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What if our Moon was instead a set of binary moons?


FungusForge

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I was just sitting around wondering how different things may be if Earth had a second moon, and my thought eventually drifted into wondering what it would be like if our singular Moon was instead two moon orbiting a common point between the two. For the sake of making things easy we can assume whatever mass difference between the two moons is, the combined mass of the two moons will be about that of our Moon. Anyway, the kind of answers I'm hoping for are along the lines of how this might have changed things, or if there'd be a notable differences at all (save, of course, for the different view in the sky).

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My gut says it would be a short-lived arrangement. The Moon is already close enough to be tidally-locked, if it were a binary pair, tidal effects would eventually (maybe even quickly) disrupt it. Possibly resulting in the destruction of one or both moons and possibly with catastrophic effects for us.

Perhaps if the binary moon pair were much further away than our current moon it would have more chance of stability.

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I read (and saw) somewhere about a theory of moon formation, with all it's surface feature disrepancy amd whatnot, that the Moon might've been two moon, one quite smaller than the other. Each was positioned more or less along the others L4/L5. Then a slow-velocity collision happened, with the lesser moon forming the far side. It was based on simulations, but simulations aren't reality and reality isn't always possible to be simulated.

So yeah, if it was there, it's gonna be short lived. Unless it's a captured moon or co-coalesced with Earth, which is a hard try.

The difference ? Not much I guess. (?) Tidal heights are mainly due to the Sun, so if any maybe the maximum tides would be slightly lower.

Edited by YNM
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5 hours ago, YNM said:

The difference ? Not much I guess. Tidal heights are mainly due to the Sun, so if any maybe the maximum tides would be slightly lower.

I'm sorry, but it's 2:1for the moon :-)

http://oceanservice.noaa.gov/education/kits/tides/tides06_variations.html

But i have no idea what would happen if there were to bodies ... weather services probably had one more excuse if things wouldn't happen as predicted ...

 

Edited by Green Baron
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I guess one would still be called 'Luna', perhaps the other would be 'Moony McMoonFace'.  It would be made of a kind of blue cheese instead of Luna's cheddar.  The man in Moony McMoonFace would be considered a brother of the guy in Luna, inspiring all sorts of pagan religions whose primary tenets was doling out noogies and pantsing.  And there would be a different view in the sky.

 

Sorry; it's a friday afternoon and I've gone loony...

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Well, the anemones would notice it, so life might have developed a little differently, but probably not much. The Space Race would probably have lasted longer, though, with the added gravitational difficulties and the extra moon to keep the enemy from claiming.

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If we had two moons.... then earthquakes would be more common and the effect of our oceans would be that much harder to sail a ship....

We are actually lucky... the Moon is JUST the right distance from the Earth to be perfect... its orbit means we can ever see the dark side from the Earth... which is also good... (asteroid impacts etc) ...

We DO have two moons in KSP.... Mun and Minnmus (spelling?) ...

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5 hours ago, Green Baron said:

Hmm... Ah well, thanks for the correction. Weirdly people here have almost always assumed tides to be semi-diurnal no matter where the Moon is (and it does appears to be that way, so on midday and midnight the tides are always the highest), so I assume so.

Still, the reduced mass and a more spread direction for the tides to point at could mean that the overall tides be a bit dampened out.

Edited by YNM
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12 hours ago, kiwi1960 said:

If we had two moons.... then earthquakes would be more common and the effect of our oceans would be that much harder to sail a ship....

We are actually lucky... the Moon is JUST the right distance from the Earth to be perfect... its orbit means we can ever see the dark side from the Earth... which is also good... (asteroid impacts etc) ...

We DO have two moons in KSP.... Mun and Minnmus (spelling?) ...

 

Hmm... I don't think that would be the case. First of all two moons wouldn't be gravitationally stable over planetary timescales. They would hit each other, the earth, or be ejected from the system.

But neglecting that and assuming it were possible, the effect on the surface of the earth would be pretty much the same.

The tides would be roughly similar, as the distance to the common barycentre of the moons would be far greater than any variations in the positions of each individual moon (okay, this would be a bit different if the moons were, say, located at L4 and L5 or something, which would result in reduced tidal rance, but that's not a stable configuration either)

It's not really the moon's orbit that means that one side only faces us, and it's certainly not luck. Any orbit relatively close to the parent body (not just at the "perfect" distance) will result in the orbiting body becoming tidally locked, because of small variations in the gravity field. But tidal locking doesn't really affect life on earth. Asteroids don't care which side of the moon they hit, or if the moon is spinning at the time, it will still get in the way. In fact, two moons would have a greater surface area for the same mass, so would present a slightly bigger target, for what that would be worth

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I disagree. The Moon is at such a distance from the Earth that its rotation matches our orbit perfectly, meaning the dark side is never seen. Assuming we had another Moon on the opposite side of the planet, the gravitational pulls would cancel each other out... dramatically affecting tides or screwing them up totally... the constant pulling from both Moons would create earthquakes... at least till the earth adjusted itself. Don't forget, this is exactly what happened when the Moon first appeared in the sky... there is a theory that it was passing by and was captured by our earth way back when... the fact that its in a perfect orbit, just at the right distance to never show its dark side AND not deviate much from its perfect orbit is in itself amazing...

Had it been an elliptical orbit then ... things would not be so stable for us here on Earth.

Another theory has it that our Moon was actually part of the Earth which was ejected into orbit thanks to an asteroid or comet impact.

Its the Moon that has protected us from more asteroid impacts than we have had, mind you, in the early days, there would have been thousands of impacts but they have all gone now thanks to erosion and water erosion. They think the asteroid impact zone off Florida was the one that killed off the Dinosaurs..

Either way... I won't argue about it... the Moon we have is gorgeous... I'm glad its there... if there was another Moon and provided it didn't make our lives hard, I'd love that one as well. :)

Edited by kiwi1960
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Heh, I remember myself, posting this question, in this same subforum. Then it turned to a debate about whether mass is real. :P

25 minutes ago, kiwi1960 said:

Its the Moon that has protected us from more asteroid impacts than we have had, mind you, in the early days, there would have been thousands of impacts but they have all gone now thanks to erosion and water erosion. They think the asteroid impact zone off Florida was the one that killed off the Dinosaurs..

Florida? I thought it was the Chicxulub Crater in Mexico...

25 minutes ago, kiwi1960 said:

Either way... I won't argue about it... the Moon we have is gorgeous... I'm glad its there... if there was another Moon and provided it didn't make our lives hard, I'd love that one as well. :)

Agreed. :D

Edited by Atlas2342
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A few weeks ago i read a text about a trinary star system, a binary couple very close to each other, like a few AE and a third one very far away (many hundrets AE) that seemed to be stable. In that case the couple might be assumed to have a single COM in respect to the far away star. Whether this is usable for smaller units with planet/moon sizes and distances, i don't know. Sun and Jupiter would probably disturb in our case.

Another thing that comes to my mind: tidal forces brake the rotation due to friction, thus over time forcing the body with less mass into a lock (the bigger one will lock as well but much slower). The moon drifts slowly away from the earth, it startet much closer (well, on the surface :-)), i recall this is because of impulse preservation while braking the earths rotation over time. How would that effect influence a binary moon-system close to a planet ? Would the larger component drift away slower cause it brakes slower, thus loosing the smaller component which might be "flung out" rather quickly that way ?

(see for example: http://sservi.nasa.gov/?question=moon-drifting-from-the-earth)

Just playing with ideas ...

 

Edited by Green Baron
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4 hours ago, kiwi1960 said:

I disagree. The Moon is at such a distance from the Earth that its rotation matches our orbit perfectly, meaning the dark side is never seen. Assuming we had another Moon on the opposite side of the planet, the gravitational pulls would cancel each other out... dramatically affecting tides or screwing them up totally... the constant pulling from both Moons would create earthquakes... at least till the earth adjusted itself. Don't forget, this is exactly what happened when the Moon first appeared in the sky... there is a theory that it was passing by and was captured by our earth way back when... the fact that its in a perfect orbit, just at the right distance to never show its dark side AND not deviate much from its perfect orbit is in itself amazing...

This isn't just a coincidence though. Tidal locking is a natural process, because the uneven mass distribution of the moon means that it will have a preferred orientation to sit in the gravity gradient. If the moon was closer in, it would rotate a little faster to keep the same face pointed towards the earth, if it was further out, it would rotate slower. Basically, if it is not sitting in its preferred orientation, gravity is trying to pull it slightly out of shape, so energy is expended and eventually it settles down with one face towards the parent body. Most moons in the solar system are actually tidally locked. Wikipedia has a pretty decent article about the process: https://en.wikipedia.org/wiki/Tidal_locking

The same for the eccentricity of the orbit, tidal circularisation is also a thing, as well as a natural circularisation due to the orbiting body hitting the interplanetary medium harder at periapsis than apoapsis. This is a very slow process, but the moon has been around a long time, and also, in the aftermath of a giant collision (the favoured theory for how our moon was formed), there would be a lot more for the moon to hit as it orbited.

With another moon orbiting opposite the first (at L3), well, this wouldn't be a stable orbit, but neglecting that, it would indeed have a weird effect on the tides, but perhaps not as big as you'd think. Currently the moon doesn't just orbit the earth, they actually both spin about a common barycentre. On the outside, centrifugal force is more important, and the water bulges towards the outside. On the inside, the moon's gravity is more important, and the water bulges towards the moon. If there was a moon on either side of the earth, the common barycentre would be the centre of the earth, but there would still be a bulge of water on either side of the earth, and you would still get two high tides a day. What would happen to the overall range, I can't take a reasonable guess at.

4 hours ago, kiwi1960 said:

Either way... I won't argue about it... the Moon we have is gorgeous... I'm glad its there... if there was another Moon and provided it didn't make our lives hard, I'd love that one as well. :)

Totally agree. More moons for everyone! :)

 

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8 hours ago, Atlas2342 said:

Heh, I remember myself, posting this question, in this same subforum. Then it turned to a debate about whether mass is real. :P

Florida? I thought it was the Chicxulub Crater in Mexico...

Agreed. :D

Yes, well, I never said I was perfect! :) I just know its out Florida way! :)

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5 hours ago, Yourself said:

It's actually closer to Texas and Louisiana than it is to Florida.

Well.... here is where I must object, the crater I am referring to is under water and it was found by scientists looking for oil...

chicxulub-crater-e1457449783890.jpg

From what I remember, THIS asteroid forms the whole of the gulf of Mexico... or.. most of it... although in the photo, there is a crater on land as well...
GulfCrater.jpg
Chicxulub-fireball-on-earth.jpg

00065m_major_extinction_1600km_central_a

 

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That's a misunderstanding :-). Not the whole gulf is the impact crater. The impact crater is buried under sediments, partly on the peninsula of yucatan, partly on the shelf.

http://neo.jpl.nasa.gov/images/yucatan.html

The crater itself is "just" 200-300km in diameter.

http://www.chicxulubcrater.org/

https://en.wikipedia.org/wiki/Chicxulub_crater

Edit: Gulf of Mexico is a tectonical structure on a larger time scale:

http://www.nature.nps.gov/geology/education/osu/GUIS_1999_GIP_Lillie.pdf

 

Edited by Green Baron
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