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Thread: A tidally locked planet

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    Guild Artisan Gracious Donor Lingon's Avatar
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    Default A tidally locked planet

    I've been thinking about a world… I've actually had it in mind for a long time, and I made an early draft for a map about a year ago, but for no special reason I decided to pick it up again, scratch everything and only keep the most fundamental part of the setting. It's a world that suddenly, without any clear reason, became tidally locked to its star. I have more details, some characters and the start of some kind of plot, but they are very fuzzy… Anyway, I'd like to write a novel about it, but I'm in the middle of another writing project, so I think I'll make a map first

    Here's a rough draft. Black is coastlines, dark grey is mountain ridges, light grey is rivers, the orange blob is the dayside and the blue blob is the nightside. I sketched the shapes by hand and then traced them in illustrator, because G.Projector does weird things to pencil lines… The first one is in the Robinson projection to give an idea what the world looks like. The second is the one I think I'll continue working with, in Equirectangular Oblique to make the inhabitable twilight band around the planet a straight line. I cropped away the extremely distorted (and rather irrelevant, frozen and burnt) top and bottom.
    Click image for larger version. 

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    What do you think about the Oblique one, is it a good choice? It gives a very unusual height:width ratio, which I rather like… but it makes it very hard to grasp the shape of the continent. As always I'll paint the map by hand, so when I've decided on a projection I'll print it and trace it on watercolor paper. That's why the mountains and rivers are so simple, and why I'll remove the color blobs first – must save printer ink…

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    I think the right map makes more sense. On the left, there is this outer rim of the map, that is actually very interesting to the setting and should not be split up, while on the right: Who needs details about the wastelands in the dayzone or the endless ice in the nightzone?

    Two things that come into my mind: The day zone will be frozen over, if this has the base parameters like earth. Look at the north pole, it does no melt even in it's summer days, when the sun constantly shines on it at a low angle. When the sun does not shine at all, there should definitly be ice.

    The dominant wind direction is easy: Air rises on the hot side and thus falls on the cold side and near the surface there will be a constant flow of air from night to day, thus the habitable rind will probably be a good bit towards the sunny side. Less "Land of the perpetual dawn" and more "Land of the perpetual 9 o'clock forenoon"

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    Guild Expert Jalyha's Avatar
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    I like the choice of projection.

    It IS unusual, but not unheard of, (I referenced a Japanese map, somewhere around here, that has near the same dimensions) If this is the only inhabitable part of your planet, I doubt they would map the rest?

    And although you're right, you can't make out the shape of the continent this way (it does take away some of the subtle intricacy) you have to start *somewhere* and not all maps are going to show the whole continent anyway... especially if it is different countries..

    I would turn it more north-south, though, just for clarity (unless I have mistaken? you turned it?)

    I'd say do this map, and when you've got it finished, and got the time, lol, you can map out the rest of the world.


    And I'm eager to see where you pull in those influences
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    This will generate a nice map/world. And I can imagine it will only allow for life on a marginal N-S strip, so the map projection fits perfectly. I'm curious to see where this goes.

    Quote Originally Posted by Maraxus_ View Post
    The dominant wind direction is easy: Air rises on the hot side and thus falls on the cold side and near the surface there will be a constant flow of air from night to day, thus the habitable rind will probably be a good bit towards the sunny side. Less "Land of the perpetual dawn" and more "Land of the perpetual 9 o'clock forenoon"
    ... adding to Maraxus ...
    Sea currents will be mighty and sea level will vary a lot in the planet, I reckon. Continuous heat on the lit side will not only evaporate a lot of water but also generate water bodies expansion on the sunny side. This, in turn, will generate a gravity fueled movement towards the dark side on all the oceans. On the other hand, very strong Dark->Lit winds will push the water surface forcing currents that bring icebergs into the light. Where these two contrary movements meet you will get perpetual rain and ubiquitous thunderstorms.
    Furthermore, since the planet is rotating very slowly, Coriolis force might be negligible so the currents (sea or air) will be mainly dependent on the mountains/coast design and the region where they mix will tend to be the same and very well defined all the time.

    (This is all guesswork, grounded, but guesswork...)

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    Guild Artisan Gracious Donor Lingon's Avatar
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    Thanks for the comments!
    @Maraxus: I haven't done any calculations, but I don't think you're completely right on the science bits. The dayside won't be frozen, it gets constant direct sunlight. The poles on Earth are not comparable because of the angle they receive light from. With the winds, the atmosphere on the night side would thicken because of the cold, pulling air from the day side, I think… I agree with you about the projection though

    @Jalyha: Yup, good points Yeah, we'll see where I can get those influences in… (Jalyha and I were talking about African fantasy settings on PM)

    @Pixie: Thanks! The weather will probably be pretty crazy, yes! I will do some research on this.

    Here's a small update… I've just added some more detail, mostly. Considering the winds, does the rivers seem ok like this?
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    Guild Expert Jalyha's Avatar
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    Ummm... idk. I'm having a lot of trouble imagining the winds, so it makes it hard

    I think the rivers look good though ^.^
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    Lingon: I'm pretty sure, the light side very close to the rim would have ice. What I wanted to say with the north pole comparison is, that: The north pole in summer has very long days of very low standing sun. That the sun goes down for a few hours (except the very middle of summer some few days) evens out the fact, that at midday it's a good bit higher than the horizon (light hits at a blunter angle thus more heat per area). This way I think the situation exactly at the rim is comparable to earth's poles. (It would be better, if earth had no axial tilt but it has to do. I guess you can imagine, that the poles would not suddenly melt if the earth would turn a bit to rotate in the solar system disk.)

    Now, how far day-wards the frost-zone would go is a different question. And of course, if your planet has a warmer average temperature (more greenhouse effect, stronger sun, anything,...), than obviously the rim can be ice free or the border of ice.

    Yes, the dark side would be one huge high pressure zone and the light side one huge low pressure zone.


    Pixie: I think you are overestimating the gravity fueled movement. I think the water would flow towards the day-side, too, to replace the water that evaporates there.
    Let's see, the hottest places on earth get around 55°C warm on their all-time hottest days. Now, if the sun would not be directly overhead at the beginning and end of summer, this would probably be even warmer. Maybe 70°C? Much more important of course is, that the sun never goes down and never shines from a low angle, thus the light energy per area should be 4 times as high per time than on the hottest day. Since hot bodies give away energy faster than not so hot once, this does not fourfold the temperature but it should still be a huge impact easily increasing the temperature to 150 or 200°C. Water boils and evaporates (As you move away from the point where the sun is directly overhead, this should fall quite quickly, so the costal lines seam possible to me, though I'd expect the innermost shores in the day zone to be boiling hot.)

    And I definitly agree with the bad weather. The hot side evaporates a lot of water, hot air can carry more water than cold air, thus as the air cools down on its way towards the dark side, their will be lots of rain. And since this rain falls from a rather high altitute, some of that will probably turn to snow and hail

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    Guild Expert Jalyha's Avatar
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    The differences between the light side of a tidal locked planet and the earth's poles are many.

    First and foremost, the poles are getting light all day for 1/2 the year. The second half is dark, and cold. The light side of *this* planet is *always* light, and warmed by the sun.

    The poles receive (even in their summer) indirect sunlight, the light side of this planet receives direct sunlight.

    The ice already at the poles from prior circumstances (Ice Age, anyone?) has a cooling effect as well. That *keeps* the poles cool, even when they would normally warm under the light. This planet doesn't seem to have had the same unique series of events, and has, therefore, no huge ice masses in the area you're comparing to the polar caps.

    I'd imagine that there would be no ice at all, well into the "dark" side of the planet, especially with such a huge landmass, which also tends to warm things up.

    And there would be high and low pressure zones on EACH side, because sunlight/heat is not the only thing that affects pressure.

    @ Pixie: Sounded dead-on from what little I did bother to check.
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    Guild Artisan Gracious Donor Lingon's Avatar
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    Thanks for your thoughtful comments. I've read some discussions on the topic on science forums now, and though I still haven't made any calculations (and probably won't – I don't want to know exact values of things as they would likely limit more than help), I think it would roughly be like this:

    1. Winds start moving as if the dayside is the equator and the nightside is a pole, i.e. cold surface winds towards the dayside, hot winds higher up towards the nightside. Nightside gets an ice cap, dayside gets hot rising oceans and a heavy cloud cover. The habitable zone is quite large, stretching both into day and night, but probably more into day. The poles still have their ice caps.

    2. As the atmosphere condenses on the nightside, winds shift to moving only "nightwards". More and more water from the dayside gets frozen into the night cap, creating huge beaches of salt (should look cool on the map…!). Water beneath the night cap is pulled by the sun's gravity towards the dayside, creating some sort of interrupted cycle where all water slowly gets deposited on the ice. The habitable zone shrinks both ways, but because of the winds that carry lots of water, the land towards the dayside is kept from drying out, making the zone remain slightly more into the sun. The old polar caps probably melt towards the dayside, giving even more water flow into the habitable zone.

    3. The magnetic field gradually weakens because of the slowed rotation and the solar wind blows even more of the atmosphere towards the nightside, where it either leaves the planet or gets trapped in the indent made by the weight of the night cap pushing the bedrock downwards. The seas are completely vaporized on the dayside, the rivers from the melting edges of the nightcap boil away in the desert. The habitable zone is very very thin and the air is hardly breathable.

    Or something like that I have no idea how long each "step" would last, they would surely overlap a lot, and it's only based on forum discussions and Wikipedia articles compared with my intuition… But does it seem to make sense, sort of? And any idea how long it'd take to get to stage 2, where I think the map and eventually the story should be set?

  10. #10
    Guild Expert Jalyha's Avatar
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    A looooong time ?


    It does make sense, but most of this is all going to be based on no more than discussion, scientific theories, conjecture and gut instinct, since no one has ever actually watched it happen.

    but it makes sense.

    I've no idea how long thaat would take, though :/

    I'll read around and see if I can find something
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