It is my understanding that in the distant past the Moon was much closer to the Earth than it is today and it is in fact moving farther away each year. Why is this so? I realize that the Moon’s traveling speed is what keeps it in orbit but wouldn’t the Earth’s gravity eventually win out and thus draw the moon closer and closer until they eventually crashed into each other? Doesn’t classical physics predict this outcome? Or does the speed that the Moon is traveling throw a wrench into this prediction? What am I missing here?
What’s the difference between the Moon’s fate and the fate of every other satellite in Earth’s orbit?
Likewise:
Is the Earth getting closer or farther away from the Sun?
See this thread for some information on this. The upshot is that tidal forces are sapping angular momentum from the Earth-Moon system, resulting in a slowing of the Earth’s rotation and and the Moon slowly receding from Earth.
The moon is actually gaining energy at the expense of the earth’s energy.
The earth revolves more quickly than the moon orbits the earth. As a result the tidal bulge in the oceans is always a little in front of the moon. This is causing the moon to speed up to catch the bulge and the faster the moon orbits the further it will get from the earth.
The energy being added to the moon comes at the expense of the earth’s rotational speed. Our days are slowly getting longer (but not by much…IIRC less than a second per century). Eventually this will all balance out and a day on earth will be equivalent to a lunar month. The moon already always points the same side towards the earth for this very reason. When the earth and moon reach equilibrium the earth will likewise always point one side towards the moon and the tidal bulge will be permanently fixed ‘underneath’ the moon. At that point the moon will likewise stop moving away from the earth.
Of course all of this will take a REALLY long time. The earth significantly outweighs the moon thus it kinetic energy is far greater. That’s a mighty big pool of energy being depleted not all that fast. Someone else will have to do the math but I wouldn’t be surprised if our sun will stop shining before this cycle completes.
Ok…I did a quick back of the napkin calculation and come up with about 1.3 billion years for the earth and moon to reach equilibrium so it would seem the sun will far outlast this occurrence (roughly 5 billion more years for the sun).
Understand that math is NOT my string point and I could easily have screwed that up or not taken into account other important factors (such as will the rate of energy transfer increase or decrease over time…my calculation assumes a constant rate but it wouldn’t surprise me in the least if that is wrong).
Thanks,
I’m glad I asked. I have always wondered what the hell I was missing and never found an adequate and reasonable explanation for the phenomenon. I defiantly thought I was going to get some complex equations thrown at me. I’m thankful for the knowledge!!
Peace
More or less correct, but the net effect of the interaction is to slow the moon down. The bulge pulls the moon forward, which will mean the moon is in a higher orbit but it will lose speed in going to the higher orbit. It’ll have a slower angular velocity too, but the increased radius means that the moon has increased its angular momentum, at the expense of the Earth’s angular momentum.
You may’ve already acknowledged this, Flynn, but I think you can see from all this that what’s going on with the moon is 100% in keeping with gravity and physics.
What has been said is accurate, though I haven’t seen an explicit reference to the law of conservation of angular momentum. The tidal forces of the earth on the moon have of course finally slowed the rotation of moon to the point where it must keep one side always facing the earth. Now the tidal forces of the moon on the earth are similarly slowing the earth’s rotation. The tidal forces act as friction, as it were, to slow the earth’s rotation. This causes a loss of angular momentum in the earth-moon system. The only way the system can conserve its momentum is for the moon to move further out from the earth.
There will come a point where the earth’s rotation slows to where one side of the earth always faces the moon. At that point, the tidal forces of the sun will come into play and the earth’s rotation will slow even further. Looking at the angular momentum of the earth-moon-sun as a constant, the earth-moon system will then be forced to move further out from the sun. But if you look at the angular momentum of strictly the earth-moon system, it still needs to make up for the slowing earth’s rotation. So the moon will begin to creep slowly back to the earth in a long death dance that will eventually result in the moon’s breakup. Or so I’ve read- I’ll try to find a cite.
BobLibDem, that’s all true, but the OP was from someone who is not a physicist. I’ve found that you can’t use conservation of energy nor conservation of angular momentum to explain anything to a layman. You have to explain it with forces. Just like the classic example of why a figure skater spins faster when she pulls in her arms. If you say “to conserve angular momentum,” this will go over the head of most nontechnical people - they’ll nod because they’ve heard of angular momentum, but they won’t really have an understanding of what’s going on. I’m not sure that even a physicist will have an intuitive understanding of AM. You should explain that the arms are really travelling fast when they’re extended, and when the skater brings them in, they try to keep their speed, but the distance around the circle is smaller, so they end up rotating faster.
To make this all really simple, start making a cake.
Use a big mixing bowl and toss in all the major ingredients. Then start stirring, moving your mixing spoon in a rapid cirlce near the center of the bowl. If your batter is viscous enough, you’ll notice that the bowl will start to spin in the same direction as the mixing. Eventually, the bowl and the mixing will be at the same rotational speed. In this sense, the moon is picking up energy from the rotating Earth, causing the moon to move faster (and subsequently to a higher orbit). This energy is stolen from the Earth, causing its rotation to slow down.
I love science.
We all know the sun spins, thus it has angular momentum. This should stay constant over time, or should it? The sun is also losing huge amounts of mass over time due to nuclear reactions. So as the sun loses mass, does it need to spin faster in order to conserve angular momentum?
I don’t know the answer and am unsure if anyone does. My gut feeling is that when a bit of matter is destroyed, then that bit’s contribution to the sum of the angular momentum is destroyed as well, therefore the total angular momentum need not remain constant as the mass decreases. If someone knows better, please inform me.
The Sun is only losing a lot of mass, if you don’t compare it to the total mass of the Sun. Over the course of its lifetime, the Sun will only burn about 10% of its hydrogen fuel into helium, and that process only turns less than a percent of the mass of the hydrogen into energy. And this is taking place over ten billion years. So in any given year, the Sun only loses about 0.0000000000001 of its total mass.
Now, angular momentum must in fact be conserved, even when we’re converting mass to other forms of energy. But the energy which is carried away can also have angular momentum. It’s not much, but it would be enough to balance the books.
But at a guess, I would suspect that the angular momentum of the Sun changes more from gravitational effects (taking it from or giving it to the other planets) than from radiative effects.
Can anyone double check this for me? After thinking about it a bit I am guessing the energy transfer will decrease over time. Garvitational attraction works on an inverse square so I am assuming the further the moon gets from the earth the tidal bulges will be smaller and they in turn (both due to distance and less mass) will exert less pull on the moon trying to drag it forward.
If the whole thing is too complex to easily calculate then don’t bother but even so is my assumption correct?
