# If the Earth stopped spinning, what's the ideal point for it to stop to ensure the most people survive?

Say the Earth were to stop spinning, and became tidally locked (or rather, stop spinning 366.25 times a year, and only once a year), through the machinations of a superpowered alien visiting down on the planet with a twisted sense of fun. If we pretend this would have no immediate consequences (which there obviously would be but go with me), my question is: what's the best point for the world to stop at so that, when the frontside gets scorched and the backside gets frozen, as few people as possible would perish or become climate refugees?

Essentially this is a game of working out what time of year and time of day would the Earth need to stop spinning such that the sunset/sunrise "band" around the planet covers the most highly populated area possible.

• I guess you want to find the sun-vector that minimizes the sum over persons of the squared dot product of sun-vector and person-vector Commented May 9 at 18:04
• @jdunlop: Why would the axial tilt vanish? For example, our own Moon is tidally locked to Earth, and yet it's axis of rotation is inclined by 6° 41′ with respect to its orbital plane. Commented May 9 at 18:12
• Apparently it shouldn't matter all that much: worldbuildingpasta.blogspot.com/2024/03/… Commented May 9 at 18:59
• @NuclearHoagie "...instead of once per ~24 hours." (Nitpick-squared!)
– Tom
Commented May 9 at 23:21
• Obligatory XKCD/What if Commented May 12 at 16:44

Here's one way you can try to answer this. I googled "interactive day night map", and while I didn't find exactly what I wanted I did find this one that's fairly close. (The only problem is it only shows day and night - you don't get a good idea of exactly where it's twilight.)

I don't have a good way to really optimise it, but just from eyeballing things I think this solution is pretty good - about 12:20 GMT on 21st June:

Compare this to a population map of the world

As you can see, this will put big chunks of China and India in the twilight zone, and that's where the population is densest. As a bonus we also get the west coast of the US and much of central America. But Europe, Africa and most of South America are toast, while Indonesia, Japan and Australia are frozen.

You can experiment yourself with other places to draw the line.

• I'd shift the time 2h earlier, to include China's fields into sunlit zone and exclude midwest US which is pretty rough, but not too far to not exclude Brazil's jungle. Note also, with this map Europe will be scorched and all its population would be as good as dead. Commented May 10 at 6:41
• @Vesper if you want to keep Europe another option is 8:30am on 16 December - the UK, Western Europe and Scandinavia are ok, and so is eastern China and also Brazil, and maybe parts of Indonesia and Japan. But you lose India that way. There isn't a perfect solution. Commented May 10 at 7:09
• I thought the atmosphere might precipitate out on the night side but if this link from @talrnu can be believed that's older thinking and newer modeling suggests the whole daylight side may be reasonably livable (for a given value of 'livable') which would mean your positioning was a good one as you more or less maximised the land on the day side. Commented May 10 at 11:31

You scenario doesn't work out as you hope.

The terminator is a narrow stripe, and even if it doesn't get too hot or too cold, it will be impossible for this narrow stripe to sustain the life of those living there: the dim light of sunset/sunrise is not enough to sustain any of the plants we humans use as base for our food chain.

And it would take years to convert agriculture to the new conditions, years that you hardly have, considering that the thing happens all of a sudden.

• Plus, as I understand it, the photosynthesis cycle relies on both day and night. If that is true and you get rid of the day/night cycle plants aren't going to cope even if they are getting a good amount of light in the current location. Commented May 9 at 18:42
• @SoronelHaetir if true there will be plants in polar regions that will do fine with 24 hour daylight (plants there already have to deal with that for months at a time), how many of them are suitable as food crops (few to none I'd guess) and how long it would take to transplant them and bring their numbers up to adequate levels is another thing of course :) Commented May 9 at 20:32
• @SoronelHaetir I'm not entirely sure that plants do 'need' a night cycle though, not on an existential level anyhoo? I do know many if not most do need a dark period for their best growth-rates and yields, yes, but I'm not so sure it would do them in entirely if they didn't get it? Commented May 9 at 20:54
• I would like to contest the claim "the dim light of sunset/sunrise is not enough to sustain any of the plants". Neglecting things like heat transport by winds or ocean currents, every place's temperature is determined by its latitude (and elevation), i.e. how much radiation per area [W/m²] it receives. So if we propose that there is a terminator zone with an agreeable temperature, this zone must be receiving an amount of radiation similar to that of places that we find agreeable on our current Earth, i.e. the plants of that zone would still have the same amount of light available. Commented May 10 at 12:54
• @L.Dutch so move closer to the sun? Obviously large portions of the planet would be getting plenty of sunlight so your objection doesn't make sense. Commented May 10 at 15:43

More is less

The last thing you'd want is to stop with the most people surviving as you'd have a bunch of people and no real way to feed them which would then lead to fighting and starvation.

Where you'd want to stop is the place where people are more likely to survive.

You'd want it where twilight runs through Norway, Sweden and Finland personally.

Lower populations, use to surviving the cold, has geothermal power and indoor farms.

Are we given a heads-up before these superaliens tidally lock our planet?

if we're given a couple of years to prepare, along with knowing where the line will be, that affects where the optimal place for survival would be.

If we get to build a huge solar farm somewhere in the late afternoon or early evening,(it's kinda cool how this scenario turns times-of-day into geographical positions) close enough to night that we could feasibly perform maintenance and transport energy without melting the cables, that could enable us to use artificial lights for foodgrowing in the night.

You'd probably also want to place as many nuclear powerplants as possible in the night, but close to the terminator.

other's have mentioned that you'd probably want to consider water vs land. They probably have a point, but it's not clear to me how exactly that works out. any water in the day will evaporate fairly quickly, and then the seabed will be scorched to a crisp. The vapors will then likely migrate to the night, where they will quickly freeze to ice, resulting in a wall of ice along the terminator.

provided we have some sort of means to generate electricity, this ice is a good source of water, which is good, but I also suspect the ice along the terminator will be quite thick(measured in km, at least), and that much ice can really do a number on any structures in the area.(this is the understatement of the century btw)

It's not impossible that our best bet for survival is actually to just put as many nuclear reactors as possible in the night, and far away from the wall of ice around the terminator. It's easier to heat up a cold place than it is to cool down a hot one.

If we lean into this theory, you absolutely want to put as much forest as possible in the day. If we're all going to live in the cold night forever, greenhouse-gasses are our friends. They wont make the night warm enough to live in on their own, but they certainly wont hurt either! be sure to keep the Arabic firmly in the night, their oil is gonna be very useful too, if we can keep oildrills working in the cold.

I wonder how cold the night would actually get. the atmosphere should carry some heat from day to night, I'm not smart enough to calculate how much exactly, but it would be interesting to know if anything can remain fluid. water will obviously freeze, and so will oil on the surface, but what about oil underground? there's a concept in my native language (swedish) "Tjäldjup", which refers to how deep underground water will freeze in winter in a given location. Where I live it's around half a meter, but further north you can get to almost a couple of meters. In our theoretical night, it would be deeper than any place currently on earth, but geothermal activities will limit the depth at some point. I'm just not sure how deep that'd be.

Perhaps our best bet is to just move into the deepest mines underground, build underground nuclear plants and slowly evolve into moles. Radiation might speed up the process if we're lucky!

Also, we'd better hope the night isn't cold enough to turn oxygen into a liquid, or even worse, a solid. if that happens there's a chance that all available oxygen accumulates in solid form somewhere, and then we're truly screwed!

Of course, if your aliens just want to watch the confusion and starvation of as many people as possible, and they don't find cooked or frozen humans to provide the ultimate quality humor, then N.Virgos answer provides the best gag. That will have the most short-term survivors.

• This is the first time I've ever been asked "are we given a heads-up before these superaliens tidally lock our planet" and I'm here for it Commented May 10 at 10:36

I'm no astrophysicist, but as I understand it, no matter where rotation stopped everyone would die. Not necessarily instantly, but every system in everything, both natural and human made, was with the assumption the planet will keep spinning. Things will go poorly.

• Yeah, pretty much. We'd either cook or freeze one way or another. Commented May 9 at 22:27

Most people would be dead within a few months as food runs out and land can no longer be farmed.

However, those with hydroponics and a source of energy could last a while in your given scenario. Ideally, they'd be right in the middle between the hot and cold side to make their lives a lot easier.

As to where exactly to stop the sun, you'd want to line up the edge with the most dense population centers like Beijing. The fact that there isn't much farm land in big cities is unimportant in this scenario as office buildings can be converted to vertical hydroponic farms and living spaces.

But really, this is the best-case scenario. No matter where it stops I'd say at least half of humanity is dead, if not a lot more.

• "Half or more" is quite a common outcome when it comes to apocalyptic scenarios. I say 1% survival rate is good enough here. Commented May 12 at 5:09
• @Vesper Hence why I said "I'd say at least half of humanity is dead". Half is the minimum dead. Commented May 13 at 9:13

You don't specify what happens to the Moon. At the moment the transfer of momentum from the earth to the moon via tides keeps the orbit from decaying. Although it would take a considerable time, the loss of angular momentum would cause the Moon to crash to low earth orbit and break up with catastrophic consequences.

• What would happen is the moon would restart the earth's spin as the angular momentum would go from the moon to the earth until both were tidally locked to one another. Then the decay of the orbit would probably stop as their is nothing major affecting the system (see Charon and Pluto) Commented May 13 at 1:13
• I agree angular momentum would transfer from the Moon to the Earth. But the Moon has only 1/81 of the mass, so there isn't much spare momentum to transfer to get the Earth back from 1 revolution per year to 1 revolution per month. This momentum can only come from the Moon spiralling in with eventual catastrophic consequences as already mentioned.
– jrrk
Commented May 13 at 10:00

Others have provided good answers as to the best place for the Earth's rotation to stop. I want to expand on that to show that the thin line of survivability might not be as thin as one might assume.

Tidally-locked bodies are not perfectly stationary. They still have a slight horizontal and vertical back-and-forth motion, called libration. This would have the effect of increasing the habitable area around the terminator, as you would still have a cyclical sunrise and sunset in those areas. You can see this not-insubstantial motion in our own moon:

• That is an important issue. Unless the axis is changed, the sun will move in the sky along the old familiar analemma. Commented May 13 at 19:41