# Multiple Moons....

I was just reading a web page about a fictional world. When they discussed the planet itself, they said it had more than one moon, which affected the tides. I was wondering, how would having more than one moon affect the tides here?

One effect of multiple moons would be a more rapid slowing of earth’s rotation due to the crust’s friction with the ocean, which would be in an even greater state of ebb and neap than it is now. I suppose it would all depend on how many, their placement (both distance and alignment) and, of course, their mass.

• Planetary systems with three or more members orbiting a single point are unstable - the more “objects”, and the closer in size they all are, the more unstable. (An object can be a planet or a moon.) Systems such as this will usually suffer collisions or throw off objects until only two remain. The planet itself may get thrown, leaving only two moons orbiting each other. The only way to keep it relatively simple is to make one object (the planet) HUGE and make all the other objects in the system (the moons) all in different sizes and quite tiny by comparison, which describes our own gas giants pretty well.
• The only way it is easily stable is if you have sets of two objects (orbiting each other) also orbiting other objects, or sets of two objects. There are triple stars - star A orbiting star B, and set (A&B) also orbiting another larger star C. There are also quadruple stars in the sky - Star A & B orbiting each other, and star C & D orbiting each other, and set (A&B) orbiting set (C&D). The requirement here is enough distance that any star in one group can’t upset the orbit of any star in the other - and so it will take millions of years for this group to complete one orbit. I don’t know of any examples with more than four members.
• Well nuts! Kinda missed the question, didn’t I? -
• Multiple moons wouldn’t have much of any effect on the tides, for the reason that such moons would have to either be very distant from the planet, or very small. - MC

Huh? Without our moon there would be no tides.

Dunno about the theory of multiple moons, but if one does, figures more would.

Man, this question is going nowhere fast.

• Well, I tried. I assumed that if we had two moons (the sme size) orbiting each other, and orbiting Earth as a group, what tidal cycles would do. The whole thing hinges on how fast you want the two moons to orbit each other, but any way you set it up the tides get smaller and of longer duration, because the gravitational influence of the moons is more spread out. There’s an unknown factor in the equation, so there is no exact figure. - MC

If there are two moons orbiting each other, couldn’t they be viewed as a single object having its center of mass somewhere between them? Tides would then work the same way they do now if the aggregate mass of the “moon-pair” and its distance to the earth (measured between the centers of gravity) were the same.

Holger

• Yes, that is correct: that’s the reason you have to have either single objects, or pairs of objects primarily influencing each other. The problem is tidal action on the moons’ cores: the gravity of Earth would vary between their nearest point in (their) orbit and their farthest point. It isn’t possible to do any more than estimate how much the moons would heat up, and if they heat up and liquify/vaporize very much that screws up your formula. The moons will get lighter, and spread farther apart until you have a random three-object system. (-A single moon around a planet doesn’t have this problem) So two moons would have to be at least four times as far away, and possibly much farther to be stable, which means their gravitational effect on the Earth’s tides would be less than it is now, spread over tidal cycles of longer duration.

Handy–without the moon, there would still be the (admittedly much smaller) solar tides.

Just thought I might bring up a thread from 1999, sorry if this might bother you.

OK, I’ll bite.

WHY did you revive this thread?

For nostalgia

How can a person who registered this month be nostalgic for a thread that’s four years old?

Don’t bump any more old threads without a good reason. This thread is closed.

bibliophage
moderator GQ

That is very frowned upon by the board. I’d strongly suggest you don’t do it “just 'cause” anymore or your going to probably have a mod chew you new one.