a lunar month. No need to buy tickets for the event just yet.

Why is the Moon Leaving Us?

Goodbye Moon. Every year, the Moon slips a few centimeters away from us, slowing down our day. Why is the Moon drifting away from us, and how long will it take before the Earth and the Moon are tidally locked to each other?

We had a good run, us and the Moon. Grab your special edition NASA space tissues because today we’re embarking on a tale of orbital companionship, childhood sweethearts and heartache.

You could say we came from the same part of town. A long time ago the Mars-sized object Theia, collided with the Earth and the Moon was formed out of the debris from the collision.

We grew up together. Counting from the very beginning, this relationship has lasted for 4.5 billion years. We had some good times. Some bad times. Gravitationally linked, arm in arm, inside our solar family sedan traversing the galaxy.

But now, tragedy. The Moon, OUR Moon, is moving on to brighter horizons. We used to be much closer when we were younger and time seemed to fly by much faster. In fact, 620 million years ago, a day was only 21 hours long. Now they’ve dragged out to 24 hours and they’re just getting longer, and the Moon is already at a average distance of 384,400 km. It almost feels too far away.

If we think back far enough to when we were kids, there was a time when a day was just 2 – 3 hours long, and the Moon was much closer. It seemed like we did everything together back then. But just like people, massive hunks of rock and materials flying through space change, and their relationships change as well.

Our therapist told us it wasn’t a good idea to get caught up on minutiae, but we’ve done some sciencing using the retroreflector experiments placed by Apollo astronauts, and it looks as though the Moon has always had one foot out the door.

Today it’s drifting away at 1-2 cm/year. Such heartache! We just thought it seemed like the days were longer, but it’s not just an emotional effect of seeing our longtime friend leaving us, there’s a real physical change happening. Our days are getting 1/500th of a second longer every century.

I can’t help but blame myself. If only we knew why. Did the Moon find someone new? Someone more attractive? Was it that trollop Venus, the hottest planet in the whole solar system? It’s really just a natural progression. It’s nature. It’s gravity and tidal forces.

And no, that’s not a metaphor. The Earth and the Moon pull at each other with their gravity. Their shapes get distorted and the pull of this tidal force creates a bulge. The Earth has a bulge facing towards the Moon, and the Moon has a more significant bulge towards the Earth.

A series of photos combined to show the rise of the July 22, 2013 ‘super’ full moon over the Rocky Mountains, shot near Vail, Colorado, at 10,000ft above sea level in the White River National Forest. Moon images are approximately 200 seconds apart. Credit and copyright: Cory Schmitz

These bulges act like handles for gravity, which slows down their rotation. The process allowed the Earth’s gravity to slow the Moon to a stop billions of years ago. The Moon is still working on the Earth to change its ways, but it’ll be a long time before we become tidally locked to the Moon.

This slowing rotation means energy is lost by the Earth. This energy is transferred to the Moon which is speeding up, and as we’ve talked about in previous episodes the faster something orbits, the further and further it’s becomes from the object it’s orbiting.

Will it ever end? We’re so attached, it seems like it’ll take forever to figure out who’s stuff belongs to who and who gets the dog. Fear not, there is an end in sight. 50 billion years from now, 45 billion years after the Sun has grown weary of our shenanigans and become a red giant, when the days have slowed to be 45 hours long, the Moon will consider itself all moved into its brand new apartment ready to start its new life.

What about the neighbors down the street? How are the other orbital relationships faring. I know there’s a lot of poly-moon-amory taking place out there in the Solar System. We’re not the only ones with Moons tidally locked. There’s Phobos and Deimos to Mars, many of the moons of Jupiter and Saturn are, and Pluto and Charon are even tidally locked to each other, forever. Now’s that’s real commitment. So, in the end. The lesson here is people and planets change. The Moon just needs its space, but it still wants to be friends.

What do you think? If you were writing a space opera about the Earth and the Moon break-up, what was it that finally came between them? Tell us in the comments below.