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

[OscarCP]

To Ken G: An orbit resonance might have a mostly (i.e. not exactly) discontinuous cause, but it is always a continuous effect.

 

[Ken G]

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.

 

[OscarCP]

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?.

 

[Ken G]

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!

 

[Nik_2213]

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....

 

[swampwiz]

My understanding is that the horrificly deadly 2004 earthquake/tsunami moved the poles by 25 cm.

 

[swampwiz]

The obliquity/tilt of Mars is 25 degrees, almost the same as Earth. It is Uranus that has an obliquity of 98 degrees.

I would imagine that the "Midnight Sun" on Uranus lasts a LONG time.

 

[swampwiz]

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.

It's only the Earth's crust that has been accelerating; the iron-nickel core could very well be decelerating (perhaps we'll see a magnetic pole swap soon?).

 

[swampwiz]

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!

I remember reading the Time-Life Nature book "The Universe" saying that the Moon would first go farther away, but then would move closer in so much so that the Moon would disintegrate and give the Earth rings. Perhaps this speculation was based on the Sun becoming more luminous as it slowly gets to its red giant phase, causing the Earth's atmosphere to thin?

 

[Bandersnatch]

I remember reading the Time-Life Nature book "The Universe" saying that the Moon would first go farther away, but then would move closer in so much so that the Moon would disintegrate and give the Earth rings. Perhaps this speculation was based on the Sun becoming more luminous as it slowly gets to its red giant phase, causing the Earth's atmosphere to thin?

No, it's just mechanics. Due to there being two pairs of tidal interactions - the Earth-Moon system, and the Earth-Sun - there is no stable configuration with both systems tidally locked as one lunar month will always be shorter than one year.
Right now, a day is shorter than both a month and a year. And, the lunar tides dominate due to Moon's proximity. As long as a day is shorter than a month, the tidal interactions with Earth raise the Moon on its orbit.
Once the Moon drifts far enough, draining Earth's rotational angular momentum in the process, and sometime before a day and a month equalise, the Earth-Sun tidal interactions will start to dominate. These will act to further slow down Earth's rotation even below the period of a month (toward one day=one year).
At this point, the Moon begins to have a shorter period of the orbit than the period of rotation of the Earth. This reverses the direction of the tidal acceleration acting on the Moon. From then onwards, the Moon will always be getting closer until it collides/disintegrates.
Although the time scale may be comparable with the Sun swelling up and vaporising both rocks, so it might not actually happen.