What Could Cause the Earth's Rotation to Dramatically Slow Down?

In summary: While Venus' thick clouds make it impossible to see its surface to time its rotation by looking trough a telescope, this has been measured using radar, that can penetrate those clouds, carried in two missions named "Magellan" and "Venus Express." Comparing their results has shown that the Venusian day became longer by 6.5 minutes between these measurements taken 16 years apart.In summary, a large enough object could slow down Earth's rotation if it didnt vaporize or rip apart the planet. The atmosphere would remain with the planet and slowly rotate at the same rate.
  • #1
garymc11
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TL;DR Summary
Aside from collision with another object, is there anything that could dramatically slow Earth’s rotation?
Hi everyone,

I’ve been thinking about this for a while and have done a bit of research but can’t seem to get a straight answer.

Aside from collision with another object in space, is there anything that could cause the Earth’s rotation to dramatically slow down? If a sufficiently massive object passed close enough to Earth, could that potentially exert enough force to slow it down (ignoring what this would mean for the moon and any other planets in our solar system)?

And if this were possible, would the Earth’s atmosphere continue to spin at its current speed, or would it slow at a similar rate?

This has been hypothetical scenario that I’ve been thinking about for a while so I’d be keen to hear any of your thoughts on it.
 
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  • #2
Not without breaking things. The kinetic energy of the earth's rotation is the equivalent of millions of H-bombs.
 
  • #3
Think in terms of angular momentum and rotational kinetic energy. In order to stop a rotating body, the energy and momentum must go somewhere else. What somewhere else can you suggest for the Earth stopping, other than a collision with another object?
 
  • #4
Giant, cosmic brakes!

The other possibility would be to build a bunch of particle accelerators that shot really big dirt clods out at reltavistic speeds to the east - and power them with billions of hampsters...
 
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  • #5
anorlunda said:
Think in terms of angular momentum and rotational kinetic energy. In order to stop a rotating body, the energy and momentum must go somewhere else. What somewhere else can you suggest for the Earth stopping, other than a collision with another object?
I’m not sure, I know the presence of the moon causes tidal friction which has (very) gradually slowed the Earths rotation, but I guess there is a limit to how much of an effect that could have.
 
  • #6
garymc11 said:
I’m not sure, I know the presence of the moon causes tidal friction which has (very) gradually slowed the Earths rotation, but I guess there is a limit to how much of an effect that could have.
That's a good example. Yes it did slow the Earth's rotation. At the same time, it made the radius of the Moon's orbit increase. So we had an exchange of momentum and energy, not a dissapearence.
 
  • #7
Concerning this text in the opening comment:
"If a sufficiently massive object passed close enough to Earth, could that potentially exert enough force to slow it down "

If a star, neutron star, white dwarf or black hole passed close enough without vaporizing or ripping apart the Earth, its rotation could slow down fast due to the eventual conversion of its rotational energy into heat through friction, by the huge tides the gravitational pull of the passing body would raise in the solid earth, the oceans and the atmosphere. It may also cause an exchange of angular momentum between this planet and the passing object, that would both speed up and move away from Earth, while this will slow its rotation further. In fact even the Moon is right now slowing Earth's rotation, very slowly, with its tides, also moving away from our planet, also very slowly, while continuing to orbit it.

As to the second question in the original comment:
The atmosphere does not rotate now at exactly the same rate as the solid Earth and oceans, or even all of it rotates at the same rate, so there is no reason why it should not do so after such an energetic encounter. If it remains a part of Earth at all, not having been ripped away by the gravitation of the passing body, or evaporated by its radiation.
 
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  • #8
Venus famously rotates slowly and in retrograde direction to her rotation, but her atmosphere rotates rapidly.
Has she always been like that, or has her rotation ever changed?
 
  • #9
snorkack said:
Venus famously rotates slowly and in retrograde direction to her rotation, but her atmosphere rotates rapidly.
Has she always been like that, or has her rotation ever changed?
The rotation of the planet beneath its remarkably thick and opaque atmosphere has been shown to change over time.

While Venus' thick clouds make it impossible to see its surface to time its rotation by looking trough a telescope, this has been measured using radar, that can penetrate those clouds, carried in two missions named "Magellan" and "Venus Express." Comparing their results has shown that the Venusian day became longer by 6.5 minutes between these measurements taken 16 years apart.

There is no settled consensus on why Venus rotates the way it does, or why is slowing down, although the fact that it has a thick atmosphere mostly of CO2 means that this must absorb enough solar radiated energy to power very fast and strong winds blowing constantly from dayside to nightside and doing so in the opposite sense to the rotation of the rest of the planet, resulting in the whole atmosphere turning more or less together once every four of our days, although the solid planet below turns approximately once every 243 days on its axis. So maybe it is the friction with these very strong winds blowing in the opposite direction to the rotation of the planet below that is slowing it's rotation down.

There is no accepted explanation as to why the rotation of Venus is opposite in sense to the one in which Venus orbits the Sun, unlike all the other planets. One rather picturesque theory put forward by a group of French scientists is that it has always turned as the others do when looked at from the North pole, but at some point Venus turned upside down for whatever reason, so it is still turning clockwise, but only when looking at Venus from its current South Pole, that used to be the North Pole.

As to why the planet rotates so slowly, one theory is that it is close to being tidally locked to the Sun, presenting always the same side to it, as the Moon does to the Earth; another, that the gravitational interaction between Venus and Earth is what has caused its rotation rate to slow down from a much faster original one, perhaps closer to that of the Earth and Mars.
 
  • #10
That's a lot of interesting information about Venus, thank you. It seems to me if the change in Venus' rotation is secular in nature (which would be the case if due to some long-range solar system interaction like tidal locking with the Sun), it must happen in concert with some kind of internal interaction that can be giving it random kicks. Otherwise, it could be an internal oscillation that is periodic rather than random. The reason it should need something other than just a secular change comes just from the timescales-- 6.5 minutes in 16 years, implies the planet rotation rate must change significantly in about a million years. If any secular change was slowing Venus' rotation that quickly (including tidal locking with the Sun, which in any event could not explain how Venus' rotation became retrograde, unless it was born that way or had some kind of early collision, in which case we don't need any other explanation for its rotation), it would hardly be rotating at all by now. Also, no orientation with anything else in the solar system could change that rapidly.

To oscillate or vary stochastically, it must be something that Venus is closely coupled to, because any long-range interaction would be secular in nature-- and the timescale is too rapid for that. So it's an oscillation that must repeat many times, or it could be a secular slowing down occurring against the backdrop of random changes that keep giving it kicks, over the timescales of change in the solar system. Thus it must be closely coupled to something internal that can oscillate or produce random kicks, either from its atmosphere, or some kind of internal change in rotational inertia. What else could it be? It's not like something hit Venus a million years ago, or it underwent some kind of internal change that recently, that's just seems too unlikely.
 
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  • #11
Ken G, Too little is known of Venus, this very hard to observe world, for people to know what its long-term change in rotation speed is likely to be.

As to the OP, that was about why the Earth rotation is genrally slowing down? Well, it should slow down, although in recent years it has been accelerating a little, so the last time a leap second was added to UTC (Universal Tine Coordinated, our Civil Time we keep track of with watches, clocks, computers and cell phones, a.k.a. Mean Solar Time), to keep it wthin 0.9 seconds of the time measured by astronomical observations of our planet's spin (UT1) and to stay in line with the millenary tradition of keeping time by the Sun, was in 2016. On average, a leap second is added every year and a half, but that has changed for the time being.

The main reason for the slowing down is the gravitational interaction of the masses of the Earth and the Moon, that causes them to exchange rotational moment, so the Earth slows down and the Moon moves farther away from us - VERY SLOWLY. Some of the Earth's rotational energy is also lost due to friction of ocean waters rubbing gainst the ocean bottom as they are moved by the tides raised by the Moon (a gravitational interaction). There are also tides raised by the Sun that do the same things, but play a much smaller role in this. As to the recent slight speeding up of the Earth? I think nobody has a good explanation for that right now.
 
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  • #12
That's also interesting, and I agree it must be very hard to establish where these tiny changes are coming from. I don't think you mean the Earth's spin has had a slight speeding up though, do you? That would require removing leap-time and you were describing the uneven rate of adding leap-time, so I think you are saying the rate the Earth is spinning down lessened a bit rather than staying constant.

I would think that couldn't be due to a change in the torques (since those are long-range and should not vary), but rather a change in the moment of inertia or in the interior differential rotation (which are local to the Earth so could respond to something in the interior shifting a little). I've heard it said that the relative spin of the core of the Earth is slowing down (such that it is no longer spinning a little faster than the rest of the Earth), which is a differential rotation effect which could cause the outer laters to pick up the missing angular momentum. That could also not be a secular change (as it's happening too quickly for that), so it sounds like the kind of oscillation or stochastic kicks I was mentioning above. Basically things that don't pile up secularly over time so don't require adjustments in the long-range orientation of anything!
 
  • #13
A well-told story, written in an accessible way, of the recent spin up is here: https://www.timeanddate.com/time/negative-leap-second-maybe.html

As to possible causes of fluctuations in spin rate, of which this has been I think the most dramatic registered so far: https://en.wikipedia.org/wiki/Day_length_fluctuations
And, according to not just Time and Date, but articles published in, for instance, Nature: well, yes!, for the first time since leap seconds were first used half a century ago, back in the Seventies, it might be necessary to introduce a negative leap second in the not too distant future.
Why has this happen? Unknown at this time, although the suspects are many. Strong winds blowing against chains of mountains and many other reasons internal and external to the solid Earth are known, or seriously suspected of making the spin of our planet vary even daily, if one measures it precisely enough. The most precise way is to use Very Long Baseline Interferometry, or VLBI for short: combining the observations obtained with arrays of dedicated radio telescopes sited at great distances from each other, tracking the less apparently moving things in the sky from our point of view by collecting the radio waves emitted by active black holes at the center of distant galaxies, known as quasars, short for quasi-stars. Fast fluctuations are also detectable by GPS and similar global navigation satellite systems (GNSS for short) with their atomic clock ensambles.
The changes are then distributed as data in various ways, the one commonly used being the tables of Bulletin A, published daily by the US Naval Observatory, and the monthly Bulletin B published by the Paris Observatory: https://hpiers.obspm.fr/iers/bul/bulb_new/bulletinb.pdf
 
  • #14
OscarCP said:
A well-told story, written in an accessible way, of the recent spin up is here: https://www.timeanddate.com/time/negative-leap-second-maybe.html
That is an excellent article, containing a chart that shows clearly everything we are talking about. It also justifies your statement that the spin (of the surface) has actually sped up a little, even though we have not removed any leap seconds, because the spinup did not persist long enough to span the interval between when leap seconds are applied. The chart also shows what I've been pointing out, which is the big difference between secular advances (the yellow trend in the chart) and the oscillation/fluctuations (the green curve showing the instantaneous spin rate). So that chart really puts things in perspective, my point is that I believe we should not expect anything that can increase the spin to come from external astrophysical sources, the fluctuations seem more likely to be internal to the Earth (especially its internal or atmospheric rotation rate as compared to surface rotation, and anything that can change its moment of inertia). Moreover, if there are changes that are of astrophysical origin, they should probably be a periodic cycle rather than a secular trend. One obvious signal to look for is the monthly period in which the Sun's tidal forces either assist or resist the spindown effect of the Moon, and I think we see that in the Wiki article you cited, but they average out so quickly they don't really matter.

OscarCP said:
As to possible causes of fluctuations in spin rate, of which this has been I think the most dramatic registered so far: https://en.wikipedia.org/wiki/Day_length_fluctuations
And, according to not just Time and Date, but articles published in, for instance, Nature: well, yes!, for the first time since leap seconds were first used half a century ago, back in the Seventies, it might be necessary to introduce a negative leap second in the not too distant future.
Why has this happen? Unknown at this time, although the suspects are many. Strong winds blowing against chains of mountains and many other reasons internal and external to the solid Earth are known, or seriously suspected of making the spin of our planet vary even daily, if one measures it precisely enough.
The culprit does seem to be angular momentum exchange with the atmosphere, but we must not forget the curious result from the last few years that there is less angular momentum being contained in the spin of the core. That acts the wrong way, as it would transport angular momentum out and seem to increase the surface spin, but we must also account for changes in the rotational inertia. I wouldn't be quick to rule out internal changes within the Earth, there's certainly a lot more angular momentum and rotational inertia in there than there is in the atmosphere! But that Wiki article does clearly hone in on atmospheric effects as the cause of the change. I would say it's even possible that we have another indicator of global climate change there, as it seems the downward trend is likely to correlate well with CO2 concentration. I was going to suggest that warmer air might lead to an effect on the way to Venus, but that works in the wrong direction-- as you point out, Venus' atmosphere carries the opposite sign of angular momentum, so if our atmosphere was starting to act in that way, it would speed up our surface rotation, not slow it down.

What is very cool about all this is that it's not clear if the slowing trend will continue for decades into the future. But if it does, that will really give an interesting challenge to astronomy classrooms, who have to explain why the Moon's orbit is getting farther away with time because of the slowdown of the Earth's spin, but oh by the way, the Earth's spin isn't actually slowing down lately! (I can't wait to see what the Young Earth folks will make of that seeming contradiction...)
 
  • #15
The Moon gains momemntum by moving away, as I understand this, because the radius of its turn around the Earth increases faster than its orbital speed decreases with the increase of distance to its common center of mass with Earth (that is sited inside our planet). As to the Earth likely to stop slowing down, well, no: tidal friction will continue to slow it down in the long turn and to move the Moon away. And, by the way, the solid earth (so to speak, as their is a molten part around its solid core, plus all that lava spewing out of volcanoes) also has a tide every day of as much, in places, as half a meter up and down that we do not notice, because it has such a long wavelength that all we can survey goes up and down practically together -- and also very gently. The atmosphere also has tides. And all this also means more friction and loss of energy as heat, plus a consequent loss of the Earth's rotational momentum.
Finally, this planet (and all the other planets with substantial moons, really) has been slowing down since its beginning (or, more precisely in Eath's case, after its big crash with Thea -- what it did before does not count.) The end point of this slowing down is a fully tidally locked Earth-Moon system with Earth and Moon showing each other the same faces for ever (or until the Sun goes red giant and evaporates both of them).

The Earth's day, for example, was four hours a mere 4.5 billion years ago, after it got its big moon out of its big crash with Thea: http://www.iea.usp.br/en/news/when-a-day-lasted-only-four-hours#:~:text=According to it, the first,and the eukaryotic cells emerged.
 
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  • #16
OscarCP said:
The Moon gains momemntum by moving away, as I understand this, because the radius of its turn around the Earth increases faster than its orbital speed decreases with the increase of distance to its common center of mass with Earth (that is sited inside our planet).
What happens is, the Moon induces bulges into the Earth, both on the near and far sides of the planet. But since the Earth rotates faster than the Moon orbits, there is some delay in the bulges' ability to stay aligned with the Moon direction, so instead they get a bit ahead of the Moon. This should mean, for example, that the high tide is not when the Moon is directly overhead, but rather, when the Moon is slightly behind. That also means the bulge closer to the Moon (which has the largest back-reaction on the Moon) pulls the Moon slightly forward, as it is slightly out ahead of the line to the Moon. Any time an object receives a continuous but weak forward pull, it slowly moves into a wider, slower orbit. That orbit is still circular, just very gradually spiraling outward, with gradually increasing angular momentum. The excess angular momentum comes at the expense of Earth's spin, because that same forward pull on the Moon is a backward pull on the bulge that is closest to the Moon.

OscarCP said:
As to the Earth likely to stop slowing down, well, no: tidal friction will continue to slow it down in the long turn and to move the Moon away.
That's the secular trend on long-term timescales, I'm talking about the possibility that astronomy classrooms over the next few decades might experience a steady increase in the Earth's spin over those (short) timescales, which would nevertheless be a little embarrassing for teachers who are saying that the Earth's spin is slowing down!
OscarCP said:
And, by the way, the solid earth (so to speak, as their is a molten part around its solid core, plus all that lava spewing out of volcanoes) also has a tide every day of as much, in places, as half a meter up and down that we do not notice, because it has such a long wavelength that all we can survey goes up and down practically together -- and also very gently.
Yes, it's not just the oceans, but water responds faster so most of the "bulge" is ocean.

OscarCP said:
The atmosphere also has tides. And all this also means more friction and loss of energy as heat, plus a consequent loss of the Earth's rotational momentum.
Agreed, this cannot be neglected and may be playing a key role in the latest fluctuations.
OscarCP said:
Finally, this planet (and all the other planets with substantial moons, really) has been slowing down since its beginning (or, more precisely in Eath's case, after its big crash with Thea -- what it did before does not count.) The end point of this slowing down is a fully tidally locked Earth-Moon system with Earth and Moon showing each other the same faces for ever (or until the Sun goes red giant and evaporates both of them).
Actually, believe it or not, when the Earth is tidally locked to the Moon, that will not be a permanent relationship-- the Moon will start coming back in again! That is because the Earth's spin period will at that point be 47 days, but its orbit will still be a year, so the Sun will be exerting tides on the system, which for some reason that is not completely obvious will end up starting the Moon back in. Not sure if it brings the Moon all the way back to be torn into a disk around the Earth or what happens there!
OscarCP said:
The Earth's day, for example, was four hours a mere 4.5 billion years ago, after it got its big moon out of its big crash with Thea: http://www.iea.usp.br/en/news/when-a-day-lasted-only-four-hours#:~:text=According to it, the first,and the eukaryotic cells emerged.
Yes, the Moon has certainly had a pretty important effect, some even think the way it creates tidal pools helped life go from the ocean to the land.
 
  • #17
Thanks for clarifying why the Moon moves away: it is because of an orbital resonance, something that happens when a force is applied repeatedly and at more or less regular intervals, especially those close to an orbital revolution, to an object orbiting another.

I also agree with the rest of your last comments. I had forgotten about the Moon coming back towards Earth, also very slowly, because of a tidal interaction with the Sun. Assuming this will happen in less than the around five billion years left before the incineration of both Earth and Moon by the Sun gone red giant, whether it engulfs both, or its surface gets sufficien=ently close to boil away all of Earth's air and water. Or such is the current extrapolation from what is known to what is assumed will happen.
 
  • #18
Ken G said:
which for some reason that is not completely obvious will end up starting the Moon back in.
If you drain energy from an object in a circular orbit, the obvious effect would be to slow the object. However, this is orbital mechanics and things do not always behave in the intuitive way. If you slow an object in a circular Earth orbit with just one bump, it will be slower than before at apogee (high point in the orbit) but faster than before at perigee (low point in the orbit).

If you drain energy smoothly around the entire circular orbit, you get a lower circular orbit, a satellite moving faster and, as a result of both of these things, orbitting with a shorter period.

Now the orbital period is faster than the Earth's rotation rate. So you have positive feedback. The rapidly orbitting moon acts to speed the earth's rotation while the slowly rotating earth puts the brakes on the moon's orbit. The effect of the braking is to make the moon's orbit lower yet and faster yet with an even shorter period.
 
  • #19
It seems to me clear that a slowing of the core's rotation directly caused the mantle to speed up. Isn't this very basic?

It is a little known fact that this occurs with neutron stars, thus suddenly changing the rate of pulsars. This was strong evidence that the cores were superfluid, as was later confirmed via observed cooling rates.
 
  • #20
The gravitational interaction of the solid core and the mantle, as the massive solid core rotation fluctuates, imay cause the spin rate of the Earth to fluctuate as well, as the core is fee to fluctuate its rotation, because it is surrounded by a layer of liquid metal that acts as a mechanical clutch between it and the lower mantle: the outer core. But the whole thing, concerning what is the core doing right now, is up for grabs. A group of scientists from Beijing University(also called officially Peking University even now) recently published a paper in "Nature Geoscience" stating that they have made observations of seismic data traveling through the core at different times, that suggest it must have paused in its rotation relative to the rest of this planet and might be about to start turning, relatively again, in the opposite direction. It does this every several decades, but there is still no consensus accepting these new results from China.

At the University of Southern California another team also has something interesting to say:
https://news.usc.edu/200185/earth-core-oscillates/

When the constitution, physical state and movements of the core came into conversation, back in the eighties, I used to say that the core could be made of French waffles and old shoes, for all that we knew.
We do know considerably more about it these days, I must admit, but still not enough to go and study something else because, by now, the core is all done.

The Moon tides move the earth axis, making it wobble very slightly, but enough to be something to take into account in work that requires knowing very precisely Earth's orientation (e.g. to the milliarcsecond).

A more important effect of the Moon on the Earth axis that is of interest to all of us, is that it stabilizes its orientation enough that it does not tilt so much, now and then, that the poles end up in the equator, as has happened in Mars, that has no Moons big enough to prevent this.

Finally, some Norwegians have something to say about this:
https://sciencenorway.no/forskningno-norway-planets/what-would-we-do-without-the-moon/1433295
 
  • #21
OscarCP said:
A more important effect of the Moon on the Earth axis that is of interest to all of us, is that it stabilizes its orientation enough that it does not tilt so much, now and then, that the poles end up in the equator, as has happened in Mars, that has no Moons big enough to prevent this.
The obliquity/tilt of Mars is 25 degrees, almost the same as Earth. It is Uranus that has an obliquity of 98 degrees.
 
  • #22
Earth has bulges due to tides from the Sun, both on the near (day) and far (night) sides, which thanks to rotation are dragged, noon bulge to afternoon.

But Earth surely also has the unpaired bulge due to thermal expansion after noon? Which is in the afternoon not due to rotational drag but inherent accumulation of heat over day?

How does the tidal torque on the thermal bulge compare to the tidal torque on the tidal bulges, for Earth and for Venus?
 
  • #23
Hornbein wrote: The obliquity/tilt of Mars is 25 degrees, almost the same as Earth. It is Uranus that has an obliquity of 98 degrees.

My bad: I should have made it clear that I was referring to something that happened a very long time ago. By now new polar caps have formed and the remains of the old ones, now near the planet's equator, are largely gone because of the planetary dissecation that left no water to speak of other than that chemically locked in the substance of rocks, and the effects on the rocky surfaces that supported the ancient ice sheets of eons of the dusty fast winds that are the main cause of erosion there.
 
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  • #24
snorkack said:
Earth has bulges due to tides from the Sun, both on the near (day) and far (night) sides, which thanks to rotation are dragged, noon bulge to afternoon.

But Earth surely also has the unpaired bulge due to thermal expansion after noon? Which is in the afternoon not due to rotational drag but inherent accumulation of heat over day?

How does the tidal torque on the thermal bulge compare to the tidal torque on the tidal bulges, for Earth and for Venus?
Yes, the heat of the Sun does create a bulge that culminates in height at 4 hours local time in the tropics, if I remember correctly. This moves air around producing what is known as the atmospheric thermal solar tide (there are also in the atmosphere the tides due to the Sun and Moon gravitational fields). It is, in much denser atmospheres than ours, such as that of Venus, a likely cause of quite significant tidal breaking and one posible explanation of why Venus is close to being tidally locked with the Sun, although it seems at first sight to be too far from it for this to happen.
 
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  • #25
OscarCP said:
Thanks for clarifying why the Moon moves away: it is because of an orbital resonance, something that happens when a force is applied repeatedly and at more or less regular intervals, especially those close to an orbital revolution, to an object orbiting another.
Actually it's not a resonance, it is just happening all the time.
OscarCP said:
I also agree with the rest of your last comments. I had forgotten about the Moon coming back towards Earth, also very slowly, because of a tidal interaction with the Sun. Assuming this will happen in less than the around five billion years left before the incineration of both Earth and Moon by the Sun gone red giant, whether it engulfs both, or its surface gets sufficien=ently close to boil away all of Earth's air and water. Or such is the current extrapolation from what is known to what is assumed will happen.
It will definitely boil off the oceans, but probably not incinerate or swallow the Earth. It is thought that the Sun will lose mass as its luminosity climbs, so its gravity will weaken, so before it gets large enough or luminous enough to threaten the Earth, the Earth will move off to relative safety. Not that it won't be a charred brick.
 
  • #26
jbriggs444 said:
Now the orbital period is faster than the Earth's rotation rate. So you have positive feedback. The rapidly orbitting moon acts to speed the earth's rotation while the slowly rotating earth puts the brakes on the moon's orbit. The effect of the braking is to make the moon's orbit lower yet and faster yet with an even shorter period.
Yes, we can say that if there were no Moon and the Earth was rotating at 47 days, then the Sun's tidal effect would slow the Earth's spin and put that angular momentum into the Earth's orbit, eventually trying to make them equal each other at something even longer than 365 days. Now if the Moon is also there, tidally locked to the Earth at 47 days, most of the angular momentum internal to the Earth-Moon system is in the Moon's orbit, not the Earth's spin, so as the Sun tries to spin down the Earth, the Moon's angular momentum keeps getting dumped back into the Earth, as you say. This makes it take a lot longer for the Sun to slow the Earth down, but more importantly, it also pulls in the Moon as the Moon loses that excess angular momentum.
 
  • #27
snorkack said:
Earth has bulges due to tides from the Sun, both on the near (day) and far (night) sides, which thanks to rotation are dragged, noon bulge to afternoon.

But Earth surely also has the unpaired bulge due to thermal expansion after noon? Which is in the afternoon not due to rotational drag but inherent accumulation of heat over day?

How does the tidal torque on the thermal bulge compare to the tidal torque on the tidal bulges, for Earth and for Venus?
Good question, I've never heard of a role for thermal expansion, but it must do something. The usual answer is that the noontime bulge is closer to the Moon so has the stronger effect, but maybe it's really that it is a larger bulge. It can't be too large of an effect though-- the heating from the Sun only penetrates a small distance below the surface.
 
  • #28
Ken G said:
But the subtle issues come from the fact that if there were no Moon and the Earth was rotating at 47 days, then the Sun's tidal effect would slow the Earth's spin and put that angular momentum into the Earth's orbit, eventually trying to make them equal each other at something much longer than 47 days. Now if the Moon is also there, tidally locked to the Earth at 47 days, you'd think the Moon's orbit and spin would also want to rise, which requires the Moon to move out farther from the Earth. So I'm not sure why the Moon comes in.
I am not sure that I follow what you are saying.

Yes, the sun's tidal effect will slow the earth's rotation -- slower than 47 [modern] days per rotation. If the moon's orbital period is unchanged, this means that the moon is now orbitting too fast for the rotating earth. This puts the tidal brakes on the moon's orbit, draining energy. The moon will not be rising to a higher, slower orbit to keep pace with the slowed earth. It will be falling to a lower, faster orbit, speeding ahead of the tidal bulges that it is producing and continuing to brake as it speeds ever faster.
 
  • #29
Wow, you caught my post mighty quick! Sometimes I write something, then think about it, then edit it a minute or two later. So yeah, ignore what I said there, and look at the edited post, which agrees with that you said. Guess I should do the editing before hitting return!
 
  • #30
Ken G said:
Wow, you caught my post mighty quick! Sometimes I write something, then think about it, then edit it a minute or two later. So yeah, ignore what I said there, and look at the edited post, which agrees with that you said. Guess I should do the editing before hitting return!
I feel you. The percentage of my posts that are adequate when I press the Post button is negligible.
 
  • #31
To Ken G: An orbit resonance might have a mostly (i.e. not exactly) discontinuous cause, but it is always a continuous effect.
 
  • #32
Orbital resonances are very different from things that happen steadily-- they occur because a small effect is repeated synchronously with something else (it takes two to tango, and two to resonate also). The typical way they happen is when two different periods are in the ratio n/m, so repeat exactly every n cycles of one and m cycles of the other.
 
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  • #33
Ken G said:
Orbital resonances are very different from things that happen steadily-- they occur because a small effect is repeated synchronously with something else (it takes two to tango, and two to resonate also). The typical way they happen is when two different periods are in the ratio n/m, so repeat exactly every n cycles of one and m cycles of the other.
Isn't that precisely what I wrote, using not very different words?.
 
  • #34
It seemed that you were saying the Moon moves away from the Earth because of an orbital resonance. There are not two periods that are in a ratio n/m that are resonating in that situation, maybe you were talking about something else. No biggie, we're all in agreement and you are bringing a lot of interesting information into play!
 
  • #35
While trying to rediscover a fascinating reference on Medieval tidal ranges, lost to a PC mega-crash a decade ago, I learned about the zoo of orbital issues and cycles of precession etc affecting the Moon's orbit, hence tidal stuff, day-lengths etc...
Remarkably, seems distant Jupiter has much more effect on Moon and, therefore, us, than near-by Venus. does...
These are the short-term equivalent of the vastly slower Milankovitch geo-cycles.

Another factor is tidal basin resonance: As continental plates move about, they may produce conditions for resonance, with higher tides leading to greater tidal dissipation plus accelerated coastal erosion etc etc. Other configurations, with lesser tidal dissipation, would slow the Moon's retreat...

On a shorter time-scale, oceans' ice-age low-stands and inter-glacial high-stands may change coastlines enough to take tidal basins in/out of resonance. Further, primary and secondary isostatic effects may change the depths of water bodies, eg parts of Northern Baltic that were submerged in historical times now stand above the shore-line. Similarly, much of Hudson Bay will become dry land or foetid salt-marsh. IIRC, funnelling aside, the famous 'Bay of Fundy' mega-tides have developed since the ice withdrew...

I never did find that lost reference but, IIRC, there was a period during early Medieval times when synergic combination of orbital factors gave tidal ranges around much of NW Europe significantly greater than at present. Our monthly 'Spring' range was their daily norm, our 'Equinoctials' their monthly 'Springs', their 'Equinoctials' the stuff of Legend, driving fearsome currents spawning maelstroms, whirlpools and over-falls, especially where channels and reefs were shallower. Plus, yes, devouring 'soft coasts', shifting sediments 'to and fro' at alarming rate....
 

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