Destroying the Moon - orbital instability?

The column:

Can we get more discussion on what would cause this instability, and how the Moon prevents it?

The one factor I can think of is moment of inertia. Sure the Moon has a small mass compared to the Earth but considering its distance from the barycenter it would have a high I which may contribute to the stability of the overall system.

Well, here is the SciShow’s take on it (YouTube video).

Basically, it stabilizes the axial tilt. The moon dampens the tendency for the Earth to wobble (due to gravitational effects of the other planets) more than it already does, which gives us more consistent and stable seasons. Without it, the wobble would be much worse, causing more extremes in tilt.

My guts say the Moon is actually causing orbital instability, and that I should lay off Taco Bell food for a while … but what do guts know, eh? The Earth herself wobbles through her orbit, and without the Moon the Earth won’t wobble through her orbit … thus the orbit is more stable … at least more stable than a half dozen soft tacos.

I’m not clear on how the Moon causes waves. I’m not a surfer so maybe I’m missing something but I thought wind causes waves. How did the Moon protect the Earth form asteroid hits, even a 1,000 mile diameter rock hitting the Moon would almost certainly not hit the Earth 250,000 miles away. If anything the Moon slung these rocks towards us.

But I agree … the Moon is not made of cheese … so it has no value to us.

The Earth wobbles, but the Moon keeps it at a mostly stable wallow.

Waves are partly, probably mostly, caused by tides, and tides are mostly caused by the Moon.

I thought most waves were caused by wind. And now that you mention it, I’m not sure what causes the constant action of waves breaking on the shore.

Normal surface waves in the ocean are caused by wind, and their amplitude is roughly proportional to the fetch (distance over which wind is free to blow). The collection of waves is called swell. Storm surges can also be caused the the presence of a low pressure system which can dramatically amplify the waves beyond normal swell conditions. Tsunami and misnamed “tidal waves” are caused by seismic impulses which can create large traveling waves (in which the water actually moves laterally rather than just transferring energy by up and down motion). Actual solar and lunar tides do very little to create waves; they just pull water up and down depending on the relative orientation of the Sun and Moon at any given time during the day.

The supposed stabilizing effect of the moon upon the inclination of the Earth to the solar ecliptic is based almost exclusively, as far as I can tell, on the work of French astronomer Jacques Laskar. Laskar uses models of solar system evolution with chaotic perturbation to evaluate the stability of the planets over time, suggesting that Earth is uniquely stable because of her ginormous moon. However, Laskar’s conclusions are disputed in the astronomy community and have little evidence to support them.

The notion that the orbital eccentricity of the Earth is dramatically affected by the Moon is nonsensical; the effect of Jupiter would be essentially the same on the Moon as the Earth, and any resonance that might pull one into a more elliptical orbit would inevitably act on the other with virtually equal influence. It may have had that effect on the more eccentric orbit of Mars but I don’t think there are any definitive conclusions on that.

However, the Moon’s influence in clearing out the near Earth area from potentially hazardous asteroids is pretty well accepted. The Earth-Moon system act like a pair of wrestlers, flinging potential opponents out of its orbit over eons to clear the local area of the most likely candidates for impact. It doesn’t protect as much against more elliptical asteroids or long period comets, but the interaction of Jupiter and (to a lesser extent) Saturn has probably done a lot to clear out the inner system, so in our celestial middle age the system is relatively stable and clean, albeit not to the extent that we shouldn’t be concerned about the occasional (on the order of 10 million year) impact doing extensive regional destruction, and the once in 50-odd millions years doing some degree of global destruction.


That video has some weirdness.

First it claims the lack of the Moon means the planets would cause Earth’s axial tilt to change from 0 to 85 deg. I would like more explanation of that. I can see how the Moon can help stabilize the Earth, as the Earth/Moon wobble together. But what justifies those other large numbers? What’s the mechanism to cause the tilt?

Second, the picture demonstrating the tides has the Moon on one side and the Sun on the other, pulling together. This is only one orientation. At different times of the month, they are at different angles to each other, causing a fluctuation in tides. I suppose that was too much detail for them to get into, but that picture is misleading because it suggests they always work together, which is not true.

He says “our planet can hold on to everything on it, except it’s water”. He glosses over the fact that the Earth itself, the rock, is also subject to tides. That’s right, the rock also stretches. It just has a much lower amplitude than the water, so the water is visible.

As for night sky watching, the biggest obstacle for most people is city lights.

But the biggest howler is when he says “Take the Moon away, and the Earth could speed up in a hurry.” How? Yes, the Moon has been working as a brake, and yes in the past the Earth rotated faster, but the slowing effect of the Moon is fairly small now, and deleting the Moon will not add energy to speed up the Earth. It just means we won’t be slowing down at the rate of 1 second every 67,000 years. Congratulations, we no longer have to worry about leap seconds.

First, there could have been asteroids coming in from the direction that the Moon just happened to occlude. In a distant, more crowded past that is a possibility, but in that situation the Earth likely protected the Moon more than it protected us.

Rather, the orbital oscillation of the Earth/Moon pair means there is a wobbling ellipse of gravity, i.e. the gravity field changes in the vicinity of the pair. Any asteroids coming close would get disturbed and potentially thrown to a higher or lower orbit. Without the Moon, Earth’s rotation would be a circle and so it would not have that wobble. Some would still be disturbed, but more would get closer.

Sure…it’s a popular science tube that’s trying to convey complex things in an easy and fun way to grasp them.

Though Stranger seems to think this is ridiculous, a quick Google search seems to confirm that at least some scientists believe this. No idea where they got that range, but the fact that some additional destabilization would occur seems pretty well accepted. I found this cite that explains the mechanism, but if you Google it you will find a ton of additional cites on it (i.e. the SciShow guys didn’t just pull this out of their ass):

Again, it’s a popular science tube presenting stuff in small bites that are supposed to be fun and easy to grasp. There are more hard core (and lengthy) science videos but I’ve found that when using a video as a cite on this board it’s best to go with the 5 minute ones if you want anyone to actually watch them. :stuck_out_tongue:

Honestly not sure about this one. Googling it, the answer seems to be that the sun/moon combination of tidal effects slows the earth down, but without the moon the slowing effect would be much less…and that the earth would speed back up to it’s previous rotational speed. I’m not sure what the mechanism is, however (interior dynamo? effect of other planets?) and I can’t find an easy to grasp cite explaining it in simple terms (that I can understand anyway). Again, though, it doesn’t seem that the Scishow guys pulled this out of their asses…there seems to be some consensus that the earth would speed up again (though over what timeline is also not clear to me…a billion years? Million? 100,000? No idea).

Gru already stole the Moon, so you can’t destroy it.

Anyway, “vaporizing” the Moon does not reduce its mass. Anything that would convert the Moon’s mass to energy would likely destroy not only the Moon, but the entire Solar System.

Destroying the Moon would more than likely create rings of matter around Earth, like Saturn. Some chunks might fall and strike the Earth, destroying not only Cleveland but a good portion of all life on the planet, but most would remain in its current orbit.

I think we are much more dependent on tides than most people realize. But then, as a shellfish farmer I might be a little more sensitive to what happens to tides than most people.

Geez. That’s awfully shellfish of you. :smiley:

Prompted by this, I did a little googling myself. The first couple of sites did merely say “astronomers have long believed” without explanation.

Then I found this:

So that refutes the actual statement even though scientists may have previously believed it.

But then I found this:

Bolding added. Oops.

Anyway, that previous cite did explain the mechanism. It involves the rotational precession of Earth versus the rotational precession of Earth around the Sun. The Moon-Earth rotation acts as an additional variable to keep them out of synch. When they fall in synch is when the wobbling gets severe.

Of course these changes occur over very long time scales.

That site is odd, too. Here it says

Ironically, that quote is actually appropriate to the idea of the Moon suddenly disappearing, but is rather wrong for the case he uses it, if the Moon hadn’t been there in the first place.

The reason a hammer thrower loses his balance is precisely because he is stable while spinning before the throw. When he releases the hammer, he is suddenly off balance because a chunk of mass and momentum are immediately removed at an oblique angle. But if the hammer thrower wanted to spin and wasn’t holding a hammer, he would be able to spin up in place and not fall over.

Like an ice skater. They can spin in one place without being suddenly off balance.

I’m gonna need you to list some of your findings or something, because I don’t know what to even google.

Here’s wikipedia’s entry on tidal acceleration, i.e. the slowing of Earth’s rotation from lunar tides. Note that it does not discuss what would happen if the Moon magically disappeared.

I think it’s possible they are supposing that the Earth would retain the same angular momentum but the Moon is missing. In that case, because the mass changed and the effective rotational radius changed, the angular velocity would have to increase.

But I’m not convinced the Earth would retain the angular momentum. I suppose that depends on the magical method of making the Moon disappear.

I’m thinking just adding heat energy to the Moon and boil it off. The mass of the Earth/Moon system would go down which most likely would change the orbital parameters. We could maybe find the edge of the Habitability Zone.

Sorry to intrude, fascinating discussion …

Yeah, that’s about a 200 gigaton bomb. I think that might have some effect on orbital mechanics all by itself.

Even this cite, however, says there would be some additional instability, just not the 0 to 85 degree shifts others posit. So, again, it seems clear that most agrees there would be SOME…how much depends on the model and assumptions. And, again, it wasn’t something weird or ridiculous or something just pulled out of Cecil’s ass wrt the original article.

Sure. It’s not going to destabilize over night, depending on how, exactly, the moon disappeared. If it was blown up, of course, all or most of the mass would still be up there. If something large passed by and dragged it from orbit then that would have other effects on the Earth. If it just disappeared into a quantum tunnel or something then that would have yet another effect. You asked about an orbital effect of suddenly not having a moon but you didn’t set up any pre-conditions, so I went the magically gone route, since it seems the least messy.

Well, I’m unsure how the earth COULD fling the moon out of orbit without some fairly drastic changes. You’d have to speed up the orbit of the moon around the earth a ton to get that effect…or, like I said, if something large came close enough it could destabilize the orbit (but probably would destabilize the earths orbit as well and gods know what else…I was playing around with this in the sand box universe simulator and I couldn’t get anything large enough to pull the moon out of orbit without wreaking havoc throughout the rest of the solar system. Granted, no idea how accurate their simulation is :p).

Most of the sites I googled asking ‘what would happen if there was no moon’ talk about or have a faster rotational speed of the earth afterwards. I didn’t dig through every one, but it seems a common theme…sort of like the idea that if there was no moon there would be some additional instability in tilt. I’ve seen everything to explain this from the sudden disappearance of the constant tidal effect on the earth itself (which would cause a huge tsunami as soon as it disappeared) to conservation of angular momentum as you note to other bizarre things. I have already admitted that I don’t get this part…seems to me that the earth would, by now, remain fairly stable, though we might obviously stop slowing down the rotational of the earth, since most of that is caused by the moon (i.e. our current 24 hours and change would pretty much be where we stayed for the next few billion years). I certainly don’t see how it would speed up substantially.

I guess, in the end, your question is going to depend on how the moon disappears (i.e. what happens to it), and who’s model/theory do you want to believe. I think your question about Cecil’s article has been answered in so far as it’s not just him who thinks that the tilt will become more extreme, whether it’s 10% change or one going from 0-85 degree…nearly every link I clicked on or video I skimmed (like this one on YouTube), seem to agree that SOME instability would happen.

wrong thread

Putting aside the equally “misnamed” Tsunami (“harbor wave.”), the expression “wave as big as a tide” is perfectly good English.

Unless you want to argue that “High Tide” is a festival day, and “tides” are misnamed using a ridiculus foreign importation…

If it were “vaporized”, it would be the initial shock to the gravitational equilibrium. I would think that if it occurred at specific various points relative to the Earth, it could throw it out of orbit… out of current orbit, that is.

However this is not the main issue. The moon is born from the Earth. Think about how Earth would feel if a dingo ate its baybay