[Space] Climate & habitability of tide-locked planets
 

There has been a long debate in planetary science about whether planets that are in 1:1 spin:orbit resonances with their stars (colloquially, "tide-locked") might have conditions on their surfaces habitable to humans.

Consideration of the temperature of different parts of the surface as though they were roughly in radiative equilibrium gave us a model in which the sunny side would be unbearably hot (and "therefore" dry), the dark side as cold as a moon of Neptune, and the twilight zone swept by perpetual gales of ice-cold, bone-dry winds from the dark side to the lit. It seemed obvious that all the water on the lit side and in the twilight zones would evaporate, pass into darkness in high-level winds, freeze out, and form an ice cap on the dark side, where it would be so cold that the ice would be in effect a rock-forming mineral. A little later it became fashionable to realise that the cold of the dark side would be deep enough that carbon dioxide would sublimate to dry ice, that nitrogen and oxygen would liquefy and even freeze. This was called "atmospheric collapse", and we supposed that tide-locked planets would be effectively airless.

Then in 1997 Joshi, Haberle, and Reynolds¹ used a general circulation model (supercomputer model of the atmosphere) to simulate the atmospheres of synchronously rotating terrestrial planets orbiting M dwarfs. This model included the advection of sensible heat in the global circulation of the winds, and it delivered the bombshell conclusion that even with an atmosphere of only 200 millibars (⅕ of an atmosphere) of CO₂, and even with a dry surface, the transport of heat from the lit side to the dark side would be enough to raise the temperature of the dark side to above the temperature at which CO₂ sublimates to a solid. Since that temperature is well above the boiling point of nitrogen and oxygen it follow that atmospheric collapse is not to be expected in the Goldilocks zone² unless the atmosphere is far too tenuous³ to support respiration anyway.

Joshi et al used models that did not include water vapour, and gave us the idea of tide-locked planets in which warm air flowed from the lit side at altitude, cooled and sank on the night side, forming cold air that blew at ground level to the lit side. We, or at least I, did not consider that though a tide-locked planet does not rotate with respect to the direction of its star, it does actually⁴ rotate, and there are Coriolis effects on its atmospheric circulation. And when layfolk informally added water to this model our conclusions were dominated by an impression of warm air flowing across the terminator at altitude, cooling in the darkness with the effect that its humidity would rain or snow out, sinking, and then flowing back to the lit side as a cold, dry, gale. It seemed that the sunlit side of a tide-locked planet would become desiccated, most of the planet's water accumulating as a super-Antarctic ice-cap on the dark side. The availability of water on the lit side would depend on the return of water into the light by the flow of glaciers into the twilight, and would be best in the twilit band around the terminator. We (or at least I) did not think that the convergence zone at the subsolar point would be rainy for the same reason that the tropical convergence is, or thought that it would be too hot for human habitation⁵.

That was the approximate state of the art in 2006, when Zeigler & Cambias⁶ wrote Space for GURPS 4th edition. Tide-locked worlds in GURPS Space emerge from the determination of rotation rate in Step 30 of the advanced world generation sequence, specifically, on p.117. That is before the habitability and population of the world are determined in steps 32 (p. 121) and 35 (p. 122). But the effects of tidal locking on surface conditions are not calculated until the section on "Special Cases" on p. 125. That means that the habitability rating is calculated on the basis of the average surface temperature, i.e. the surface temperature in the twilight zone near the terminator. i.e in GURPS Space 4th edition the human population of a tide-locked planet is assumed to settle in the twilight zone. Following the state of the art about 2006, the rules for tide-locked worlds on p.125 apply adjustments to the day face and night face temperatures that reflect the transportation of sensible heat in the circulation of the atmosphere and therefore depend on the density of the atmosphere. Very dense atmospheres keep the night face very nearly as warm as the day face and effectually prevent any freezing out of either water or atmosphere; very thin atmospheres freeze out, cause the oceans to freeze out on the dark side, and allow the day face to heat up by 20% (that nearly doubles outgoing thermal radiation) while the night face is cooled to only 10% of equilibrium temperature. Thin to dense atmospheres produce various intermediate cases, with a degree of atmospheric transport of heat producing smaller contrasts of temperatures, reductions of hydrographic percentage by partial freezing-out of water, and less or no reduction of atmospheric pressure by freezing-out.

The result is that the star system and planet generation procedures in GURPS Space 4th edition put the human populations on those tide-locked planets where it is the twilight zone that has an equable temperature, and not on those where the dark face or the subsolar region has an equable temperature.

Since that time there has been further progress in modelling surface conditions on the synchronously-rotating planets of K and M stars. In 2010 Merlis & Schneider⁷ examined models in which the planet was acknowledged to be rotating and in which the surface was modelled as a "slab ocean" (which provides as much water as will evaporate, but that does not transport heat). They found dramatic differences in the patterns of winds and rainfall depending on whether the planet was orbiting and rotating quickly or slowly⁸, but that in either case that the temperature of the dark side was much more uniform and warmer that I had expected. Where Cambias & Zeigler [2006] suggested that the dark side of a tidally-locked Earth should be about 230 K, Merlis & Schneider's models showed the dark side of a tidally-locked Earth to be a huge patch of about 250 K (-23 C), which is not as cold as Antartica at night/winter⁹. That is warm enough that substantial glacial flow can be expected, especially with basal warming of the glaciers from geothermal flux. Where Cambias & Zeigler [2006] suggested that the sunlit side of a tidally-locked Earth should be about 325 K, Merlis & Schneider's models showed the subsolar region of a tidally-locked Earth to be little more than 300 K.

Merlis & Schneider's models also show little precipitation of rain or snow on the dark side. The details are substantially different in the fast-rotating and slow-rotating cases (see Merlis & Schneider [2010], figure 2), but generally the subsolar region is very rainy (kind of like the tropics, but perhaps more so) and is surrounded by a broad region in which potential evaporation exceeds precipitation (kind of like the Sahara, Arabian, Thar, Namib, Atacama, and Australian deserts in Earth's horse latitudes), surrounded by a huge area on the dark face and in the twilight zone where the net of precipitation minus potential evaporation is mostly positive but small (the significant exception is in the fast-rotating case, east of the subsolar point).

But wait! There's more!

In 2013 Hu and Yang¹⁰ published a paper reporting the results of modelling synchronously rotating planets using a model that included ocean currents and the transportation of heat in them. They assumed a uniform ocean as deep as the average of Earth's ocean, nowhere interrupted by continents nor oceanic shallows, and point out that this leads to stronger and more symmetrical effects than are to be expected on any habitability candidate. That being acknowledged, their models suggest that the transportation of heat from the sunlit side to the dark side of a synchronously rotating Earth would dramatically reduce the difference in temperatures between them — to less than 50 K. Moreover, the sea ice would be thin — 3–5 metres at most, allowing free currents below it and no ice-cap. A tidally locked planet with an equable temperature and plenty of water might even have ice-free oceans on its dark side.


So it all seems to me as though the adjustments to temperatures and hydrographic percentage for tidally-locked planets that are provided in GURPS Space 4th edition, at least inasfar as they are applied to planets with substantial atmospheres and water that are in the Goldilocks zone, are probably too large.

Since permanent human settlement requires (a) photosynthetically active illumination, (b) an excess of precipitation over potential evaporation, and (c) an average temperature between 273 K and 303 K it seems to me that the habitation candidates among synchronously rotating planets are not those that are about as warm as Earth (settled in the twilight zone), but those that are on average a little cooler than Earth, which will be settled in the subsolar region where it rains and where the light is brighter.

Re: [Space] Climate & habitability of tide-locked planets
 

¹ Joshi, M. M., Haberle, R. M., and Reynolds, R. T., 1997 Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability.

² When the entire planet is colder than what Joshi et al considered, naturally the dark side is colder than they concluded. CO₂ does freeze out on ice moons.

³ The modern consensus seems to be that 10 mbar of CO₂ will not freeze out with Earth-like solar heating, but that 3 mbar is unstable, and may.

⁴ In any inertial frame of reference.

⁵ Humans do not form permanent settlements where the average temperature is above 30 C or below 0 C. The reasons are probably agricultural, and would also rule out habitation where the photosynthetic photon flux density was too low to support the growth of crops.

⁶ Zeigler, J.F. and Cambias, J.L., 2006 Space. Steve Jackson Games, Austin TX.

⁷ Merlis, T.M. and Schneider, T., 2010 Atmospheric dynamics of Earth-like tidally locked aquaplanets.

⁸ I consider it unfortunate that the cases they published involved (a) one rotation per 24 hours, which is a lot faster than we expect for any habitability candidate, and (b) one rotation per 8 640 hours, which is a good deal slower than we expect for any habitability candidate.

⁹ The reason seems to be that Antarctica is isolated from tropical air by the Ferrel cell and polar easterlies, features produced by Coriolis effects. Since the anti-subsolar point in the tidally-locked Earth is not a rotational pole is it not similarly isolated from the global circulation, and there is no pool of cold air trapped on the dark face.

¹⁰ Hu Y and Yang J, 2013 Role of ocean heat transport in climates of tidally locked exoplanets around M dwarf stars.

Re: [Space] Climate & habitability of tide-locked planets
 

Tide locked worlds are probably unlikely to be habitable by humans outside of the thin twilight zone (this is why orbital resonant worlds are better). The near side will always be covered by clouds due to evaporation and the near side will always dark, so photosynthesis is impossible without technological assistance. The difference in dayside and nightside will also not help.

Now, cooler worlds with tidelocking could potentially allow for more habitable area, but the nightside would be absolutely frozen in the case. With an average atmosphere, the dayside is +12% while the nightside is -20%, meaning that a 240K world would be 271K on dayside and 192K nightside. One problem though is that carbon dioxide snows at the nigh side temperature, so the biosphere would collapse, as there would be no carbon dioxide in the atmosphere to allow for photosynthesis.

Re: [Space] Climate & habitability of tide-locked planets
 

Really? I just finished explaining why that belief is outdated.

Not according to the scientific papers I cited in the OP.

Earth's average surface temperature is about 288 K.

Merlis & Schneider (op. cit.) used a global circulation model and found that a synchronously rotating Earth would have a temperature a little over 300 K in the middle of the daylit side and about 250 K over a huge stretch of the dark side. And I told you about that in the message you are replying to.

Dude! Did you even read my post?

The belief that CO₂ will snow out on the dark side was debunked by Joshi et al in 1997. I explained that and both cited and linked my source in the very post that you are replying to.

Re: [Space] Climate & habitability of tide-locked planets
 

I have to wonder how many people reply to a thread after only reading the title, and at most skimming the opening post. This is far from the first time I've seen someone appear to have done that.

Re: [Space] Climate & habitability of tide-locked planets
 

Does that mean that there'd be a gradient of habitable worlds, with a habitable band located anywhere from the subsolar point to the twilight zone, depending on their average temperature (and hydro load, etc.)? Or is there absolutely going to be an arid band around the subsolar pole?

Re: [Space] Climate & habitability of tide-locked planets
 

The people of the British Isles will be surprised to hear that cloud cover prevents habitation.

Re: [Space] Climate & habitability of tide-locked planets
 

Indeed, it's a little cloudy here today, and I'm still alive :)

I think part of the issue with things like GURPS: Space is that our understanding of the science behind the way planets form and interact with their environment is increasing rapidly as the initial point was so low. Back when I got my first sci-fi RPG in the 1990s no extrasolar planets had been discovered, so the underlying assumption was that every planetary system was like ours, not allowing for things like hot Jupiters. I'm sure I remember reading last year that observations suggest there could be Neptune-size exomoon, which is a common trope in sci-fi but there was no evidence for in reality until now.

I've not had time to read the linked journal articles in the OP, but it's a very interesting topic to me - it'd be a very different alien feel than your typical Earth-like world, and thus an interesting planet type for worldbuilders to consider or characters to visit in an RPG.

Re: [Space] Climate & habitability of tide-locked planets
 

I haven't seen or done studies, so I don't actually know.

In the slowly-rotating limit (as illustrated in Merlis & Schneider (op. cit.)) there is a fairly simple pattern of winds and temperatures which seems to be readily understood in terms of a thermally-direct toroidal cell of circulation that is absolutely dictated by the fact that the subsolar region is hotter than what surrounds it. So you're definitely going to get a convergence and humid upwelling at the subsolar point, with a ring of downdrafts some way terminator-wards of it, and a pattern of brisk, relatively dry winds towards the subsolar region. In the case where the torrid zone is too hot for agriculture and permanent human habitation then the band of equable temperatures will be comparatively dry. But that doesn't necessarily mean that all the land will be non-arable. I live in the horse latitudes at 31°5' south, and the area around here is usually pretty green and well-vegetated. The is a bit of semi-arid grassland to the west, and an awful lot of desert west of that, but geographical features interrupt the straightforward patterns of winds and ocean currents — the South Equatorial Current that ought to flow to the west is diverted to the south by a continent in its way, the resulting East Australian current makes the seas here warmer and the air more humid than would be if the entire world were covered by a uniform ocean 5m or 4 km deep.

So I suspect that in the slowly-rotating case you start on a planet 30 K cooler than Earth with an "eyeball" world: only the subsolar region is free from ice. Then as you consider gradually warmer worlds the ice around the subsolar optimum gives way to an expanding ring of arid to semi-arid that is streaked and speckled with fortunate lands where local and geographical effects produce adequate rainfall. Once you get to a point a little warmer than Earth the subsolar region starts to become uninhabitable owing to heat — but there remain some favoured areas in the torrid zone that are cooled by diverted ocean currents etc and remain habitable and wet. At that stage, the zone of habitation is like, say, Australia: largely arid to semi-arid but with some good bits. Eventually the zone of equable temperatures gets pushed towards the terminator. In the twilight zone it isn't as windy, but the low slanting sunlight gets too dim to support agriculture.

Worlds that rotate rapidly enough for the Coriolois effects to be significant (and I don't know how fast that has to be) are more fortunate. The pattern of the winds and rainfall is a lot more complicated, yielding a warm/wet patch that is shaped like a lobster with its tail stretching towards, perhaps even across, the terminator east of the subsolar point. Screw that pattern up with a few mountain ranges and you have an excellent chance of finding pretty much any climate you need, albeit probably in a small area. Take a look at the map of surface temperature on p.4 of the paper by Merlis & Schneider, and the map of potential evaporation minus precipitation on p.5. The fast-rotating cases (on the right of Fig. 1 and Fig. 2 respectively) are charmingly quirky.

Re: [Space] Climate & habitability of tide-locked planets
 

One rotation per 24 hours is fair enough- it allows them to easily compare to Earth, isolating from other factors. And it might be close enough to good candidates. Looking at TRAPPIST-1e, chosen because it's terrestrial size and in the habitable zone (albeit lacking water), it has a 6 day period, so it's Coriolis effect would be within an order of magnitude of Earth's.

Actually, 1d appears it might have watery oceans, despite being inside the habitable zone (however that happens), and it has a 4 day period.

ETA: Another planetary heating mechanism that wasn't mentioned is geothermal heating. This Wikipedia entry suggests that 1d might have geothermal activity to due tidal locking with the primary, so that could be another mechanism for warming the nightside.

ETA2: Agemegos, I refer you to Carone, et al's paper, Stratosphere circulation on tidally locked ExoEarths, for some in-depth discussion of different polar-equatorial transport regimes. It might give you some useful insights.

Re: [Space] Climate & habitability of tide-locked planets
 

I don't know if this is scientifically valid--I'm not enough of an expert--but it looks plausible--certainly sufficient for gaming. Here's a well thought out tide-locked world which keeps in mind the fact that orbits are usually not circular.
http://www.worlddreambank.org/L/LIB.HTM

http://www.worlddreambank.org/P/PLANETS.HTM has a lot of neat concepts :)

Re: [Space] Climate & habitability of tide-locked planets
 

Figure 2 in Merlis & Schneider (op. cit.) shows a small area (at the middle of the day side of the slow-rotating planet, made up of two narrow arcs north-east and south-east of the subsolar point on the fast-rotating one) with 62.5 mm of precipitation per day. That is very rainy. That is twice as much rain as the rainiest place on Earth. But it falls over only a very small area. Most of the zone indicated as getting an excess of rainfall over potential evaporation gets between 12.5 mm per day and 62.5 mm per day. That seems like a lot of rain to me — the town where I live gets an average of 1128 mm per year or 3 mm per day. Seathwaite Farm in Cumbria gets 3350 mm per year (9.2 mm per day). No doubt I would find it dismal. But even Megahalaya in India is far from being incapable of supporting plant life.

Re: [Space] Climate & habitability of tide-locked planets
 

Well, there are a lot of things about it that I would like to ask the author about. I would have thought that the tides that braked that world's rotation would also have eliminated its obliquity. The eccentricity (0.2) seems rather large (it's equal to Mercury's, with stronger tides), and large enough that I would have expected a 2:1 spin:orbit resonance rather than synchronous rotation. The pattern of winds on Librata has equatorial easterlies, whereas the modelling I have seen shows superrotation (i.e. equatorial westerlies).

It has.

Re: [Space] Climate & habitability of tide-locked planets
 

I've never had the impression in 4th edition that people who live on tide locked planets are living in the twilight zone anyway. More like in the northern and southern latitudes where things are cooler than they are at the east pole anyway. The only effect on habitability is the -25% to hydrosphere.

Which overlooks the biggest issue impacting habitability there of course, the periodic blasts of radiation from the sun.

Re: [Space] Climate & habitability of tide-locked planets
 

Have you calculated the Habitability scores and Dayside temperatures of a lot of randomly-generated tide-locked worlds? I've had quite a lot of them drop out of my generator, and I don't like the look of them. The ones with high Habitability scores have uninhabitably hot day sides.

The thing that brought this issue to the forefront of my mind just now is trying to generate a society (and adventure) for Gliese 370 II "Persatuan", the highest-Habitability tide-locked world in Central Sector of my randomly-generated universe. It has an average temperature of 27 C, and a calculated dayside temperature of 67 C.

I don't understand this. What do you mean by "East Pole"? The subsolar point? The intersection of equator and terminator east of the subsolar point?

What is it in Space that makes you think that latitude makes a significant difference? Did I miss something big, or are you bringing in more recent discoveries from other sources?

Yes, those x-ray flares and solar ion storms are nasty. The stripping of atmosphere by the intense solar wind is bad, too. Then there is the prolonged intense heating of the inner system as the star develops along the Hayashi track. Given those three challenges, the fact that the sunlight is rich in IR and poor in photosynthetically active radiation (meaning slow photosynthesis and delayed development from "Ocean" type to "Garden" type) is almost trivial by comparison.

There is still a lot to find out about the planets of M and late K -type dwarf stars — twenty years ago it was still not clear that they were common at all! — but I'm not really holding my breath on the discovery of planets habitable by humans in the system of anything cooler than about K5.

Re: [Space] Climate & habitability of tide-locked planets
 

Well of course your random generator produces a huge number of tidelocked worlds with much higher average temperatures than the by the book world generation technique. When I randomly generate systems for smaller M-Class dwarfs, on the rare occasion when I actually get a stanard-sized world in the habitable zone they are always on the cold side of things. I've never gotten a "normal", much less a "warm"

That's a bit on the warm side. I'm guessing that you rolled really high on hydrosphere.

Why would the intersection of the equator and the terminator be east of the subsolar point?

The average temperature is roughly the seasonal average at a latitude of 45 degrees. Obviously it will be colder on average as you approach the poles and warmer as you get closer to the equator. But in the case of a tidelocked worlc that will actually be a roughly circular band of tolerable temperature all around the sun-side. Frankly I didn't didn't consider the possibility of a warm or even tropical average temperature sun locked world because it's apparently impossible to randomly generate them using the unmodified gurps method.

Oh, honestly M0s aren't that bad.

Re: [Space] Climate & habitability of tide-locked planets
 

One possibility for a tide-locked planet that is not often talked about is a planet with a 90-degree axis that 'rolls' as it revolves around its star (the north pole is always pointed at the star, the south pole is always pointed away, and the equator is the terminus). In recent reports though, there are some rather massive differences in temperature between the night side and dark side of directly observed tide-locked planets with very dense atmospheres. I am not sure if the mathematical models of previous research actually reflect reality.

For example, there is a planet where molten iron replaces water, and it possesses a day side with temperatures 150% higher than the night side. The molten iron vaporizes in the dayside, rains down at the terminus, and freezes on the night side. Of course, iron gas is much less reflective than water vapor, so that would drive up the day side temperatures, but it is an amazing planet.

Another thing to consider is the placement of continents. A 'polar' ocean would moderate day side temperatures through evaporation and reflective cloud cover, as well as being an effective method of transferring heat to the night side. Conversely, a 'polar' continent would be an inferno, and the rest of the planet would likely be uninhabitable outside of the terminus because there would be insufficient heat transfer. Of course, the tidal bulge caused by the tide locking would likely result in a polar continent, meaning that most tide-locked planets are likely uninhabitable outside of the terminus.

Re: [Space] Climate & habitability of tide-locked planets
 

A tide-locked world would have 6 identifiable poles:

- The subsolar and anti-subsolar points (which I like to call the day-pole and night-pole). The equator passes through both these points.
- The north and south poles. These point up and down from the planet's orbital plane, and are located on the terminator. The planet's rotational axis, due to its orbiting its star, passes through these poles.
- The east and west poles (for lack of clearer names). These would be the leading and trailing points in the planet's orbit. They'd be on the intersections of the equator and terminus. They might be significant for climatic patterns to gather at due to Coriolis forces.

These points might vary due to orbital eccentricity, libration, etc.

I'm not sure if the academic literature uses these terms exactly though.

Re: [Space] Climate & habitability of tide-locked planets
 

Why? It's a pretty damn faithful implementation.
Ah. I thought you were talking about people living on tide-locked worlds, because that was the topic. Referring to the geography of a normally-rotating world was rather confusing in context. So you were saying that people live in the mid-latitudes rather than at the east pole. What's the east pole on a normally-rotating planet?
There's got to be one such intersection to the east and the other to the west.

Re: [Space] Climate & habitability of tide-locked planets
 

No, that is not a possibility, because it violates conservation of angular momentum.

Yes. Hu & Yang acknowledged that in their paper, and I mentioned it in my original post. You will have continents and oceanic ridges and shallows deflecting currents, and mountain ranges deflecting winds, besides a patchwork of land where the models assume water. It's hard, perhaps impossible, to estimate the effects at the level at which the Space planet generator works, but I suppose that one near-universal is that geographical features will result in a greater variety of surface conditions than the models indicate.
According to the modelling by Hu & Yang that I linked in the OP the westerly current at the equator is most important for advecting heat from the light side to the dark side. Flows in the polar regions are much less significant.
The mantle material conforms to the same geoid as the oceans do. Water conforms more quickly than mantle material does, producing the tides where applicable, but the mantle goes to the long-term average equipotential surface. You don't get a band of continents around the equatorial bulge of a planet like Earth, and you won't get continental bulges at the subsolar and anti-solar points on a tide-locked planet. Remember that tides affect the oceans.
What do you mean by "outside the terminus"?

I really don't believe that the twilit band near the terminator is likely to be highly habitable, for reasons that I have explained at length. That area is going to be poor in light to drive photosynthesis (a problem exacerbated by the low photosynthetic efficiency of the light from cool stars). And the modelling all shows that it is likely to be arid, too.

The results of global circulation modelling suggest to me that the best prospect for settling on tide-locked worlds is on the sunlit faces of comparatively cool ones.

Re: [Space] Climate & habitability of tide-locked planets
 

It generates worlds closer to the sun than is possible under the dice generation rules.

Normally rotating planets don't have an east pole or a west pole at the far side


I suppose if you want to draw your map that way.

Re: [Space] Climate & habitability of tide-locked planets
 

I wasn’t aware of that. I shall have to work out why, because it oughtn’t to.

You still haven’t said what you mean by “east pole”, and it’s not a standard term.

It has nothing to do with maps. On the actual planet there is a north pole and a south pole defined with respect to the rotation in the usual way, with an equator halfway between them in the usual place. The is no difficulty in defining latitude in the usual way, and that is what the planetary scientists who study and simulate these things do. There is no difficulty in defining east and west in the usual way, and that is what the planetary scientists do. What they discuss easterly and westerly winds and currents it is zonal ones, following parallels of latitude, that they refer to.

The equator and terminator are great circles, so they intersect at two points. The subsolar point stays very close to the equator because the tidal effects that reduce rotation rate also reduce obliquity. So however I draw a map, indeed whether I draw one or not, one of the intersections of the terminator and the equator is to the east of the subsolar point and the other is to the west of it. That is, using “east” and “west” in their usual senses, as the planetary scientists who study and model these planets and hypothetical planets do.

If you are using “east” and “west” in some unconventional way that you refuse to explain even when I ask, I have to conclude that you are not trying to communicate.

Re: [Space] Climate & habitability of tide-locked planets
 

I did in fact mean the subsolar point when I said "east pole" and it seems to me the that the place to put the terminator on a tide locked planet would be running through the subsolar point and of course it's opposite.

Re: [Space] Climate & habitability of tide-locked planets
 

Huh? The subsolar point experiences permanent noon. The terminator permanent twilight. They are as far away as you can possibly put them on a sphere.

Re: [Space] Climate & habitability of tide-locked planets
 

That’s just bizarre. The subsolar point is the middle of the sunlit hemisphere, and the terminator is the edge of the sunlit hemisphere. The middle of a hemisphere can’t be at its edge.

Re: [Space] Climate & habitability of tide-locked planets
 

Does "terminator" perhaps have another meaning of being the edge of a map? If so, I couldn't find any support for it though.

Re: [Space] Climate & habitability of tide-locked planets
 

No, I was just blanking on the concept.

Re: [Space] Climate & habitability of tide-locked planets
 

What it looks like to me is that there is enough uncertainty to make any of several different choices depending on what you want for gaming, and be able to justify them. It's even possible to use several depending on the sun, the eccentricity, and more.