"Bad Astronomy" question re Earth and Moon

RichardR

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RichardR
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I've searched the web quite a bit but can't find answers to this. Maybe the BA or someone else knows. (Of course the BA knows! :D)

In Phil Plait's book, and on the BA website, we have this:

Eventually, the Earth's rotation will slow down so much that the bulge will line up exactly between the centers of the Earth and the Moon. When this happens, the Moon will no longer be pulling the bulge back, and the Earth's spin will stop slowing. But when this happens, the time it takes for the Earth to rotate once will be slowed to exactly the same time it takes for the Moon to go around the Earth once! If you were to stand on the Moon and look at the Earth, you would always see the same face of the Earth.
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Does anyone know how long it wil take to reach this state? And when it happens, will the Earth and Moon then stay the same distance apart, instead of moving further apart each year as they do now?

Also, I seem to remember reading somewhere that the Earth will also eventually be locked with the same face towards the Sun. Again, does anyone know how long before this happens?

Thanks
 
The sort of time frame for the Earth to be tidally locked to the Moon is billions of years - roughly the same as the Main Sequence lifespan of the Sun.

In other words, some time between 1 and 10 billion years in the future, the Earth will become tidally locked with the Moon, and get toasted by the Sun.

I don't think the Earth can become tidally locked with the Sun and the Moon, as this would require the Moon to take a year to orbit the Earth, and that doesn't sound possible.
 
I'm no scientition, but this doesn't sound right, the moon currently takes 28 days to orbit the earth, give or take. It is slowing the earth down to steal energy so it can recede further from the earth, thus taking even longer to orbit, so our days gradually get longer, the moon also orbits more slowley, I would have thought that it would escape earth orbit long before the days became like a month long. Or we get burned up by the sun before then aswell.

Like I said, just my opinion
 
...when it happens, will the Earth and Moon then stay the same distance apart, instead of moving further apart each year as they do now?
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The orbit of the Earth-Moon is based upon the energy in the Earth-Moon system, not the tidal locking between them.

Tidal locking has to do with the fact that the tides are "dragged" behind the Earth's rotation, and eventually this dragging will slow the Earth's rotation. Admittedly, this will take billions of years.

The Moon's orbital speed around the Earth is the primary reason for the receeding Moon. Since there are no tides on or in the
Moon (that I'm aware of), the tidal dragging shouldn't affect the Moon's orbital speed at all.

Also, tidal drag affects the rotational speed of the Earth with respect to the Moon. If there were tides on the Moon, they'd still not impact the orbital speed of the Moon at all. They'd only affect the Moon inasmuch as causing a force towards tidal locking with the Earth - a condition that already exists.

The orbital speed remains unaffected.
 
SquishyDave said:
I'm no scientition, but this doesn't sound right, the moon currently takes 28 days to orbit the earth, give or take. It is slowing the earth down to steal energy so it can recede further from the earth, thus taking even longer to orbit, so our days gradually get longer, the moon also orbits more slowley, I would have thought that it would escape earth orbit long before the days became like a month long. Or we get burned up by the sun before then aswell.

Like I said, just my opinion
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No opinions :D included below:

Geostationary orbit (right now) is 35,786 km above the surface.

The moon is 384,400 km above the surface.
 
I was taught at my amateur astronomy class, that the moon shows only one face to earth because it has a bulge, it is far from a sphere, or even a squashed sphere, there is a big lump on one side, and that means this lump gets locked facing earth, I believe it could also get locked facing away but the lump ended up pointed towards earth.

I was also taught it was receding, because if the earth is slowing down from tidal drag, the energy must be transferred somewhere, it doesn't just go away, so it goes to the moon, which then recedes.

As for the eventually only showing one face to the moon like I said earlier, the earths day would have to slow down to be in excess of 28 current days (672 hours), now they reckon when the moon first formed the earths day was like 4 hours long, but as the moon recedes the rate of slowing recedes. So we are rotating 6 times slower than when the moon first appeared, and the effect of moons slowing is GREATLY reduced now than when it first appeared, and we would have to slow down a further 28 times for this to happen. Is there enough time for this even assuming we freeze the sun so it doesn't kill the whole thing, would not the moon leave before this could happen?

Anyone?
 
RichardR said:
I should have said that the BA information can be found here.

Also, additional information here.
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Actually, I just realized the answer to my first question was in the second link above:

Fifty billion years in the future the Earth day will equal 47 of our current days and the Moon will take 47 of our current days to orbit the Earth. Both will be locked with only one side facing the other
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Don't know how I missed it. As the Sun is due to become a red giant in 5 billion years, I guess the point is moot.
 
RichardR said:
Actually, I just realized the answer to my first question was in the second link above:

Don't know how I missed it. As the Sun is due to become a red giant in 5 billion years, I guess the point is moot.
Click to expand...

I completely missed that quote too, and I had to do a search to find it. :)

I guess that answers our questions.
 
About 30 years ago, (back in the Pliocene), I read a paper by (I think) Moorbath & Runcorn on Growth structures of Devonian Corals which indicated a 400 day (tide) year. The cause was theorised to be the smaller diameter of the Earth - Moon system. There were one or two inconsistencies with the model, as pushing it back in time seemed to imply far higher tidal ranges, while other fossil data (Stromatolites and banded Ironstones) implied a smaller tidal range for the precambrian.

Anyone read anything more up to date on the question?


Nb. There are tides on the Moon. They are Earth (oops) Moon tides. Not so obvious as sea tides, but quite real.
 
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