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A thought just came to me, and I want to comfirm it here:

If we figured out a way to harvest the energy from the orbit of the Moon, would that be perpetual?

The Moon has been orbiting the earth for billions of years without disappearing for any reason. The problem of the Moon losing 4 meters per year might be able to be solved by the energy of its orbit.

Would this form of energy be perpetual? What if it happened elsewhere? What if this planet with a Moon is in a hydrogen cloud?

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    $\begingroup$ @Bobthezealot actually the question is perfectly fine here. (Sure, one could have asked it at Astronomy but if you're implying that it's not on topic here, that's not the case.) $\endgroup$ – David Z Jan 30 '15 at 22:53
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    $\begingroup$ Why do you think it could be perpetual? The potential and kinetic energy of the moon is obviously huge but finite. (On a general level: Why do you think anything (macroscopic) could be perpetual?) $\endgroup$ – Peter - Reinstate Monica Jan 30 '15 at 23:26
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    $\begingroup$ The thing is, the Moon actually steals power from the Earth's rotation. Earth's period of rotation is slowing down in part due to the energy it's passing on to the Moon. $\endgroup$ – Hot Licks Jan 31 '15 at 3:10
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    $\begingroup$ Perpetual implies that it's infinite. Clearly that's not the case. No matter what energy source you're using, it can't be a "source of perpetual power" unless it contains an infinite amount of energy. Clearly, the orbit of the moon does not. $\endgroup$ – Ajedi32 Jan 31 '15 at 5:20
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    $\begingroup$ (And it should be noted that "power from the Moon" is already being harvested, in the form of tidal power generators used along the western coast of Europe (though in fact that's actually harvesting power from the Earth's rotation).) $\endgroup$ – Hot Licks Jan 31 '15 at 20:10
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If you could take from orbital energy, then it would decrease, until at some point in the future it would zero. Hence, it can't be perpetual.

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  • $\begingroup$ What if this was in a hydrogen cloud, and you put a really tall windmill out of carbon nanotubes and used the wind energy? $\endgroup$ – Pyraminx Jan 30 '15 at 21:39
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    $\begingroup$ What difference would a hydrogen cloud make? $\endgroup$ – HDE 226868 Jan 30 '15 at 21:40
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    $\begingroup$ @Pyraminx, your windmill would harvest energy but by doing so it would slow down the carrier (the moon), so as said in the answer, it wouldn't be perpetual ... $\endgroup$ – Hoki Jan 30 '15 at 23:25
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    $\begingroup$ @Pyraminx: you're going down the path of "X is not perpetual, but what if we take it to the extreme and do Y." Perpetual energy is not possible under current scientific beliefs, all you can do is find larger sources of energy to tap. You could suck energy from an entire galaxy's rotation, but you'd still imperceptibly slow the galaxy down. Billions of years later, the difference would start to be noticeable. $\endgroup$ – Cort Ammon Jan 31 '15 at 2:19
  • $\begingroup$ This argument is a bit strangely phrased because according to the usual conventions, orbital energy is already negative for many situations. Perhaps an explicit consideration of a lower bound would be appropriate. $\endgroup$ – Stan Liou Jan 31 '15 at 4:34
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We already harvest energy from the Moon. It causes the tides and stress and strain and motion throughout the Earth. As a result, the Moon keeps getting farther away. (And it causes some heating in the Earth).

The Moon at one time had a spin that was not locked to the Earth, and the tidal bulges in the Moon's shape caused by the Earth generated heat in the Moon's interior, and the rotation slowed. That rotational energy has now been used up.

In other words, work was done on the Moon through deformations of the rock or friction of the regolith, which required energy which was lost to heat. If the Moon were an ideal elastic, and did not get hot from being bent, it would not have changed. But also no work could be extracted from the rotation.

As the Moon gets further away, the gravitational effect gets less and the rate at which the Moon retreats gets less. Sorry, I don't recall if there is a stable end-game for this scenario, at least before they are engulfed by the expanding Sun.

So, the direct answer is no, you can not use the motion of the Moon as a perpetual source of energy.

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    $\begingroup$ If we'd harvest energy from the moon it should come closer, not get further away. (Getting things to higher orbits needs energy, as any rocket shows.) The moon, on the contrary, is harvesting energy from earth, specifically from earth's spin via the tidal bulges which are running ahead of the nominal position they'd have if earth weren't spinning. They pull the moon forward, because the moon orbits in the same direction as earth spins, but with a slower angular speed. Cf. en.wikipedia.org/wiki/Orbit_of_the_Moon. $\endgroup$ – Peter - Reinstate Monica Jan 30 '15 at 23:23
  • $\begingroup$ Yeah, seems like the Moon is "harvesting" energy from Earth. $\endgroup$ – Hot Licks Jan 31 '15 at 3:11
  • $\begingroup$ Yes, net loss for the Earth. I wonder how much is the other way from the tidal deformation and friction. The Moon orbit wiki doesn't mention deformation losses and also implies the Moon being locked is from the same effect as the Earth-->Moon transfer. But that is pretty negligible without water and continents. $\endgroup$ – C. Towne Springer Jan 31 '15 at 3:26
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    $\begingroup$ Can we extract the energy from the Moon orbit at the same rate it is draining energy from the Earth? Our days would still get longer, but at least the Moon wouldn't fly away. $\endgroup$ – John Dvorak Jan 31 '15 at 14:24
  • $\begingroup$ @JanDvorak I don't know if that would violate a conservation law, but it looks like anything you did to take power from water tides will only increase the lag between the tidal mass and the Earth-Moon axis and add to the rate the Moon speeds up. If a tether were used to lift masses to the Moon, and you had enough of them, perhaps you could. If such a tether were possible you could just let it drag in the atmosphere with a parachute on the end. It would add heat tot he atmosphere and case all kinds of great conspiracy theories. $\endgroup$ – C. Towne Springer Jan 31 '15 at 15:40
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You could harvest lots of energy from the moon but not an infinite amount. Taking orbital energy from the moon will cause its orbit to decay with time.

This offers its own problems. The closer the moon got to Earth, the more extreme tides would become on Earth with potentially destructive consequences. And, in the end, if you continued to take energy from the moon's orbit, it would collide with Earth.

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  • $\begingroup$ The earth rotates on its own axis faster than the moon orbits the earth. Tidal friction transfers angular momentum from the earth to the moon. So the earth slows down (loses energy), and the moon (in this case) gains it. The moon is moving away from the earth. If the earth was rotating slower than the moon's orbit, tidal friction would accelerate the earth's rotation, draw the moon towards the earth and your answer would be correct. $\endgroup$ – Level River St Jan 31 '15 at 19:16
  • $\begingroup$ If you took exactly that much energy out of the orbit of the Moon that it would stop the Moon from getting farther away, you could both harvest energy and stop the Moon from getting away. Not perpetual, but on human time-scales almost indistinguishable from perpetual. $\endgroup$ – vsz Jan 31 '15 at 22:35
  • $\begingroup$ @steveverrill, I don't understand how my answer is incorrect. I wasn't saying that my answer is the situation that actually happens. I am just saying that if we were able to extract orbital energy from the moon that it would get closer to Earth. $\endgroup$ – NeutronStar Feb 2 '15 at 1:48
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The first law of thermodynamics (through conservation of energy) precludes that there can ever be an infinite energy source. However one must consider infinity as theoretical. We humans might consider an energy source that could power the whole of our society for a million years infinite, but on a cosmic timescale it is merely a dot on a very, very long line.

There is also the matter of efficiency. In order for a power source like the moon to be even usable, you must be capable of extracting at least as much energy from it as you used up extracting it.

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If the moon generates a magnetic field, then theoretically you can generate an electrical current simply by having a conductor within the magnetic field. This is called "Flux". When a conductor exists within a changing magnetic field, then a current will be generated based on the rate at which the field is changing. Then, because the moon is in constant motion, the strength of the field will constantly be changing based on your location, albeit SLIGHT. I think that the most effective, granted non-viable solution, would be to create a network of orbital wires that are arranged perpendicular to the pathway of the moons orbit, as close to the moon as you could get them, without having their structure compromised by the gravitational force of the moon. As the moon passed over the wires, a current would be generated in the same way that a car's alternator functions.
I apologize if my explanation is inadequate, or unclear. I'm sure the physics grads could explain it better.

edit: I was thinking more on this, and had a thought: because these conductors would cause the moon and the conductors to attract(magnetically), could the slow change in the moons average distance from the earth be counteracted, while simultaneously harvesting energy?

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The answer from the mid 1700s has been that the tides are caused by the Moon, and the tides move the Moon away.

Well maybe, but, probably not.

Both the Moon and the Earth gain mass by accretion, so both have increasing cummulative gravitational force. The Earth is just over 81.3 times the mass of the Moon, so it gains more mass at a faster rate. This gradually changes the balance of power. That is, the Earth applies a gradually increasing force on the Moon that is far larger than the gradually increasing force that the Moon applies on the Earth. There would be four equations to solve, but, two (Lunar) are very small, and 2 (Earth) are large.

The result is that the Moon moves away from the Earth at a net increase in distance of 38.2 +/- 0.7 mm per year. A simplified equation that uses the 81.3 / 82.3 and the 1 / 82.3 would be Earth moves about 0.5 mm down plus the Moon moves about 38.7 up, so the net change is the 38.2 up (away).

When using the concepts of conservation of (anything and everything), we always forget to use the limitation "in a closed system".

The Earth and the Moon, as well as all other objects orbiting a larger sphere anywhere in the Universe, are all exposed to an "open system".

The exposure to "open system" conditions to both materials, and energy means that conditions, and "ratios" of things change. The mass ratio changes, the applied gravitational force ratio changes, the orbital distances change, the angular momentums change, the kinetic energies change. The potential energies change.

What must remain relatively constant is that the alignment must remain co-linear. That is Earth, E-M Barycenter, Moon must remain co-linear, while at the same time the orbital distances and velocities are changing due to the changing mass and gravitationally applied force ratios.

Back to the original question, we can use the Solar and Lunar applied forces as they create winds, and waves, and thermal heating, but there is an upper limit to how much we can use. We could never reach the point of using all that is available, so it would never reach the definition of perpetual motion machines. There are always inefficiencies and losses in any and every system.

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  • $\begingroup$ But how much mass are Earth and the Moon accreting these days? $\endgroup$ – Hot Licks Jan 31 '15 at 3:14
  • $\begingroup$ The forces are always equal, surely? $\endgroup$ – Oliver Charlesworth Jan 31 '15 at 14:24
  • $\begingroup$ I think that's wrong. The difference in accretion would not produce violations of Newton's 3rd law. In any case the retreat of the Moon is due to concervation of angular momentum. $\endgroup$ – JDługosz Sep 16 '15 at 9:52

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