This is specifically about the climate, and where on the planet it would be mildest.

Imagine Earth, rotating at the usual distance from the sun. Except, it is tidally locked to the sun; a so-called "Eyeball Planet".

One spot on the earth is always directly facing the sun; lets call this the "Day Pole".

Opposite that spot would be the "Night Pole", eternally dark and impossibly cold.

To my understanding the Day- and Night-Poles' extreme climate would make them entirely uninhabitable, unless I'm mistaken.

What we will call the "Equator" on this earth is the band exactly between these two poles. At any point somewhere near the "Equator" the light would be like different levels of permanent twilight/dusk.

My question is, Where on the planet would the climate be most hospitable for earth-life, in relation to the Poles and "Equator", and where in the sky would the sun be if you were there?

  • $\begingroup$ @L.Dutch It seems very obvious to me how those two questions are different? This is very specifically about earth, and where the climate would be the mildest if it was tidally locked. NOT what organisms would be like there. $\endgroup$ – Fred the John Jun 29 '17 at 14:47
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    $\begingroup$ Earth's crust, deep sea. This is where the tidally locking will hardly be noticed at all. Given your recent questions, it seems like you are working on quite a project. I like weird stuff, may I ask what it is about? $\endgroup$ – Raditz_35 Jun 29 '17 at 14:54
  • $\begingroup$ @Raditz_35 There's two things I'm researching for; one with explorers on alien worlds, the other about parallel universe earths. Hence the weird questions, haha! Will hopefully end up as some web-based multimedia things. :) $\endgroup$ – Fred the John Jun 29 '17 at 15:01
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    $\begingroup$ The "sunlight equator" (also @kingledion) is called the "terminator"). $\endgroup$ – Spencer Jun 29 '17 at 17:30
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    $\begingroup$ Welcome to Worldbuilding, if nobody did that already! While every Stack Exchange site has its own distinct differences, Worldbuilding is “more different” in some ways. In particular, you ought not Accept an answer before waiting at least 24 hours. A full explaination can be found on this meta post. $\endgroup$ – JDługosz Jun 30 '17 at 8:13

The sunlight equator, sort of

First off, let be sure to distinguish between the sunlight equator that you are talking about, and the planetary equator that we have on our planet.

The Earth is already at a distance that supports an optimum average temperature for life (~14 C). Therefore, if you keep the Earth the same distance from the sun, but make it tidally locked, you will end up with a hotter day pole, and a cooler night pole.

However, another thing to consider is that the angle of the sun's rays is still pretty large at the northern and southern latitudes. Therefore, those regions will not receive as much solar radiation per surface area as the planetary equator.

The last thing to consider is that there is still a Coriolis effect of sorts. The rotation of the Earth matches its rotation around the sun. This will cause warm air rising from the day pole to move from west to east, thereby making the areas to the west of the day pole warmer than the east.

Put this all together, and your optimal temperature regions will be roughly in a gradient from the day pole to the sunlight equator, with the gradient shallower along the planetary equator and the in the westward direction.

See this answer here regarding winds on a tidally locked world for more information.

Regarding the sun's location in the sky, it would be up to 60 degrees off the horizon to the north and south of the day pole, and as low as on the horizon, along the planetary equator to the west of the day pole. In all cases, the sun would be in the direction of the day pole.

Side note, at ground level you would expect a cold breeze from the night side to be blowing towards the day pole, since the warm air escapes the day pole at high altitude. So everywhere is going to have a chilly breeze.

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    $\begingroup$ The planet is still rotating around its axis, and its axis goes from the north pole to the south pole, as we currently conceive of the poles. The planet must do a full rotation once every year to match the Earth's movement around the sun to keep the same part of the planet facing the sun at all times. Therefore, the cardinal directions, north and south poles, and planetary equator all have the same meaning on a tidally locked world as they do on ours. $\endgroup$ – kingledion Jun 29 '17 at 16:27
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    $\begingroup$ As for the Coriolis effect, since the planet is still rotating in the same direction, it still exists, just much weaker since the rotation rate is lower. Warm winds flowing north from the day pole will be deflected clockwise or East, and warm winds flowing south from the day pole will be deflect anti-clockwise or East. $\endgroup$ – kingledion Jun 29 '17 at 16:30
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    $\begingroup$ @FredtheJohn Lastly, I avoided speculation about temperatures because a lot of things like greenhouse effect, rate of wind movement, reflection of sunlight by the atmosphere, and ocean circulation will distribute heat around the planet. I would honestly not expect the day pole to me much hotter than the Amazon jungle, nor the night pole to be any colder than Antarctica. But given the numerous factors involved, I'm not in a position to provide a solid answer. $\endgroup$ – kingledion Jun 29 '17 at 16:32
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    $\begingroup$ Tidally locked bodies don't generally rotate about the axis that points towards their parent body. If a tidally locked body was rotating about this axis (or near to it), I suspect that the axis of rotation would precess like a gyroscope due to the torque on the tidal bulge. It might not even be possible for such a body to stay tidally locked, though I'd have to work through the equations to be sure. $\endgroup$ – Michael Seifert Jun 29 '17 at 16:51
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    $\begingroup$ I think the Coriolis effect on this planet will be pretty negligible. The angular velocity of this tidally locked Earth is 1/365th of real Earth. Since the magnitude of the apparent Coriolis force is linearly dependent on angular velocity, the force will be 1/365th in magnitude. The maximum force felt at the North pole of this planet will be about equal to the force felt within 0.25 degree of our equator! Hurricanes and other Coriolis-dependent phenomena just aren't observed so close to our equator, so you'd have virtually no Coriolis phenomena anywhere on the tidally locked planet. $\endgroup$ – Nuclear Wang Jun 29 '17 at 20:31

The name of what you and the first answer call "the (sunlight) Equator" is actually the terminator. The terminator is the line that separates the day and night sides of any astronomical body. On an airless place like the Moon the terminator is clear-cut, while on a body with an atmosphere, like Earth, it's fuzzy.

Even on a tidally-locked planet the terminator would not be an unmoving line, since unless the orbit is almost exactly circular the terminator will oscillate as the planet moves. This is called libration. Earth's orbit is not very eccentric but it is not circular, so as the planet moves in its orbit it sometimes goes faster (near perihelion) and sometimes slower (near aphelion). The speed of Earth's rotation around its axis, however, does not change, so the terminator will sometimes get ahead and sometimes lag behind its supposed, average "fixed" position.

The libration zone will be a thin ring around the great circle that separates night and day; inside that zone the sun will appear, go up a bit, then stop and then go back down. This area, I think, would be a very interesting place to inhabit. It will never be too hot or too cold and it will experience some of that alternation between light and darkness that is essential to many biological creatures. It will also have an interesting weather. It should in fact have something like (very mild) seasons.

Since this is an alternate Earth, you could maybe choose to make its orbit more eccentric and thus make the libration zone broader.

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    $\begingroup$ The part near the equator would not necessarily be hotter than the part near the poles. Earth's equatorial regions are hotter than the poles precisely because the Earth is not tidally locked. Thus, a point on the poles is sometimes at a high angle to the sun (noon) and sometimes at a low angle (dawn and dusk), while a point near the poles is always at a low angle. For a synchronously rotating Earth, every point along the terminator is always at a low angle to the sun, just like a point near the poles. $\endgroup$ – Logan R. Kearsley Jun 29 '17 at 17:31
  • $\begingroup$ You're absolutely right about that. I've removed the paragraph from the answer. I'm thinking however that since Earth's axis is tilted, the terminator will not go through the poles. I'm having trouble visualizing what things would look like inside the polar circles. $\endgroup$ – pablodf76 Jun 29 '17 at 22:23
  • $\begingroup$ It would pass through the poles sometimes- at the equinoxes. Things would look pretty much the same as they do anywhere else in the terminator zone, with the sun bobbing up and down over the course of the year, but due to inclination rather than eccentricity. Between the poles and equator, the movement of the sun will be a combination of eccentricity and inclination effects. Since the Moon's rotation is inclined relative to its orbit, you can see what this looks like from space just by looking up time-lapse videos of the libration of the Moon. $\endgroup$ – Logan R. Kearsley Jun 29 '17 at 22:58

Life would exist everywhere on the planet. We currently find life in boiling geysers, in volcanic vents and at the South Pole. High-flying test aircraft have captured mold spores in the stratosphere. Everywhere we look on Earth there is life.
If Earth was tidally locked to the sun, the side facing the sun would have a constant updraft. The pole facing away would have a constant downdraft. In between would be a stable, continuous planetary cyclone. This would have a cooling effect on the sunny side and a warming effect on the winter side, though the extremes would be "extreme." This constant airflow would provide a means for life adapted to the dark side to exploit the nutrients generated on the sunny side. Mauna Kea has insects living on its glaciers that eat the freezing bugs blown up from below. Glaciers several miles thick would form on the dark side and would flow towards the sun, where they would melt and continue the cycle.


I think that in a tidally locked planet, the night side gets cold enough that any atmosphere freezes, and ends up frozen on the ground. On the hot side, any liquid would eventually evaporate and, along with any atmosphere on that side, will eventually make its way to the dark side, where it would permanently freeze in place.

Even at the terminus, and even if the planet wobbles a bit, every molecule that can get off of the ground in the warmth, will eventually get permanently deposited onto the cold side.

So, a tidally locked planet is a planet without any atmosphere or liquid anywhere on its surface. The only possible place for life to exist would have to be sealed underground.

The above means, that some NASA scientist is likely to say, "There is a 100% certainty that life is there". ;)

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    $\begingroup$ IIRC, most of the models I've seen suggest that this will probably not happen, assuming that the planet has a sufficiently thick atmosphere (and/or a hydrosphere) to efficiently transfer heat from the day side to the night side. $\endgroup$ – Ilmari Karonen Jun 29 '17 at 20:17
  • $\begingroup$ I see. If it is thick enough, then the cool side gets a continuous flow of warm air, and stays warm enough, that it does not get cool enough for things to condense out. $\endgroup$ – cadcoke5 Jun 29 '17 at 20:37
  • $\begingroup$ You're not accounting for wind, see the example of planet HD 189733 b $\endgroup$ – Valentin Brasso Jun 30 '17 at 6:36
  • $\begingroup$ @altvali That's a gas giant though. $\endgroup$ – JollyJoker Jun 30 '17 at 8:14

I liked Cadcoke5's comments that anything volatile would generally end up on the dark side, but there are even worse implications of a tidally locked planet. The spinning of our planet, coupled with the convection of the molten core (which is heated by decaying potassium, thorium and uranium isotopes) powers a magnetic field. This field keeps the solar wind from stripping our atmosphere and water vapour from the planet. A tidally locked planet would become more barren than mars (which has such a thin atmosphere that liquid water can not exist and even if it was pure oxygen, you would die in seconds). It may take a while to dry up by our standards. I don't know if this would be 10 million years or a billion years but compared to the 3+ billion years our planet already is, it is instant death.

  • $\begingroup$ Welcome to WorldBuilding cam! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Secespitus Jun 30 '17 at 6:20
  • $\begingroup$ Thanks for your answer! Do you know how satellites would affect the molten core? Would a moon or two make a difference in maintaining the core's convection for longer? $\endgroup$ – Fred the John Jun 30 '17 at 8:45

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