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Author Topic: Will we eventually lose the moon?  (Read 6237 times)

Vector169

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Will we eventually lose the moon?
« on: 20/01/2011 01:30:04 »
Vector169 asked the Naked Scientists:
   
Hi gang! 

Love you all and love the show

I am a mechanical engineer in Fort Wayne Indiana.  I just recently (a few months back) found your show on Itunes and I have raced back as far as Itunes would let me go in episodes and am feverishly trying to catch up.

I am in your section now with the year of astronomy and have a question that I have wondered about.  I heard years ago on a tv show that reflectors on the moon and lasers have shown that the moon is moving away from the Earth.  My question is in multiple parts.

1) Will we eventually loose the moon, or is the rate at which it is moving away decelerating.
2) If we loose the moon, will it enter a Earth crossing orbit or will it spiral towards the inner solar system?

If you have already answered this than I look forward to hearing it when I get to that episode. 

Thanks Naked Scientists!

What do you think?
« Last Edit: 20/01/2011 01:30:04 by _system »


 

Offline Soul Surfer

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Will we eventually lose the moon?
« Reply #1 on: 20/01/2011 11:23:18 »
The moon is moving away from us because it is gaining orbital energy from the tidal friction of the rotating earth. This is slowing down the earths rotatation eventually this will stop when the earth and moon always face each other. However the sun will go red giant before this happens.
 

Offline graham.d

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Will we eventually lose the moon?
« Reply #2 on: 20/01/2011 13:55:02 »
SS, do you mean "when the earth and moon always face each other"? I would say they do that now. One side of the moon always faces the earth. Do you mean when the moon is also fixed over one point on the earth's surface?
 

Offline Soul Surfer

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Will we eventually lose the moon?
« Reply #3 on: 21/01/2011 00:08:51 »
Yes
 

Offline graham.d

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Will we eventually lose the moon?
« Reply #4 on: 21/01/2011 14:49:19 »
I assume the "yes" is to the second of the 2 questions.
 

Offline chris

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Re: Will we eventually lose the moon?
« Reply #5 on: 25/06/2015 11:42:59 »
Just bumping this topic to the top, because we could really do with a clear explanation for this.

Can anyone help?

Chris
 

Offline chiralSPO

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Re: Will we eventually lose the moon?
« Reply #6 on: 25/06/2015 15:37:18 »
The Earth is transferring angular momentum to the moon, so the moon is moving away from the Earth (very slowly, it's on the order of 4 cm/year) and the Earth's rotation is slowing down (the day is getting longer--about 2 ms per 100 years). Eventually the length of the day will be equal to the length of the month (doubly tidally locked system).

https://en.wikipedia.org/wiki/Tidal_acceleration#Quantitative_description_of_the_Earth.E2.80.93Moon_case

But this is when things get a little more interesting. The moon is moving away from us because it is further than the distance of the geosynchronous orbit, but as the Earth's day slows down, the geosynchronous orbit grows, and it is growing faster than the moon is shifting, so eventually they will be the same. (when the length of the day will be equal to the length of the month). After this point, any loss of energy from the system will result in the moon moving CLOSER to the Earth, as it will be lower than a synchronous orbit. So ultimately the moon will crash into the Earth. (but don't worry, the sun will turn into a red giant first, so the moon probably won't kill anybody)

EDIT: this all assumes no intervention and no major asteroid impacts that would distrub the system. A large enough impact could cause the moon to crash earlier, leave orbit (away from Earth), or be completely destroyed (depending on magnitide and direction of impact)
« Last Edit: 25/06/2015 15:41:01 by chiralSPO »
 

Offline evan_au

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Re: Will we eventually lose the moon?
« Reply #7 on: 26/06/2015 09:54:59 »
Quote from: ChiralSPO
ultimately the moon will crash into the Earth
...but it won't crash as a single piece (barring external influences).
The Moon will start to disintegrate when it passes within Earth's Roche limit.
So, for a while, Earth would have some rocky rings (if the expanding Sun hadn't melted it all first).
 

Offline PmbPhy

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Re: Will we eventually lose the moon?
« Reply #8 on: 26/06/2015 10:42:17 »
Quote from: chiralSPO
The moon is moving away from us because it is further than the distance of the geosynchronous orbit, ...
What does the distance of geosynchronous orbit have to do with the moon moving away?

Quote from: chiralSPO
...but as the Earth's day slows down, the geosynchronous orbit grows, and it is growing faster than the moon is shifting, so eventually they will be the same. (when the length of the day will be equal to the length of the month). After this point, any loss of energy from the system will result in the moon moving CLOSER to the Earth, as it will be lower than a synchronous orbit.
What does the distance that an object will be in geosynchronous orbit have to do with the moon moving closer to the Earth? It's not as if the slowing of the Earth's rotation slowing down changes the strength of its gravitational field.
 

Offline chiralSPO

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Re: Will we eventually lose the moon?
« Reply #9 on: 26/06/2015 13:57:56 »
No, no one is suggesting that the gravitational field is changing. The issue at hand is how the system responds to a mismatch between the orbital period (OP) of the moon and the rotational period (RP) of the Earth.

For any object in orbit above the synchronous orbit (OP>RP) tidal interactions transfer angular momentum from the rotating planet to the orbiting body--slowing the rotation of the planet, and increasing the altitude of the orbiting body (OP and RP both increase, but RP increases faster)

For any object in orbit below the synchronous orbit (OP<RP) tidal interactions transfer angular momentum the other way, decreasing the rotational period of the planet, and decreasing the altitude of the orbiting body. (like the Martian moons)

I don't have time to crunch the numbers now, but my recollection from my orbital dynamics course (so many years ago), was that the stability of the synchronous orbit is dependent on the relative sizes of the planet and moon. The closer in mass the two are, the more stable the orbit will be (like pluto and charon), whereas when the masses are very different (like Mars and its moons), the system does not self-correct. Our case is intermediate, and my recollection was that the OP>RP case converges to OP=RP, but that the OP<RP case decays...

http://assets.zombal.com/7f59e7d3/TidalEquations.pdf
http://www.askamathematician.com/2015/05/q-why-is-our-moon-drifting-away-while-mars-moons-are-falling/
 

Offline jeffreyH

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Re: Will we eventually lose the moon?
« Reply #10 on: 26/06/2015 22:59:39 »
Can this tell us anything about gravitational interactions at the atomic level? The probability distribution of the electron may well have a gravitational component, although very small in magnitude. The electron can sometimes be regarded as being coincident with the nucleus where the effects of gravitation may be much more significant.
 

Offline chiralSPO

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Re: Will we eventually lose the moon?
« Reply #11 on: 29/06/2015 13:40:18 »
Unfortunately, I don't think we gain much information about atomic scale physics from studying astronomic interactions...


There is definitely a relationship between mass and probability of being very close (or in) the nucleus. If you replace an electron with a muon (also negatively charged, but more massive) the average distance falls significantly. I don't think this is an effect of gravity though, as it can be modeled perfectly (for hydrogen-like atoms) by wave functions generated without including gravitational terms. There probably is some extremely tiny contribution that gravity has at the subatomic scale, but I don't know how we would go about testing that...
 

Offline jeffreyH

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Re: Will we eventually lose the moon?
« Reply #12 on: 30/06/2015 11:41:43 »
Unfortunately, I don't think we gain much information about atomic scale physics from studying astronomic interactions...


There is definitely a relationship between mass and probability of being very close (or in) the nucleus. If you replace an electron with a muon (also negatively charged, but more massive) the average distance falls significantly. I don't think this is an effect of gravity though, as it can be modeled perfectly (for hydrogen-like atoms) by wave functions generated without including gravitational terms. There probably is some extremely tiny contribution that gravity has at the subatomic scale, but I don't know how we would go about testing that...

I have been looking at the Compton equations. Especially the Compton wavelength. Something is nagging me about them but I can't put my finger on it. I know that it is concerned with photons and nothing to do with gravity.
 

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Re: Will we eventually lose the moon?
« Reply #12 on: 30/06/2015 11:41:43 »

 

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