Why is the moon drifting into space

[sylas]

...

This thread is a bit over four years old.

 

[pegasus980]

.moon gravitational pull
↓
↑
.earth bulge from moons gravitational pull

.but because the Earth is rotating faster than the moon is orbiting

.the Earth bulge gets ahead of the moon


.moon position
. ↓
.↑ ←earth rotation
.earth bulge


.so the moon is pulled towards the Earth bulge increasing its speed
.← moon orbit
.→
.and the Earth bulge is pulled towards the moon slowing it down


do this help?
or just add confusion?

confusion for me.

 

[ideasrule]

I know this thread is 4 years old, but for people reading it, I have to mention one thing.

Where does the energy come from? It is bled off from the Earth's rotation.
Those tidal bulges cause friction. The water acts against the ocean floor and the continental shelves.
This slows the Earth's rotation.

No! Tidal bulges are not the tides that we see on the beach; Earth's oceans have negligible volume compared to the Earth itself.

Because the Moon's gravity is stronger on the side closer to the Earth than on the farther side, Earth is slightly elongated. This is the so-called tidal bulge; it's a bulge in the rock, not a bulge in the ocean.

 

[sylas]

I know this thread is 4 years old, but for people reading it, I have to mention one thing.



No! Tidal bulges are not the tides that we see on the beach; Earth's oceans have negligible volume compared to the Earth itself.

Because the Moon's gravity is stronger on the side closer to the Earth than on the farther side, Earth is slightly elongated. This is the so-called tidal bulge; it's a bulge in the rock, not a bulge in the ocean.

The original post to which you are responding is completely correct.

The bulge in the ocean IS a tidal bulge, and this IS the tides you see on the beach. You can also get a bulge in rock, but it is much much smaller. Which matters for for slowing rotation depends on the circumstances. For Earth, it is the ocean tides which are most significant for the drag that slows Earth's rotations and transfers energy to the Moon, leading to it moving gradually further away.

That the volume of the ocean is smaller is irrelevant. There's still a tidal bulge with the ocean, which corresponds to tides.

Cheers -- sylas

 

[D H]

The Earth tides are smaller, but not much smaller, than the ocean tides. Much smaller means smaller by at least an order of magnitude. The Earth tides are about 1/3 of the oceanic tides in magnitude. The "lossiness" of the Earth tides is much smaller than is that of the oceanic tides.

Right now, the lossiness of the oceanic tides is particularly high because the oceanic tides run into two north-south barriers: the Americas and Africa+Eurasia. This configuration is rather unusual. The average change in the Earth's rotation rate over the last billion years is smaller than the present rate. If the change was due to the Earth tides the change in length of day would have been a lot closer to constant.

 

[sylas]

The Earth tides are smaller, but not much smaller, than the ocean tides. Much smaller means smaller by at least an order of magnitude. The Earth tides are about 1/3 of the oceanic tides in magnitude. The "lossiness" of the Earth tides is much smaller than is that of the oceanic tides.

Thank you! I did not realize that they were that close in magnitude. I've learned something.

The rest is my understanding as well. Much more energy is dissipated in the oceanic tides. That's probably how I jumped too quickly to the bulge size. I've also been aware that energy dissipation from ocean tides varies with the locations of continents, and is particularly high in the present epoch.

Cheers -- sylas

 

[ideasrule]

I stand corrected (and interested). I really feel this should go in Wikipedia somewhere; do any of you have sources you could cite, DH and sylas?

 

[sylas]

I stand corrected (and interested). I really feel this should go in Wikipedia somewhere; do any of you have sources you could cite, DH and sylas?

The wikipedia article Tide seems pretty good, to me, at a quick glance. I am a registered editor on wikipedia, although I have done hardly anything there for a couple of years now. (Anyone can register; it is very easy to do, and has some advantages when you edit articles.)

There's a section on Dissipation, and that links to a more detailed article on Tidal acceleration, which covers this particular aspect of tides.

Cheers -- sylas