Will the centre of mass of the Earth-Moon system eventually be outside the Earth?

[Trulialia]

Sorry for the possibly stupid question, but how come that the Moon continues to slow the Earth when it's moving away from us? Does that mean our tidal bulges are shifting the centre of mass of the Earth-Moon system away from the Earth? Will it end up like Pluto and Charon then, orbiting the centre of mass outside Pluto?

[grant hutchison]

The tidal bulges raised by the moon always lag a little behind the position of the moon in the sky. So the moon's gravity drags on them, and they act like a friction brake on the rotation of the Earth. Meanwhile, the bulges drag on the moon, and their gravity flies the moon into a higher, slower orbit.
Eventually (billions of years), we'll get to a situation in which the moon and Earth revolve around each other while keeping the same face always turned towards each other. At that time, they'll be about 1.4 times farther apart than they are at present, which will move the centre of gravity of the system outside the Earth.

Grant Hutchison

[Ara Pacis]

If they survive the sun's red giant phase, right?

[grant hutchison]

If they survive the sun's red giant phase, right?

Yes. The time scale to the doubly synchronous state is actually in the tens of billions of years.

Grant Hutchison

[EDG]

Yes. The time scale to the doubly synchronous state is actually in the tens of billions of years.

Grant Hutchison

True, but getting to the point where the centre of mass of the system is outside the Earth would happen before then, wouldn't it?

[grant hutchison]

True, but getting to the point where the centre of mass of the system is outside the Earth would happen before then, wouldn't it?

It would.

Grant Hutchison

[Jeff Root]

The tidal bulges raised by the moon always lag a little behind the
position of the moon in the sky. So the moon's gravity drags on
them, and they act like a friction brake on the rotation of the Earth.
Meanwhile, the bulges drag on the moon, and their gravity flies the
moon into a higher, slower orbit.

As I see it, the tidal bulges raised by the Moon always race ahead
of the Moon's position in the sky, because the Earth is rotating
faster than the Moon orbits, so the Moon moves backwards through
the sky relative to the direction it is orbiting, and its gravity drags
on the bulges, slowing Earth's rotation. Meanwhile, the bulges pull
the Moon forward, and their gravity makes the Moon go faster,
throwing it into a higher orbit, which causes it to slow down to a
speed lower than it had to start with.

-- Jeff, in Minneapolis

[AriAstronomer]

I find that explanation also resonates with me well, Jeff.

[grant hutchison]

I presume you guys do get that they're exactly equivalent explanations?

Grant Hutchison

[jfribrg]

According to my calculations, the Earth-Moon barycenter will be at the Earth's surface when the centers of the Earth and Moon are 523,160 km apart, assuming that the ratio of the masses of the two bodies stay the same ( Mass of the Earth is 81 times more than the mass of the moon). That won't happen for many billions of years. They are currently about 384000 km apart (it varies a little depending on the time of the month). The barycenter will probably never leave the Earth because when the Sun expands in its red giant phasethe Earth and Moon will probably fall into the Sun.

[Ken G]

I presume you guys do get that they're exactly equivalent explanations?

The old "heliocentric or geocentric" distinction, eh? I did think we were past having to specify which framework we were adopting before choosing terms like "lag behind" or "race ahead." Indeed, the advantage of your perspective that it is a "lagging" is that, well, that's just what it is-- if the Earth's oceans could respond instantly to a changing gravitational potential, then we wouldn't have that effect at all. Instead, their response lags the changes in the gravitational potential (the latter following the Moon-- oops, there's that geocentric language again). The simple fact is, these calculations really are easier in the geocentric frame, so I agree that such language does not require correcting! But maybe it's useful to hear it both ways, and see whichever clicks better. If Galileo could have just said that, it might have saved him a great inconvenience!

[Ara Pacis]

Of course, this also depends on whether the water is in ocean form in basins or locked up in polar ice caps. Ice Ages reduce the braking action, do they not?

[Ken G]

It might depend on details like whether or not the Bay of Fundy is frozen. I once heard that the amount of braking that happens due to the strong resonance in the Bay of Fundy represents an appreciable fraction of the total, though I'm rather dubious that could really be true.