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Old April 16th, 2009, 10:38 PM
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Default Season on tidally lock planets

It occurs to me that if a tidaly locked planet is axially inclined it would experince seasons. At least it seems like they would. Am I incorrect in thinking this?
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Old April 16th, 2009, 11:20 PM
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I think you might. Pretty sure actually but I wonder if the seasonal variation would be at all significant compared to the tidally locked effects for the sun side and the dark side. Maybe along the terminator. Giving a narrower region of temperate conditions at the equatorial terminator with seasonal migration of the temperate conditions north or south of that. Would make an interesting world.
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Old April 17th, 2009, 12:50 AM
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There are no seasons on a tidally locked planet, there is no rotational axis to be inclined.

A tidally locked planet is defined by the condition that one point on the surface perpetually faces the star (it doesn't face a fixed direction in space). There may be minor peturbations as experienced by Earth's Moon, so the terminator may shift slightly, but no day/night and no seasons.

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Old April 17th, 2009, 01:16 AM
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Right. Tidally locked by definition means no or very little axial tilt. I wonder though, nothing in the dance of the planets is fixed, it all varies over time through interaction. Could a world not be in a very long, practically tidally locked orbit, and still retain a significant axial tilt? A point sometime before becoming entirely tidally locked, still having some small relative rotation and not having spun it's axial tilt up to near zero.
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Old April 17th, 2009, 01:29 AM
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Quote:
Originally Posted by far-trader View Post
Could a world not be in a very long, practically tidally locked orbit, and still retain a significant axial tilt? A point sometime before becoming entirely tidally locked, still having some small relative rotation and not having spun it's axial tilt up to near zero.
Not for long.

First of all, tidal lock implies a close orbit, not a far one; the strong gravity across a relatively small distance (as compared to the satellite's own diameter) is what kills the satellite's own rotation (unless it is a perfectly radially-homogenous sphere -- good luck with that).

Secondly, any "nutation" in the satellite will dampen out quickly unless it is in the same plane as the orbit; the satellite will tumble as necessary to settle into such a configuration. Luna, for example, has a slight nutation even to this day, mostly because its primary, Terra, is an irregular, lumpy mudball with an uneven gravitational field that is rotating on its own axis, tugging Luna back and forth slightly every day when the fattest part of Terra turns by.
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Old April 17th, 2009, 01:33 AM
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Agreed, probably not for long. So most likely a captured world. And by most likely of course I mean a very rare event to find. Not that it wouldn't happen often enough to be unique, but that Traveller's finding it at that stage would make it unique.
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Old April 17th, 2009, 01:50 AM
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Agreed, probably not for long. So most likely a captured world. And by most likely of course I mean a very rare event to find. Not that it wouldn't happen often enough to be unique, but that Traveller's finding it at that stage would make it unique.
Well, now, a captured world raises some possibilities.

Especially if it is an elongated body, such as the TNO Haumea in the Sol system. It would have to be Plutoid-sized or smaller (implying no or little usable atmo), since larger objects tend to be spheres by virtue of their own gravity.

On the other hand, Mars is notably "out of round" due to a primordial impact basin, and might be a larger subject to model.

Either way, upon capture (say, by being perturbed from its outer system orbit by a passing star or whatever) such a body could enter, for a few million years at least, an orbit with a notable eccentricity. This eccentricity would cause a variation in its orbital velocity as it passes closer and farther from its primary, and could introduce a noticeable nutation (or "wobble") for several million years until tidal forces finally damp it out and the satellite settles into a circular orbit.

Still probably more of a navigational curiosity than a usable plot point, at the end of the day... but hey, tidally-locked T-prime worlds in close orbit around M-dwarfs are currently predicted to be fairly commonplace in the universe, and -- if the computer models are to be believed -- their weather and climate are, pardon the expression, "out of this world" alien and rife with perilous adventure possibilities...

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Old April 17th, 2009, 02:52 AM
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Axial inclination would probably not last long enough to matter, but...

Orbital eccentricity would. It will produce mild climactic change (significant only if the eccentricity is significant) and drastic vulcanism (as Io experiences; the stretch of tidal change throug tiny orbital eccentricity is blamed for Io's internal heating).
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Old April 17th, 2009, 08:29 AM
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The thing that started me wondering was seeing an animation of the moon undergoing libration. Further reading revealed that there where 3 conditions that contributed to this, at least 2 of which I think would contribute to a hypothetical tidally locked planet shifting its facing during a year.

here is the link if you care to have a look:

http://upload.wikimedia.org/wikipedi...e_Oct_2007.gif
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Old April 17th, 2009, 11:32 AM
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Quote:
Originally Posted by spank View Post
The thing that started me wondering was seeing an animation of the moon undergoing libration. Further reading revealed that there where 3 conditions that contributed to this, at least 2 of which I think would contribute to a hypothetical tidally locked planet shifting its facing during a year.
Indeed, but not very much.

Bear in mind that a planet orbiting a sun will have a bigger difference in mass, and this will dampen the nutation effects a lot more, so there will only be a very small amount of observable libation.

Also note that Luna's libation is not strictly East-West because her orbit around Terra is inclined slightly with respect to their orbit around Sol. That is what causes nutation along two axes, and therefore the somewhat circular observed libation. A face-locked planet around a sun would only have one axis of motion, and it would be in the orbital plane.
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