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Imagine we took earth and slowed it down so that a day lasted 100 years, could humans (or any life) survive? Let's assume that there are no ill effects of slowing it down, no massive tidal waves, no continents melting as they slide across the surface, and the atmosphere stays in its relative location. In short, every particle just decides to take a break from spinning. Would life be able to survive?

I have this image in my mind of people living on the band between the hot and cold hemispheres and gradually moving their settlements along with the rotation of the earth. In one regard, energy would seem to be relatively easy to harvest - just put a boiler in the sun-facing region and a condenser in the dark region and you could have a nice steam engine. Would people be able to use this easy energy to overcome the challenges presented by the scorched and frozen earth that surrounds them?

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    $\begingroup$ it's likely also relevant how quickly the earth rotates around the sun. on that note, rather than just slowing down the earth, you would actually need to spin it in reverse, because if you slowed it to a stop, rotating around the sun would cause a "day" to pass every year, because the part of this stopped earth facing the sun faces away after half a year when its on the opposite side of the sun $\endgroup$ – Destructible Lemon Oct 16 '18 at 0:16
  • $\begingroup$ Are we talking about if this were always the case or are we actually suddenly changing it, because the latter will wipe out most life on the planet. $\endgroup$ – John Oct 17 '18 at 4:26
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The big problem I see for survival is not energy, it is growing plants for food and feed. The dark side is going to turn into a desert, no grass or crops or trees or any photo-synthetic life is going to survive even a year of night, and there is no time for them to adapt. Which means all the wild animals and insects that depend upon plants are dead, and all the predators that eat those are dead.

You have a similar problem with the oceans; the photosynthesis that creates the basis of the food chain stops. Most of our oxygen comes from that activity, When the Pacific is on the dark side, you just created a low-oxygen planet, and that is going to kill trillions of fish.

Some plants will do fine with 24/7 daylight, and may adapt to it, but every year, 1% of these high-light plants move into night and will die there. Coming out of night into the day, there is no guarantee that what has become desert is going to suddenly sprout with life. Just as the Sahara was once green, but rain doesn't help it grow again.

Or, the constant sunlight may turn the bright side into a desert, too.

When the big agricultural regions in the US and Asia go dark, there goes the food supply for the world, and here come the food wars to figure out which 20% of the population gets to live on 20% of the former supply.

As for the energy, solar energy is not usable for all daylight hours, the sun is only high enough for about 10 hours a day. You will still have the dawn and sunset bands where insolation (that is the technical term) is strong enough to extract useful energy from sunlight. So similarly, only 37% to 42% of the slow earth, at any given time, has sufficient insolation to use for either photovoltaic power or solar concentration (thermal) power.

I don't think "wild" humans survive this. A high tech civilization could, the power generation could be mobile, crops grown indoors and given artificial night by simple shading mechanisms, oxygen generated the same way. But there is no huge advantage to having 24 hours of sunlight versus 9 or 10 hours, in terms of energy this is just a linear relationship. The bright side isn't going to boil, the atmosphere is a fluid and will act as a heat conductor, the flow will circulate hot air to the dark side, where it will cool, and that will inevitably push cool air to the bright side.

But the lack of photosynthesis: That's going to kill us all, we will be starving for both food and oxygen pretty quick, and there is going to be mass extinctions of wild life that may well destroy the ecosystem on Earth.

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  • $\begingroup$ Could the plants on the light side ever adapt to be able to handle both? Or time their development such that they don't ever have to grow in the dark? $\endgroup$ – user45266 Oct 16 '18 at 3:19
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    $\begingroup$ @user45266 Not without magic. Without light (artificial or natural) there is no photosynthesis, and I suspect a century is likely far too long for a plant to "hibernate". Also, if you have any wild life at all (insects, rodents, worms) they will consume the plant, and it will have no energy for defense or repair. A plant is an engine that runs on photosynthesis. They will die in the dark. (Mushrooms and other fungi might grow in the dark). Animals and bacteria that do not rely on photosynthesis will eat it. None of our major green-leaf food crops will survive. $\endgroup$ – Amadeus-Reinstate-Monica Oct 16 '18 at 14:01
  • $\begingroup$ I think 100 years day rotation is slow enough that the plants will adapt to marching with the sun, colonizing temperate areas and leave deserts and permafrosts to the elements. $\endgroup$ – Alexander Oct 16 '18 at 17:07
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    $\begingroup$ @Alexander As RonJohn's map (another answer to this question) shows, there will be times when the dark side covers entire continents and the "dawn" line is entirely over oceans. 100 years is not enough time for plants (especially slow growing trees) to colonize across 1000s of miles of ocean, and 12,000 miles of land from pole to pole. Perhaps if we had a wide continuous band of land circling the Earth, but we do not. When dawn hits the eastern desert of the Americas, where do the plants "march" from? Not from the ocean! They are all dead. $\endgroup$ – Amadeus-Reinstate-Monica Oct 16 '18 at 17:17
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In addition to what @ChristmasSnow said, 7.7 billion people "gradually moving their settlements along with the rotation of the earth", when 70% of the Earth is ocean, and a big chunk of that is the Pacific is -- at best -- impossible.

I'm dubious as to whether anyone could do it, since the ocean storms caused by those winds will be stupendous.

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    $\begingroup$ It's pretty clearly an established consequence that billions of people are going to die before an equilibrium can be established, though. $\endgroup$ – jdunlop Oct 15 '18 at 23:45
  • $\begingroup$ @jdunlop even 1M "living on the band between the hot and cold hemispheres and gradually moving their settlements along with the rotation of the earth" is impossible. $\endgroup$ – RonJohn Oct 16 '18 at 0:08
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    $\begingroup$ I would imagine the ocean on the dark side would freeze over - or at least sufficiently enough near the poles such that people on the sunrise side could walk along the ice when needed. And yes, I'm assuming pretty much all life would die out but I'm trying to think of anyone could actually survive this, maybe a group of a few thousand near the north or south poles where things might be a little more temperate. $\endgroup$ – BobtheMagicMoose Oct 16 '18 at 13:58
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Edit: clarifications in bold

Over these long distances (dark side DS and bright side BS) trapping heat and conveying it from BS to DS is impractical because of heat losses along the steam pipes You would rather harvest electric energy on BS and export it to DS. solar stations converting heat to electricity would do better, but the length of power lines cannot stretch to infinity

Put it another way: a thermosolar power plant concentrates heat by mirrors to a solar tower. This tower is now hotter than its surroundings . This heat difference relative to the environment is sufficient to generate electricity and conduct it to DS. resistance losses through power lines is small at urban scale but too large if we stretch lines across the globe so producing power locally is better

Second way: this slow rotation creates an almost eyeball Earth where strong winds rise up and move from BS to DS, cool and descend and move back to BS. So, wind energy will be more available in DS and will not require moving any energy source from BS to DS. it also encourages local production

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  • $\begingroup$ You make a lot of claims without justification. Could you provide some reason that I should believe you? Sources would be nice. Explanations of your assumptions would also help. $\endgroup$ – BobTheAverage Oct 15 '18 at 22:09
  • $\begingroup$ @BobTheAverage If you speak about regular line losses, before distribution losses, it's 2 to 6%, but if you talk about planetary scales like moving power to the other side of the globe then this amount would go up drastically. This is wire resistance. The longer the wire the bigger is the loss! insideenergy.org/2015/11/06/… $\endgroup$ – Christmas Snow Oct 16 '18 at 11:31
  • $\begingroup$ That's a good point about wind currents, hadn't really thought about that but I agree, there would probably be heavy winds on the surface from DS to BS that could be used for energy. $\endgroup$ – BobtheMagicMoose Oct 16 '18 at 13:55
  • $\begingroup$ @christmasSnow You are using electricity to model heat loss from a pipe carrying steam or some other heat transfer fluid? $\endgroup$ – BobTheAverage Oct 16 '18 at 16:18
  • $\begingroup$ To all who commented above, thanks for pointing-out ambiguity. I clarified in bold (changes). $\endgroup$ – Christmas Snow Oct 16 '18 at 22:03
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What you're essentially doing here is tidally locking the planet: making one half almost permanently day and one half almost permanently night.

For the first week or so, the effects would not be that noticeable, but plants on the night side would start to die from inability to perform photosynthesis.

After the first week or so, things start to get weird. The temperature on the night side will gradually drop until the night side freezes over completely and basically becomes Antarctica during the winter. The temperatures on the day side will gradually rise until the oceans, lakes, and rivers all dry and that side of the planet becomes a desert.

To survive, humanity would have to live on the boundaries between these two zones, which would be a temperate area permanently in sunrise/sunset. All farming would have to take place here, and people would only be able to grow dark-tolerant plants that could survive in reduced sunlight. As these would probably not provide enough biomass by themselves to feed the human population and their livestock, fungus farming would take off, with various types of mushroom replacing most of our food crops.

The only use I could think of for the night side of the planet would be for penguin or seal farming if the population decided to invest the necessary resources to protect themselves from the cold to do that. However, the day side would probably be used as a mining venue, since all plant and soil cover would be gone, leaving the sand and minerals exposed.

In short, your civilization would be nomadic fungus farmers, chasing the sunsets around the planet over its long rotation period.

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While you've already selected an answer, I thought I'd respond.

As has been pointed out, your power idea doesn't work. The relatively large separation required means you get unacceptable losses in the connecting pipes. By the time hot material gets to the cold sink it's no longer hot. Plus (not mentioned) this enormous (hundreds of miles) pipeline network has to be moved every few years. I suppose you could propose burying the pipes, but every 100 years you're going to get a glacier advancing over the terminals and grinding them away. Possible, I suppose, but very expensive.

Furthermore, water is going to be a big problem. The dark side will essentially be a big ice cap (and if you're not lucky an atmosphere cap) with cold winds at ground level constantly blowing from the dark side and returning at higher altitude. This will tend to lock up all water in the cold regions, with only a small amount of water vapor available from the sunrise zone. This will restrict the habitable zones significantly.

While animals and people can move faster than the terminus, this is not true for plants. At the equator sunrise will move at about 250 miles per year with lower speeds at higher altitudes. There simply aren't many plants whose propagation mechanisms will cope with that. Add to that the narrow habitable zones implied by the water problem, it's just not clear that the habitable zones will be able to support any significant amount of plant life - and hence no animals or people, either.

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