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Old 04-17-2013, 03:30 PM   #21
Agemegos
 
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Admittedly that sounds insanely improbable, but I'm not sure where conservation of angular momentum come in.
Conservation of angular momentum requires a planet's north and south poles to point in constant directions. As its orbit carries a planet around a star, the direction to the star is constantly changing. Therefore the north pole of a planet cannot point constantly at the star (or any of the stars) in its system ó because of conservation of angular momentum.
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Old 04-17-2013, 03:31 PM   #22
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Angular momentum is a vector quantity. If Dinom's north pole points at its primary as it revolves, then the direction of that angular momentum changes by 180 degress in half its orbit, pointing in the opposite direction from which it started, and then changes again, reversing itself twice per orbit -- all without any external force acting on the planet.

Or think of the planet as a gyroscope. The pole isn't going to change where it's pointed relative to "the fixed stars" unless something pushes on it. If the north point points at the primary at 0 degrees in the orbit, then at 180, it should be pointing directly away from the primary.

Conservation of angular momentum means you can't just go from L to -L back to L. You're stuck at a constant L unless you have a way to dump 2L onto some other body, and get it back later.
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Old 04-17-2013, 03:32 PM   #23
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Conservation of angular momentum requires a planet's north and south poles to point in constant directions. As its orbit carries a planet around a star, the direction to the star is constantly changing. Therefore the north pole of a planet cannot point constantly at the star (or any of the stars) in its system ó because of conservation of angular momentum.
Isn't our direction to Polaris constantly changing?
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Old 04-17-2013, 03:40 PM   #24
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Isn't our direction to Polaris constantly changing?
Yes, by about 50 arc seconds per year, or 26000 years to return back to its starting position. This precession is due to the fact that the Earth's rotational axis is tilted WRT its orbit, and also that it's a bit oblate. Tidal forces from the Sun thus torque the planet a little bit.
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Old 04-17-2013, 04:03 PM   #25
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Isn't our direction to Polaris constantly changing?
Yes, owing to the effect of tidal torques on Earth's tidal and equatorial bulges, which transfer angular momentum between Earth's rotation, its orbit, and the orbit of the Moon; the orientation of Earth's axis precesses with a period of 25,700 years.

Yes, Dinom might well be precessing too. If it were rotating rapidly, if it were close to its star, and if it had a large moon in close it might precess several times faster than Earth.

No, there is no way it could precess around an entire great circle of the sky with a period equal to its year.
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Old 04-17-2013, 04:41 PM   #26
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Conservation of angular momentum requires a planet's north and south poles to point in constant directions. As its orbit carries a planet around a star, the direction to the star is constantly changing. Therefore the north pole of a planet cannot point constantly at the star (or any of the stars) in its system ó because of conservation of angular momentum.
Sure it can. Just run a giant pole through the middle of the planet and the middle of the star.

As far as Dinom goes, pretty sure that's atmosphere code E, ellipsoid, from CT.
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Old 04-17-2013, 04:48 PM   #27
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No, there is no way it could precess around an entire great circle of the sky with a period equal to its year.
Or if there was, it should certainly rate a mention in the planetary writeup, as that setup would be a lot more interesting than just the planet...

What's wrong with Youghal? Just the scale of the effect (from "too thin" to "comfortable")?
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Old 04-17-2013, 04:52 PM   #28
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What's wrong with Youghal? Just the scale of the effect (from "too thin" to "comfortable")?
Any effect that elongates the planet will similarly elongate the atmosphere.
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Old 04-17-2013, 05:31 PM   #29
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Dinom, in Behind the Claw.
Dinom is a rogue planet caught in the trailing trojan point of the systemís only native planet, a gas giant named Inina. Dinom orbits with its north pole constantly pointing at the systemís very bright giant star. The northern hemisphere is thus eternally baked, while the southern is in frozen night.
BtC got that wrong:
"The revolution of Dinom around its central star means that the polar axis will not always point towards the star. The 1600 year orbit of Dinom advances the pole about one degree in four years; in 200 years, it will advance 45 degrees.

Ultimately, the world will have ordinary days and nights and the frozen gas of the Cold Face will sublimate to give Dinom an atmosphere. In 800 years, the Cold Face will have become the new Bright Face.

Settlement of this world took place only 200 years ago, just as the world was settling into a Bright Face period. The cities were established in the predicted twilight zones to take advantage of the temperate climates."
Across the Bright Face, p. 18
I've no idea if that's any better. A 1600 year orbit sounds as if it's quite a distance from the star. Can it really burn that bright at that distance? But it is at least different.


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Old 04-17-2013, 05:58 PM   #30
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Any effect that elongates the planet will similarly elongate the atmosphere.
Yep. If the planet is notably oblate because e.g. of rapid rotation the atmosphere will be deeper at the equator than at the poles, but because the centrifugal effect reduces effective gravity it will lighten the air column to just the same effect as it deepens it, resulting in the air having the same pressure and therefore density all over the surface. It has to work out that way because the geoid is an equal-energy surface, which means that gravity-cum-rotational effects will not cause air to flow over the surface, which means no pressure differences from that consideration.

That leaves, I guess, the proposition that the land surface might not conform to the geoid (the water surface has to if it's connected), but that's inconsistent with hydrostatic equilibrium in the mantle, and it would correct itself in only thousands to about 20,000 years. I suppose that it's possible that there might have been truly enormous ice-caps that weighed down the polar regions and displaced mantle material to under the topics. If such caps melted very recently (in the last couple of thousand years) then I suppose that the planet might be in the midst of adjusting from a large-scale disequilibrium. But then I think you would expect to see large polar oceans and equatorial land-masses, with the oceans about as deep as the highlands are elevated. There would be progressive flooding of the equatorial land as it sank towards equilibrium, and vigorous seismicity. Even so, I feel intuitively that the amount of ice necessary to make this effect noticeable to the extent implied is implausible. I'll have to ask a geophysicist about that. In any case, this situation is hardly compatible with there being any land at the poles.
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