zompist bboard

How many moons does Almea have? Are they small like Mars's satelites or larger like Earth's?

3, I believe, smaller moons. Means Almea has much more complicated tides. Though I'm curious to see how that would work...and on that note, how a tides (if at all) could fuction without a satelite.

Nikolai(College) wrote:3, I believe, smaller moons. Means Almea has much more complicated tides. Though I'm curious to see how that would work...and on that note, how a tides (if at all) could fuction without a satelite.

Without any satellite of significant size, tides would be far too small to notice. The Sun and other planets do of course have a tidal effect on Earth, but as one might imagine, it is very small.

I would also think that a moonless planet would have rougher waters than one with a satelite?

Nikolai(College) wrote:I would also think that a moonless planet would have rougher waters than one with a satelite?

Difficult to say; I'm not really sure what effect tides have on overall ocean behavior once you get away from the shore. My guess would be no, the absence of tides wouldn't make waters rougher; that's mostly wind's job, as well as a whole slew of tectonic effects.

So if a planet was alone in a system, and was trhe only mass arder than lets say a few hundred miles, except for the sun, and the planet had no moons it would have no tide?

Shades Of Grey wrote:So if a planet was alone in a system, and was trhe only mass arder than lets say a few hundred miles, except for the sun, and the planet had no moons it would have no tide?

No noticable one anyway.

Dudicon wrote:

Nikolai(College) wrote:Without any satellite of significant size, tides would be far too small to notice. The Sun and other planets do of course have a tidal effect on Earth, but as one might imagine, it is very small.

From some quick Googling: http://www.clockwk.com/tides/

The planet with the biggest influence on our tides is Venus (and only, of course, when it's close to us). Its maximum tidal influence is just 1/10,000 that of the sun. The sun's influence, however, is half that of the moon. So a moonless planet should still have tides.

Almea does have three moons (in Verdurian, Iliazh?, Iliac?sh, and Naunai). Iliazh?, the largest, is about the size of Ceres: ~ 600 miles wide, or ~ 1/4 the diameter of our moon. (In the sky, however, it's about 1/2 the diameter of our moon, since it's closer to Almea.) Iliac?sh is about 2/3 its size, and Naunai is only just large enough to register as a disk.

I am just realizing how long it's been since I played with orbital dynamics. Given an orbital period, can one calculate a moon's distance? And if so, what's the formula?

Kepler's 2nd law, or rather its derivation from Newton's law of gravitation, gives us... I don't think I can do equations here very well.

(Since the orbit's elliptical, there's no one distance. This formula gives the semi-major axis a, which will be the radius in the case of a circular orbit.)

a = ((T(Gm)^(1/2))/2pi)^(2/3)

Semi-major axis = (Orbital period times (Square root of (gravitational constant G times mass of planet)) divided by 2 pi) to the power of 2/3

Well, I hope that was moderately clear. The mass of the moon doesn't matter as long as it's small relative to that of the planet.

Maybe someone could check my formula?

Dudicon wrote:

Nikolai(College) wrote:I would also think that a moonless planet would have rougher waters than one with a satelite?

Difficult to say; I'm not really sure what effect tides have on overall ocean behavior once you get away from the shore. My guess would be no, the absence of tides wouldn't make waters rougher; that's mostly wind's job, as well as a whole slew of tectonic effects.

Don't forget that Earth's tectonics are also influenced by the moon . . .

So Haleza Grise wrote:

Dudicon wrote:

Nikolai(College) wrote:I would also think that a moonless planet would have rougher waters than one with a satelite?

Difficult to say; I'm not really sure what effect tides have on overall ocean behavior once you get away from the shore. My guess would be no, the absence of tides wouldn't make waters rougher; that's mostly wind's job, as well as a whole slew of tectonic effects.

Don't forget that Earth's tectonics are also influenced by the moon . . .

Of course; a good point. I guess the general idea then is that it would be, on the whole, very complicated to determine exactly the effects of the absence of a major satellite on an earth-like planet.

Thanks for the formula, butsuri... I have to find the exact length of the Almean day, which I remember setting but can't find on VV. Still, I was mistaken in assuming that the moon could be placed at any distance. Since the period of Iliazhe is close to that of our moon, its distance has to be about the same too.

There is a section on calculating data about moons at World-Builders.org, which I assume many people here are aware of. That site was really helpful when I was doing all this for my own planet. The equations for this particular problem given there are different to butsuri's, but they look derived from the same source.

zompist wrote:Since the period of Iliazhe is close to that of our moon, its distance has to be about the same too.

This would depend on the mass of the planet around which it orbits, wouldn't it? Since Almea is smaller than Earth, I imagine it will be different to some degree, though I don't know what degree that might be.

Yah, I'll have to work out the exact difference. But the main correction is that instead of just using the planet's mass M, you use M + m, where m is the satellite's mass. Since the Earth is about 80 times the mass of the moon, it doesn't make a huge difference. The difference between Almea and Iliazh? is even more lopsided.

Dudicon wrote:Of course; a good point. I guess the general idea then is that it would be, on the whole, very complicated to determine exactly the effects of the absence of a major satellite on an earth-like planet.

There's a pretty good book called, creatively enough, What if the Moon Didn't Exist? Detailing speculation on this and other astronomical counter-factuals. Pretty good on the hard stuff, don't like some of his biology, but . . .

One more point is the importance of tidal braking on the rotation of the Earth. Comins says that the rotation was about once every 6-7 hours before the moon was formed, and has been slowed down primarily by the moon's tidal braking action. I can't remember what he speculates it would be by now if not for the moon, but still much less than it is. Which would lead to much stronger winds.

Speaking of which, an old friend of mine just got a book published called The Big Splat, or How Our Moon Came to Be. But I haven't had a chance to look at it yet.

On your close in moon, Zompist, I'd watch out for the Roche limit, but I can't remember any of the calculations on it right now, it might not be a problem.

One thing that is worth noting is that for any intelligent life at all a bigish moon is quite important. Firstly some biologists think life began in the tidal pools formed by early moons huge tides. Secoundly the Moons tidal force locks Eathrs axis so it doesn't wouble around wildly causing massive ice ages and desserts that destroy all life so far devolped. For example Mars doesn't have a massive moon and its axis has been quite wild during its history.

gach wrote:...desserts that destroy all life so far devolped.

Like evil blancmanges from the planet Skyron in the Andromeda Galaxy?

I found that unless you want the moon really really close, Roche's limit isn't all that much of a problem. Basically, provided the planet and moon have the same density, the moon must be beyond 2.423 times the planet's radius or it will disintegrate. The denser the moon compared to the planet, the closer it can be; the denser the planet the further away the moon must be.

gach wrote:One thing that is worth noting is that for any intelligent life at all a bigish moon is quite important. Firstly some biologists think life began in the tidal pools formed by early moons huge tides.

Yeah, though it's an idea that's falling somewhat out of favor these days, with new discoveries about extremophile bacteria.

gach wrote:Secoundly the Moons tidal force locks Eathrs axis so it doesn't wouble around wildly causing massive ice ages and desserts that destroy all life so far devolped. For example Mars doesn't have a massive moon and its axis has been quite wild during its history.

That, I think, is the much more important part. I had forgotten about that. Though Mars wouldn't have had quite such a hard time of it if it wasn't also so small. Too small to hold in a proper insulating atmosphere. Those swings on a more Earthly planet might still have caused huge extinctions, but would almost certainly have left plenty of life. Bacteria, if nothing else. Though that doens't help exactly when talking about intelligent life.

On the other hand some significant climatic change is important for developing intelligent life. Humans only exist because an icehouse (cold and dry) period contracted the forests greatly and forced a bunch of hapless apes to adapt to a new situation. And mammals in general developed their relatively large brains partly in response to the icehouse of the mid- to late-Triassic.

It just occured to me that the possibly recent origin of the Zone of Fire on Almea could be related to a fluctuation in axial tilt. The closer to straight up and down, the more extreme the heat gradient from pole to equator will be. but, there's probably to many other, unwanted ramifications of that to be of use.

eloqui wrote:I found that unless you want the moon really really close, Roche's limit isn't all that much of a problem. Basically, provided the planet and moon have the same density, the moon must be beyond 2.423 times the planet's radius or it will disintegrate. The denser the moon compared to the planet, the closer it can be; the denser the planet the further away the moon must be.

You're right, you're right. I remember now doing the calculations and being suprised it so far in.