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I am envisioning a habitable world with an atmosphere comparable to earth's in which one day is extremely long (>100 earth years). Human settlers have successfully colonized a small portion of the planet near the equator, and have adapted to living in perpetual sunlight. They have not, however, been around long enough to experience nighttime on this planet.

When the colonist's part of world finally does enter nighttime, how will the climate change (specifically temperature)? Or will it change at all? How would it affect the colonists? I have read that Venus's lack of dramatic temperature shifts has something to do with its slow rotation. It seems to me like the more time a portion of the planet spends in darkness away from the sun, however, the more time that portion has to cool down. The way I understand it, the colonists would eventually be plunged into several decades of freezing temperatures.

Let's assume that a habitable planet with 100-year-long days isn't scientifically infeasible, and that all other life on the planet has adapted to these conditions. How does the length of a day impact temperature variations between day and night?

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    $\begingroup$ Being unable to grow any crops for a century would seem like a major (and deleterious) change in the natural conditions of the colony... (Hint: photo synthesis doesn't work at night.) $\endgroup$ – AlexP Jun 25 '18 at 1:57
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Well, the day/temperature here can be extreme, easily 30+ degrees celcius, particularly with no cloud cover. I'd have to imagine that on a planet with no night, that it would be even worse.

There are a couple of bodies in our solar system with little to no rotation, and they have massive temperature differentials. Venus may be an exception due to how thick and active the atmosphere is.

The atmosphere would conduct some heat through to the night side of the planet, and the planet's core temperature would provide so me heat. However you'd still be looking at well below freezing, probably in the realm of -80 degrees Celsius. The center of the day side may turn into a desert due to the unrelenting heat of the sun focuses on it for decades at a time.

So, possible adaptations I can see to a decades long night:

  • Extreme Hibernation - your colonists may be in for a bit of a shock when all the plants die off or enter a deep hibernation to await the return of the sun.
  • Cryostasis - There are examples of animals on earth that can survive being frozen and thawed as an adaption to arctic conditions. The animals on your planet may use a similar system
  • Head North/South for the night! Depending on the axial tilt of the planet, either pole may not become fully dark during the night. Life may retreat into the polar region to take advantage of the meager remaining light.
  • Move! if the rotation is slow enough, the life may move around the planet in a never ending slow migration, continents and oceans permitting. Imagine trees that move several feet to the left every year, demolishing all in their path
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    $\begingroup$ Good answer, but don't forget the extreme weather effects on the day night border, with the extreme cold parts meeting the extremely hot parts, its going to create some pretty impressive weather systems, although i like your idea of migratory trees $\endgroup$ – Blade Wraith Jun 25 '18 at 9:42
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Let's add to the above answer that you get very strong storms moving from dayside to nightside, terminator line wouldn't be really a nice play to stay. Any ocean on the dayside would be very perilious to navigate as heat would form superstorms on a continous basis.

On the bright side, you could channel all that wind & solar energy to get some power toward the nightside.

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