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Our moon has been slowly drifting away from Earth over the past 2.5B yrs

phys.org

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Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#81

Post by ben_w »

AprilArcus wrote: post #80
ben_w wrote: post #56

Earlier quoted context omitted.

Third option, we can take advantage of repeated gravitational slingshots to transfer orbital kinetic energy from Jupiter to Earth via an asteroid going repeatedly between both planets, gradually increasing Earth's orbit. (I'm not an astrophysicist, I just read about this idea a few years back and it stuck in my mind).

The limit on earth's habitability is determined by permanent sequestration of atmospheric CO₂ through the carbonate-silicate cycle, not insolation. Once CO₂ drops below a certain level, photosynthesis will no longer be possible and all remaining ecosystems will collapse. (Don't mistake that as an endorsement of burning fossil fuels — climate change is operating at a rate measured in decades, CO₂ drawdown via the carb…

True, I had forgotten that. But I assume we can fix that over this kind of timescale.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#82

Post by dotancohen »

redact207 wrote: post #12
FiatLuxDave wrote: post #10

Earlier quoted context omitted.

Sure. The earth is spinning faster than the the moon is going around the earth (earth rotates in one day, the moon goes around in about a month). When the tides transfer energy and momentum to the moon from the earth's spin, it slows down the earth's spin and gives the moon more energy. This moves the moon to a higher orbit, so it moves a bit farther away from us. The source of energy for the whole process is the ear…

If the moon wasn't drifting away and was in a static orbit, would the Earth's rotation slow and the moon's orbit speed up to the point where they're in sync?

It's already happened! More at the end.

This is called tidal locking, and if the universe consisted of only the Earth and moon, this would in fact happen. However, the big heavy Sun also affects both the Earth and moon's rotations.

So why do I say that it has happened? Because the moon, having significantly less mass than the Earth, is almost tidally locked to the Earth. That's why we always see the same side of the moon. So the Earth's rotation hasn't synchronized with the moon's revolution, but the moon's rotation has nearly synchronized with the Earth's revolution (actually both the Earth and moon revolve around their common barycenter).

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#83

Post by sliken »

I was disappointed in this article that doesn't really explain what's going on.

Yes, the moon is moving away from earth, because the energy transferred by tides. Said energy transfer is slowing the spin of earth, which lengthens the day. This transfer will stop once a day = a lunar month. At which point the moon will slowly decrease it's orbit until it hits the Roche limit and becomes a ring.

Fun to think about, granted it's billions of years and the sun may become a red giant before then and either alter earths orbit or consume it.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#84

Post by sliken »

soliton4 wrote: post #58

-- quote -- These have shown that the moon is currently moving 3.8 cm away from the Earth every year. If we take the moon's current rate of recession and project it back in time, we end up with a collision between the Earth and moon around 1.5 billion years ago. -- end quote -- 3.8 cm * 1.500.000.000 = 5.700.000.000 cm = 57.000.000 meter = 57.000 km distance between earth and moon = 384,399 km according to https://en…

Yes, but the rate of energy transfer is related to the ratio of an earth day (which used to be shorter) and the lunar month (which also used to be shorter).

Taken in the opposite direction that moon will stop moving away once the a earth day = lunar month. At which point it will start getting closer, until it hits the roche limit and becomes a ring.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#85

Post by sliken »

ncallaway wrote: post #25
chowells wrote: post #19

Earlier quoted context omitted.

If the moon's orbit is increasing in radius, doesn't that mean its orbital velocity is decreasing? Which way is energy being transferred? This stuff confuses me.

I haven't done the math, but I strongly suspect that the potential energy (from moving away from the source of gravity) is increased more than the kinetic energy (from the reduced velocity) decreases. So, I would think when the moon moves away from the earth, its total energy increases. Thus, the earth's energy decreases (in the form of slightly reduced rotational kinetic energy)

Earths rotational velocity is decreased (longer days) and added to the moon which increases it's potential energy (distance relative to earth), increases it's orbital height, and increases the period of it's orbit (the lunar month).

But once the lunar month = earth day the transfer will stop and the moon will slowly approach earth again, until it hits the roche limit and becomes a ring.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#86

Post by sliken »

yellowapple wrote: post #20
redact207 wrote: post #12

Earlier quoted context omitted.

If the moon wasn't drifting away and was in a static orbit, would the Earth's rotation slow and the moon's orbit speed up to the point where they're in sync?

Eventually, yeah; just as the Moon is tidally locked to Earth, Earth would be tidally locked to the Moon. I don't know offhand whether that would happen before the Moon drifts too far away to remain in Earth's orbit, however.

Moon is below escape velocity, so it's not leaving. In fact when 1 day = 1 lunar month the movement away will stop, and reverse. It will continue to get closer to earth until it hits the Roche limit and becomes a ring.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#87

Post by sliken »

redact207 wrote: post #12
FiatLuxDave wrote: post #10

Earlier quoted context omitted.

Sure. The earth is spinning faster than the the moon is going around the earth (earth rotates in one day, the moon goes around in about a month). When the tides transfer energy and momentum to the moon from the earth's spin, it slows down the earth's spin and gives the moon more energy. This moves the moon to a higher orbit, so it moves a bit farther away from us. The source of energy for the whole process is the ear…

If the moon wasn't drifting away and was in a static orbit, would the Earth's rotation slow and the moon's orbit speed up to the point where they're in sync?

Yes, but it will take a long time. Then very slowly the moon will slow down, decrease it's orbit, until it hits the Roche limit and becomes a ring.

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#88

Post by soliton4 »

snowwrestler wrote: post #75
soliton4 wrote: post #71

Earlier quoted context omitted.

"If we take the moon's current rate of recession and project it back in time" is a very straight forward way of saying, lets assume the moon receeded from the earth at the current rate.

No it’s not because you are projecting backward in time in the presence of a gravitational field that varies with square of radius. The path of the moon away from the Earth is not a straight line, it is a tightly wound spiral.

given your insight in the matter, would you care to post the correct calculation then?

Re: Our moon has been slowly drifting away from Earth over the past 2.5B yrs

#89

Post by zaarn »

sidewndr46 wrote: post #69
zaarn wrote: post #43

Earlier quoted context omitted.

The Moon Kerberos of Pluto is actually very very close to the size of the sun in the sky (of by being 0.0099° compared to the suns 0.0089°, around 11%), so the earth isn't the only planet (if you count Pluto as planet, otherwise, but it's still a solar body you can stand on where a moon is roughly the size of the sun).

Doesn't Pluto have some weird elliptical orbit? Does it not matter enough to change the apparent size of the sun as seen from Pluto's surface?

I used the current values, they should be valid for a reasonable timeframe.

Edit: only by a few percents; the distances are too large to appreciably change the visible size of the sun in the sky.

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