The moon...no...not the moon...

[Jelly]

I was watching a program and nearly burst into tears. Very nearly. Apparantly, the moon is going to drift away from Earth. The fact that we wouldn't be able to cope with the temperature changes after that is irrelevent...it's the moon. It looks so beautiful at night..and...we can't lose it! Yes, it's in a couple of million years, but still..

What do you think?

[HotW Moiraine]

Actually, eventually the moon will stop receding from the Earth.

The recession is due to the Earth's rotation slowing down due to friction from the tides (about .002 seconds per century). To compensate for that slowing rotation, the moon is receding (the law of conservation of momentum is the reason). Eventually, the Earth's rotation will slow to the point where it is rotating at the same speed as the moon is orbiting us, and there will be no more tides and therefore no more friction. At that time, the Earth and moon will present the same face each other all the time. And momentum will remain stable, so the moon will no longer recede.

 

However, this slowdown of the Earth's rotation is not happening fast enough for it to get to that point. About 2.1 billion years from now, the continual increase of the Sun's radiation will cause the Earth's oceans to boil away - that will remove the bulk of the source of tidal friction.

Even without this, 4.5 billion years from now the Sun will evolve into a red giant and likely destroy both the Earth and Moon.

 

The sun is expanding at the rate of about 1 to 2 inches per year.

[Twitchy]

Yeah, Earth is dead anyway you look at it.

 

[chillimous]

No point in worrying about global warming then, eh.

[HotW Moiraine]

Global Warming is not caused by man.

 

 

[Alaksuleiel]

There are two different phenonema, the cyclic fluctuation in temperature that occurs naturally and that which is caused by the release of man created pollutents.

  Global warming is caused by both, accelerated by the latter, and the latter is the one we can do something about. 

[cosmicpanda]

personally, I think in the very long range we'll be controlling the atmosphere and weather, and people then will look back on us and call us idiots for the very idea of burning stuff in the open air. 8)

[Dunbar]

*howls at the moon*

[Guest Far Dareis Mai]

I'm with Demi. The way I look at it, I've got (if I'm very unlucky) roughly 75 more years on this earth (if I lived to be 100). My main worry is stupid people blowing things up with big bad bombs, rather than whether or not the moon is going to float away in millions of years. Do you really think the human race will survive that long on Earth?

[Jelly]

*Shakes head*

None of you understand...

THE MOON IS GOING TO DISSAPEAR AND LIFE ON THIS PLANET WILL NEVER SEE IT AGAIN!

 

Of course we'll live that long.. Doesn't it horrify any of you to think of the moon as just..gone? Lost?  :o

 

[Pukah]

Good!  No more moon trying to block out the stars.

[Jelly]

Since when did the moon block out the stars??

[Pukah]

Hey!  It's not my fault the moon is a big fatty.

[Jelly]

Leave the moon alone! Do you want it to drift off or something?!?

 

 

[HotW Moiraine]

Didn't you read my post?

 

Everyone is going to be DEAD before the moon goes ANYWHERE.

[Jelly]

We aren't going to be dead.  :P

[Canukistani]

yes we will, silly lightfuls thinking such nonsensical romantic thoughts like the human race existing past this century.

 

as for climate change, it is a natural occurrence.  what you should be worrying about is what will the magnetic pole shift this century do to the guidance circuitry all those nuclear missiles...

[Jelly]

*Glares*

Lightful?

 

And yes, the human race WILL survive along with everything else on this planet.

[Pukah]

Canuk (and I think Moir also):  Climate changes have  been atributed to more then just a natural cylcing.  For years a lot of scientists have been almost dismissive of the idea of global warming being due to man.  But that idea has flown out a few years ago.  The evidence is to strong now to just say "It is the natural way of things."  While there most certainly exists a cyclic nature to our global temperatures (which causes ice ages and what not) there is a most definate influence due to man.  And not a small one either. 

 

 

And I do want the moon to go away.  It hates me.  I think it spit on me once.  GO AWAY MOON!  SHOO!

[Jelly]

NO! Why are people discussing Global Warming?!? It's depressing! After a while, when the idea of global warming has gotten drilled into your head, you have no motivation to try and do anything about it; if we even can. It's mind numbing!!!

Now, hug a tree!  ;D

 

Anyway. The MOON!!!

So, I'm assuming NOBODY cares about this?  >:( You can tell me all your lovely scientific theories all you want, but it doesn't change the fact that the moon is going to float off!

 

I mean, if Venus or something dissepeared, wouldn't we be a bit upset? Really. No, it wouldn't change a thing; Venus has no effect on our planet whatsoever, but we'd still CARE! What makes the moon so different?

[HotW Moiraine]

I SAID THE MOON IS NOT GOING TO FLOAT OFF.

[Jelly]

AND I BEG TO DIFFER!

The person on the program said it would. They would not let someone say something like that in front of thousands of people if it wasn't true.

[HotW Moiraine]

Do you want me to give you the math to it?

[Jelly]

::) Go on then.. :D

Not that I'll understand it

[HotW Moiraine]

 

Once you've added the energy, the balance between kinetic and potential energy is automatic. In other words, if you only accelerated the Moon once, the Moon would have to return back through that same point (the one where you accelerated it) every orbit. You're affecting the opposite side of your orbit - how far away the Moon will reach on the opposite side. As that distance increases, the balance shifts from kinetic energy towards potential energy.

 

In reality, the Moon is accelerated at every point in its orbit (not just one point). You see the end result of an infinite number of little accelerations, which is why it appears you added speed to wind up with a slower speed (for any given point in an orbit, you're looking at the opposite side of the orbit than where the acceleration took place).

 

There's a theorem called the virial theorem which says that the kinetic energy of an orbiting body with an inverse square law force law is -1/2 the potential energy.

 

(The potential energy is negative, the kinetic energy is positive, that's why the factor is negative).

 

So when the moon goes to a higher orbit, if you increase the potential energy by 2 units, making it less negative, the kinetic energy decreases by 1 unit, and the total energy increases by 1 unit.

 

See for instance Goldstein, "Classsical mechanics", pg 83-85.

 

For a sanity check, consider a circular orbit.

 

The potential energy is -GM/R

 

Setting the centripetal acceleration equal to the force of gravity gives

GMm/R^2 = mv^2/R, so mv^2=GM/R, thus .

 

5*m*v^2 = kinetic energy = GM/2R

 

which confirms the relationsip

 

kinetic energy = -1/2 potential energy

 

Note that the factor of -1/2 is specific to the inverse square force law.