Please either confirm my suspicions, or expose my ignorance. It’s my understanding that the moons gravitational interaction with the earth is largely responsible for earths ocean tides and the consequent waves. It’s also apparent that waves carry considerable energy, which is given up thrashing the shores among other things.
Since the first law of thermal dynamics states that energy cannot be created or destroyed, is it safe to conclude waves are generated at a cost to the orbital energy of the moon?
After finding a related article about why the same side of the moon always faces the earth, I believe my assumptions to be correct. Does this also imply that other celestial bodies loose orbital energy and grow further apart? Perhaps global warming will be a good idea one day after ages of drifting further from the sun.
A quick google found this. The answer being the Moon is slowly moving away, but it’s orbit will stabilize eventually.
I’ve read elsewhere that theoretically, assuming the Sun’s death or the Big Crunch/Rip didn’t destroy the Earth and Moon first they would eventually begin to draw together again. This would be due to energy loss via gravity waves; the analogy used was that of the synchrotron radiation emitted by circular particle accelerators as the particles are forced to curve; in the same way, the Earth and Moon radiate gravity waves as they are forced to move in circular orbits. And eventually, the Moon would break up and the pieces impact on Earth - we are talking many, many billions of years though; quite likely the universe won’t be around that long.
No, because the sun is ( and has always been ) slowly warming. Earth will eventually, barring intelligent intervention overheat and become another Venus; I’ve heard estimates as low as 100 million years for this.
Note that the Moon is moving away because its orbit is in the same direction that earth rotates (prograde orbit). If the opposite case were true (retrograde orbit) the Moon would be moving closer.
Can I ‘bump’ this part of the OP? Der Trihs’ link says that the moon will drift outward to an optimum ‘tidally locked’ in their orbits at some point in the future - will there be no drag (earthly tides/bulge) at this point?
Do you have a cite for this? I see no reason why this should be so. Regardless of the direction of the Earth’s rotation, the tidal drag will remove rotational energy from the Moon, which will force the Moon to recede in order to conserve angular momentum.
Would all this happen any differently if Earth didn’t have oceans to have tides? Or would the same energy be somehow used in doing things with less dramatic effects?
I’m fairly certain that waves are caused by wind, earthquakes, the deflection of currents off of a body of land, and anything else that could possibly shake it up. There isn’t a single cause.
Actually, it’s a bit more complicated than either you or Der Trihs have it. If a moon is closer to its primary than the “geo”-synchronous orbit (and moving prograde), tidal forces will cause it to approach the primary. If the moon is further than the geosynch orbit and prograde, tidal forces will cause it to depart from the primary. At precisely geosynch orbit, tidal forces don’t transfer energy one way or the other. In the case of the Earth-moon system, the Moon is receding and the Earth is slowing - and the Earth’s slowing moves the geosynch orbit outward. At the point when the geosynch orbit “catches up” with the Moon’s orbit, the Earth and moon will permanently face each other.
A retrograde orbit always leads to a moon approaching its primary. One way to think about it is to think about the orbital angular momentum of the moon - call it Mm, and the rotational angular momemtum of the primary - call that Mp. If Mp is greater than Mm then tides will slow the planet (to reduce Mp), and move the moon outward (to increase Mm) until Mp=Mm. If Mm is greater than Mp, then tides will move the moon inward and speed up the planet, again until Mp=Mm. If the moon’s motion is retrograde, Mm is negative, and tidal forces will attempt to increase Mm to equal Mp, but when Mm gets to zero, the moon will crash. See here for more Tidal acceleration - Wikipedia
It may seem counterintuitive, but as an orbiting body loses energy, it approaches its primary; it does not recede. Think of those “gravity wells” you used to only see at science museums, but now entertain us when we drop a coin in the chute of a charity-donation display sitting on a counter (shaped somewhat like the bell of a trumpet). As the marble or coin rolls around in the well, it loses energy due to rolling friction and air friction, falling deeper and deeper into the well. At the same time, it appears as though it is gaining energy, since its speed increases.
Some of the more technical folks can correct me if I’m wrong here, but I believe that the Moon moving out to a farther orbit is an increase of its orbital energy. So if the sloshing of the oceans loses energy due to entropy, and that energy goes to heat, where does the energy come from that powers the tides and the Moon’s increased orbit?
The answer is that it comes from the rotational energy of the spinning Earth. The Earth is gradually slowing down, powering both of the other two.
OffByOne has it right. Energy is being transferred to the moon (coming from the slowing of the earth’s rotation–all those leap seconds add up–and the moon, gaining gravitational potential energy, is moving further away. This has been going on for several billion years. At one point, the moon was about 1/10 as far away as it is now and the length of the day was about five hours. The tides vary as the inverse cube of the distance so what is now, say, a five foot tide, would have been a 5000 foot tide, making the Bay of Fundy look like a child’s bath. But then the Bay of Fundy, had it existed in the form it does today, would have had ten mile high tides. But the waves are wind driven and would not necessarily have been much higher than now, just the tides.
As to when the earth will become uninhabitably hot, I don’t think anyone has a really good estimate. The facts are that the sun is gradually getting hotter and, as it does, its diameter will gradually increase so that in another 5 billion years it will swallow (or perhaps just miss swallowing) the earth and moon. We will certainly be gone before then. Terraform Mars, anyone?
The point about the tides being predominatly caused by the Moon needs a bit of expansion too. The Sun exerts a non-trivial tidal force as well, though being both larger/more massive and farther away, it’s not a precise comparison.
Rather, the effect of solar tides is to modify the lunar tides, giving rise to spring and neap tides. When the Sun and Moon are exerting a tidal pull in the same dimension (i.e., at and around new and full moon), high tides are proportionately higher and low tides lower. This constitutes spring tides. On the other hand, when Sun and Moon are at roughly right angles to each other relative to earth (the quarter moons), high tides are proportionately lower and low tides higher, the neap tides.
Essentially “the tides coming in and out” are governed by the Moon, but the degree to which they vary is modified by whether Solar pull is added or subtracted to the lunar pull.
More like trillions of years, at least. For things as low-density and distant as the Earth and the Moon, gravitational radiation is pathetic. They’d probably spiral in quicker from drag from the interplanetary medium.
As the link mentions, there will be a permanent, unmoving bulge.
As for energy; as I understand it, objects orbiting each other lose energy via gravity waves, although it takes a looong time to do much.
Hmmm, maybe. On the time scale we are talking about, the interplanetary medium will also likely thin as well, I’d think. The Sun will be long dead, as will most stars; in general the universe will have wound down before this becomes a concern. There’ll be less going on to replenish the medium as it gets drawn to objects massive enough to hold onto gases.
Specifically, what happens is that the tides from the Moon raise two bulges on the Earth. One of them is under the Moon, and the other is on the opposite side from the Moon. But the Earth is rotating faster than the Moon is orbiting it (it completes one rotation in 24 hours, while the Moon takes a month to go around it), and it’s rotating in the same direction that the Moon is orbiting. The tidal bulge that started out right under the Moon gets carried ahead by the Earth’s rotation.
That tidal bulge has mass, so it exerts a gravitational pull on the Moon. That gravitational pull pulls the Moon ahead in its orbit, so the Moon is gaining energy from the interaction. The tidal bulge that was on the other side of the Earth is also pulling the Moon back in its orbit, but it’s farther away, so it does so with less force.
The Earth is also losing energy from the friction caused by that tidal bulge moving around. Eventually, that loss of energy will mean that the Earth will always show the same face toward the Moon, just as the Moon does toward the Earth now. The Moon was probably originally rotating at some other rate than it is now, so it showed different faces to the Earth, but tidal friction has stopped that rotation and made the Moon always show the same face to the Earth.
I always thought that it was remarkable that the size of the moon and the sun were so close as to allow the remarkable total eclipse of the sun. According to Wikipedia, due to the fact that the moon is moving away from the earth, we will one day have no more total solar eclipses!
Yeah, I get something like 10[sup]26[/sup] years for orbital decay from gravitational radiation, which is several orders of magnitude longer than my estimate for drag from even the interstellar medium.