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Old 06-18-2009, 06:32 PM   #21
Agemegos
 
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Default Re: Celestial Bodies

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They are the places where gravity forces cancel out.
Not even approximately. L1 involves a partial cancellation of the gravities of the planet and moon, but L2 and L3 involve a direct addition of those gravities.

The Lagrange "points" are circular orbits in a two-body system in which the gravity forces sum in a vector fashion to provide exactly the correct net centripetal force to give an object of neglible mass a circular orbit with period equal to the period of the orbit of the two significant masses around each other.
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Old 06-18-2009, 06:41 PM   #22
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As for the paired moons orbiting the planet...don't know. I would suspect that it would be possible, with the right mass and distance from the primary.
Yeah, it is okay provided that the smaller moon is within about 1/2 or 1/3 of the Hill Sphere radius of the larger moon.
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Old 06-18-2009, 07:11 PM   #23
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Yeah, it is okay provided that the smaller moon is within about 1/2 or 1/3 of the Hill Sphere radius of the larger moon.
So, if both moons were equal size... they'd need to be within each other's Hill Sphere...?
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Old 06-18-2009, 07:25 PM   #24
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So, if both moons were equal size... they'd need to be within each other's Hill Sphere...?
That's right. And well inside. The Hill sphere is defined by neglecting the mass of the third body; even so orbits in the outer half or two thirds are not stable over geological time. Two moons of equal mass are likely to excite nasty three-body-problem effects. You want to place them snugly to stop them from wandering off.
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Old 06-19-2009, 12:42 PM   #25
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That's right. And well inside. The Hill sphere is defined by neglecting the mass of the third body; even so orbits in the outer half or two thirds are not stable over geological time. Two moons of equal mass are likely to excite nasty three-body-problem effects. You want to place them snugly to stop them from wandering off.
Since they would be so close, would the planet (for a lack of better words) treat them as being one single object for the purposes of determining where they'd need to be to be within the planet's Hill Sphere?

I'm imagining that instead of putting the moons at a certain distance, I'd have to base the distance from the planet upon some sort of midway point between the two moons... the 'axle' of the moons' lunar-lunar orbit if that helps to illustrate what I'm saying. I'm not sure if that my imagination's assumption is correct though.
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Old 06-19-2009, 11:36 PM   #26
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Since they would be so close, would the planet (for a lack of better words) treat them as being one single object for the purposes of determining where they'd need to be to be within the planet's Hill Sphere?
Yep, that should be good enough. The whole thing is a bit of a rule of thumb anyway.
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Old 06-20-2009, 04:30 AM   #27
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Default Re: Celestial Bodies

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I could have the two moons just orbit the planet, but I was trying to do something a little more exotic. Also, I wanted some reason for the tidal waves to be a little more extreme and for ocean travel to be a little more trecherous than on Earth... a way to explain why, even after long periods of time, certain races and societies still hadn't come into contact with other races and societies who were based on other continents or land masses.
Late though I may be, there's an existing published setting that at least considers this same thing: The Iron Kingdoms, by Privateer Press. I don't remember the exact angles and whatsits and so forth, but the general gist is that the planet has three moons in orbit around it, which are highly different from one another in orbital inclination, mass, and the "severity" of their orbits' elliptical shape. This was used to explain the unpredictable, severe, and very dangerous tidal behavior of the setting's primary ocean, and thus in turn to explain why it had virtually never been traversed.

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Old 06-20-2009, 08:37 AM   #28
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Late though I may be, there's an existing published setting that at least considers this same thing: The Iron Kingdoms, by Privateer Press. I don't remember the exact angles and whatsits and so forth, but the general gist is that the planet has three moons in orbit around it, which are highly different from one another in orbital inclination, mass, and the "severity" of their orbits' elliptical shape. This was used to explain the unpredictable, severe, and very dangerous tidal behavior of the setting's primary ocean, and thus in turn to explain why it had virtually never been traversed.
Three highly different orbits will result in tidal effects that usually tend to cancel each other out. You'd need to get them regularly pulling together to make additive effects to produce high variation.

Underwater geography tends to have more to do with high and/or swift tides anyway. The Bay of Fundy (79 ft tide) doesn't have any special celestial body that affects it alone.
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Old 06-20-2009, 08:44 AM   #29
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Default Re: Celestial Bodies

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Originally Posted by Landwalker View Post
Late though I may be, there's an existing published setting that at least considers this same thing: The Iron Kingdoms, by Privateer Press. I don't remember the exact angles and whatsits and so forth, but the general gist is that the planet has three moons in orbit around it, which are highly different from one another in orbital inclination, mass, and the "severity" of their orbits' elliptical shape. This was used to explain the unpredictable, severe, and very dangerous tidal behavior of the setting's primary ocean, and thus in turn to explain why it had virtually never been traversed.

Cheers.
You're not too late. I always appreciate input.

Besides, the thread is open for discussing other arrangements as well.

Essentially, what I was going for with the oceans was an exaggeration of the reason for America's isolation from the rest of the world before it was 'discovered' by the European countries of the world at that time.

I imagine that tides would be the most severe during a single full moon on this world because both moons would be lined up and pulling from relatively the same spot. I'm not sure if this is completely correct, but I imagine it like a game of tug of war; when the two moons are lined up, they combine their pull. I'm not 100% sure if that thinking is correct, but, in my mind, it seems like it would be.

During a 'double moon' I imagine that the tides would be somewhat rough as well, but I imagine they'd be less severe yet more spread out than they are during a single moon. I'll try to illustrate what I mean.


o .. (full moon; lunar-lunar eclipse)

o : (double moon)



So, if you include the sun in the illustrations, some of the possible alignments over a year include some of the following -


O o .. (sun, planet, and moons all lined up in a row; a lunar eclipse)



O o : (I think this might create some sort of lunar eclipse in which the
phase of each moon's individual eclipse would be the mirror of the other's)



O .. o (an eclipse which would look similar to a solar eclipse on Earth I think; also a time when I think the tides would be the most severe because you'd have three bodies pulling from the same direction)



O : o (I think an eclipse like this would produce an interesting visual effect. I imagine it appearing similar to an hourglass.)
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Old 06-20-2009, 08:47 AM   #30
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Y

Essentially, what I was going for with the oceans was an exaggeration of the reason for America's isolation from the rest of the world before it was 'discovered' by the European countries of the world at that time.
"Sea monsters" might not look as elegant as special celestial arrangements but in a fantasy world might be more practical. :)
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