PDA

View Full Version : Two Moons: Effects, Implications, Technicalities



Akuma
11-26-2008, 11:31 AM
I have an setting very similar to earth, without all the pollution and history we've become familiar with. However, this earth has two moons, (one large, one smaller) and as far as my internet searching goes, I've read the obvious:

1) Nights will be brighter

2) Tides will be more complicated

3) Eclipses will be more frequent

That being said, these points strike me as very vague. How bright would the average clear night be? Twilight? Bright as friggin' day? And what does "complicated tides" mean? I assume the high tide would work on a busier schedule, currents might be different, women might strike that time of the month a couple times a month (God forbid :D).

I'd be extremely grateful if someone would be willing to clarify these things, or chime in any factors I've overlooked.

Happy Writing,
Akuma

Puma
11-26-2008, 03:21 PM
Hi Akuma - I'm not a scientist, but - a lot would depend on the location and rotation/revolution of the two moons. If they are the same size, directly opposite each other, the same distance from the planet, and revolving around the planet at the same speed - you'd have two nights and two days in the same planet rotation period, the tides (from what I understand) would be half as high because the gravitational pulls of the two moons would counter each other out.

But, if the moons are not equal, the effects would be very different - two moons on the same side of the planet at the same time would create giant tides (and giant low tides); the days and nights would be much longer if they were both together. If the revolution patterns of the two are different so they are sometimes together and sometimes completely opposite, you'd have a scenario that would change in the factors you mentioned continually in each revolution cycle. Does that help at all? Puma

euclid
11-26-2008, 06:03 PM
Hi Akuma - I'm not a scientist, but - a lot would depend on the location and rotation/revolution of the two moons. If they are the same size, directly opposite each other, the same distance from the planet, and revolving around the planet at the same speed - you'd have two nights and two days in the same planet rotation period, the tides (from what I understand) would be half as high because the gravitational pulls of the two moons would counter each other out.

But, if the moons are not equal, the effects would be very different - two moons on the same side of the planet at the same time would create giant tides (and giant low tides); the days and nights would be much longer if they were both together. If the revolution patterns of the two are different so they are sometimes together and sometimes completely opposite, you'd have a scenario that would change in the factors you mentioned continually in each revolution cycle. Does that help at all? Puma

...with Puma's post. I would add that two moons the same size orbiting on opposite sides of the planet, is a very unlikely possibility, and may even be impossible. What is more likely, is that one moon would be big - like our own - and the other would be small (maybe half the size). The two moons would orbit independently, so you would get all sorts of effects, depending on whether their orbits brought them close together or not. You could probably work out a rough sequence of events, based on two orbits which are either (a) multiples of each other (moon 1 orbits 12 times per year, moon 2 orbits 24 or 36 times per year) or (b) relatively prime (moon 1 orbits 12 times per year, moon 2 orbits 23 - or 57 - times per year, say). There is great potential there for haphazard or predictable chaotic events. In addition, of course the two moons would affect each other, causing "wobbles" in their orbits. The smaller moon would obviously be most affected by this. I think I remember a really good TV program about the moon and how it influences the Earth (and life on Earth). May have been David Attenborough, I'm not sure. Google might help.

euclid
11-26-2008, 06:12 PM
I have an setting very similar to earth, without all the pollution and history we've become familiar with. However, this earth has two moons, (one large, one smaller) and as far as my internet searching goes, I've read the obvious:

1) Nights will be brighter

2) Tides will be more complicated

3) Eclipses will be more frequent

That being said, these points strike me as very vague. How bright would the average clear night be? Twilight? Bright as friggin' day? And what does "complicated tides" mean? I assume the high tide would work on a busier schedule, currents might be different, women might strike that time of the month a couple times a month (God forbid :D).

I'd be extremely grateful if someone would be willing to clarify these things, or chime in any factors I've overlooked.

Happy Writing,
Akuma

My own approach to this would be to refer in passing to extra high tides (when the two moons are in conjunction), mention double shadows at night, and the smaller moon racing across the sky (compared to the bigger one). It would be great fun to build mythologies and superstitions around the two moons. They would have names and distinct identities - personalities - in people's minds. You should have a lot of fun writing this!

I should have said in the earlier post that the two moons would probably orbit in the same plane (given enough time, this is what would happen). If they are orbiting in different planes, the effects on the planet could be chaotic.

Nights would not be so much brighter, as the smaller moon would make a small contribution.

HeronW
11-26-2008, 07:24 PM
I think mass and distance of the moons from the planets would have the most effect. A least-likely planet-damaging scenario would be the 2nd much smaller moon orbiting the 1st since it would be caught in the larger moon's gravity. Small or no effect on tides, you'd see the smaller pass over the larger in regular intervals, possible spiritual customs coincide with a 2 moon eclipse, and even more with a 2 moon + 1 planet lineup.

Chase
11-26-2008, 07:33 PM
In response to one part of your query, it's a well known fact that the women of Jupiter have sixty-two "curses" per month, one for each moon.

The main effect has been a very low market for romance novels, but sales of pads and such are way up.

C.bronco
11-26-2008, 07:34 PM
Doesn't Saturn's moon Titan have its own moon? I could be completely wrong.

As for orbits, I'd look at the other planets in our solar system that have more than one moon.

RJK
11-26-2008, 08:47 PM
Mass and distance would determine the orbits, How about a smaller moon caught at the L1 lagrange point (I believe that's the point where the pull from earth is the sam as the pull from the moon). the second moon would be forever caught in an eclipse. If it were caught at L2, (the point on the opposite side of the moon, where it would be stationary and the first moon would always eclipse it). People on earth would never see it. Wouldn't that be a surprise to the first lunar explorers?

Akuma
11-26-2008, 10:39 PM
Thanks so much for your input, guys! Definitely beginning to understand a bit more, but it looks like I got some more homework ahead of me. Regardless, I'm getting excited about the possibilities. If you have anything more to add, please, don't let me stop ya.

Till then, reps for everyone. :D

FennelGiraffe
11-26-2008, 11:23 PM
Consider the moons of Mars. They don't serve as great models for your scenario, since they're both quite small and one has a funky orbit. Besides, Mars has no oceans to have tides. However, some of the details should be suggestive.

Phobos is larger (although quite small compared to our Moon) and extremely close. It orbits about 9400 km out. Its orbital period, as viewed by a distant observer, is about 7 1/2 hours. Someone on the surface of Mars, however, would see it rise about every 11 hours. The very close orbit means that it appears to move the wrong way across the sky. (Incidentally, Phobos's orbit isn't stable. About 11 million years from now it will either crash into the planet or break up to form a ring.)

Deimos is smaller and more distant (although quite close compared to our Moon). It orbits about 23,000 km out. Its orbital period, as viewed by a distant observer, is about 30 hours. However, for a viewer on the surface of Mars about 2 1/2 days pass between one Deimos-rise until the next. Because it's so tiny, it will be difficult to make out a disk with the naked eye; Deimos will look more like a bright star.

Some basic satellite principles:
Solar eclipses - It's pure coincidence that our Moon is the same apparent size in the sky as the Sun. If the Moon were either smaller or farther out, we would get a transit, not an eclipse: the Moon would move across the face of the Sun, but not blank it out. If the Moon were either larger or closer, it would more than hide the Sun and we wouldn't be able to see the corona during an eclipse.
Tidal force scales with the mass and inversely with the cube of the distance. That means if you double the mass of a satellite (while keeping the distance the same), you double the tidal force. If you halve the mass of the satellite you halve the tidal force. However, if you double the distance (while keeping the mass the same), you get 1/8 the tidal force. Or if you halve the distance, you get 8 times the tidal force.
Changing the tidal force doesn't relate directly to how high the ocean tides are. There are many other factors, ranging from the surface gravity on the planet through the overall size of the oceans to the shape of a particular coastline. But it should give you a general comparison of the comparative effects of the two moons.