Season on tidally lock planets
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far-trader
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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.
Icosahedron
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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.
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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far-trader
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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.
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.
far-trader
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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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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).
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).
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/wikipedia/commons/8/86/Lunar_libration_with_phase_Oct_2007.gif
here is the link if you care to have a look:
http://upload.wikimedia.org/wikipedia/commons/8/86/Lunar_libration_with_phase_Oct_2007.gif
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.
Another interesting possibility would be to consider orbital resonances.
Mercury is close enough to Sol to be face-locked, but instead has a 3:2 resonance -- its day is 2/3 of a year long, and it presents alternating hemispheres to Sol each perihelion.
Mercury appears to be in a stable resonance (General Relativity may be helping, since it causes Mercury's perihelion to slowly precess around Sol), but eventually, as Sol grows larger in old age, Mercury may fall out of this resonance and become face-locked (just in time to get swallowed up)...
I am unaware of any reason why a planet could not fall into such a resonance close enough to a small-enough sun and with a large-enough eccentricity; there might be some damping effects from the tidal slosh of any oceans (water or otherwise) present, but I suppose that theoretically, General Relativity effects could compensate for this and keep the resonance from being damped out... I do not know if the M-dwarf computer simulations have played with such a set-up...
Mercury is close enough to Sol to be face-locked, but instead has a 3:2 resonance -- its day is 2/3 of a year long, and it presents alternating hemispheres to Sol each perihelion.
Mercury appears to be in a stable resonance (General Relativity may be helping, since it causes Mercury's perihelion to slowly precess around Sol), but eventually, as Sol grows larger in old age, Mercury may fall out of this resonance and become face-locked (just in time to get swallowed up)...
I am unaware of any reason why a planet could not fall into such a resonance close enough to a small-enough sun and with a large-enough eccentricity; there might be some damping effects from the tidal slosh of any oceans (water or otherwise) present, but I suppose that theoretically, General Relativity effects could compensate for this and keep the resonance from being damped out... I do not know if the M-dwarf computer simulations have played with such a set-up...
Isn't that how it is on Regina/Regina/Spinward Marches? Or does Regina rotate on its own axis?
Removing axial tilt only removes one of the factor, latitudinal libration, but there still remains longitudinal libration cause by orbital eccentricity.
The Moon experiences this to at about +/- 7 degrees, Titan +3/-3, Io about 4 degrees, I don't know if this is total or +/-4. But I think that unless the orbit were exactly circular that the case could be made for oscilaction of the palnet in an at least an east-west direction. While the shift might be small the length of time that it takes place over would lead to a more extreme temperature change.
The Moon experiences this to at about +/- 7 degrees, Titan +3/-3, Io about 4 degrees, I don't know if this is total or +/-4. But I think that unless the orbit were exactly circular that the case could be made for oscilaction of the palnet in an at least an east-west direction. While the shift might be small the length of time that it takes place over would lead to a more extreme temperature change.
The length of time is not going to be that great; if you are looking at something at least marginally habitable, then you are talking about nothing brighter than an M-class dwarf star, and a planetary year in the habitable zone that is only a few standard days long, a few weeks at the most.
What primarily drives the climate on such a planet (assuming it can somehow retain an atmosphere) is the huge, stationary, permanent hurricane directly underneath the primary, and the heat flow to the sunless-but-still-warmed-by-the-atmo dark side.
And don't forget the frequent high-intensity solar flares bathing the dayside in lethal radiation. Probably the only safe place to live is on the dark side, drawing power from geothermic taps and/or unmanned dayside solar collectors.
What might be interesting is to make such a system part of a binary solar system, with the companion star the DM orbits large/close enough to drive seasons on the face-locked mainworld as it and the DM orbit their big sister with high eccentricity...
I haven't lost interest in this, but I had to migrate computers this week. I wanted to do a couple of word builds, but it will probably be a few week before I get around to moving all my stuff.
One thing, In order to have any chance of a regular day/night cycle in a two star system you would probably have to employ some form of orbital resonance.
One thing, In order to have any chance of a regular day/night cycle in a two star system you would probably have to employ some form of orbital resonance.
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