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The concept of flying cities was discussed here and that got me thinking. In a world where every animal has to migrate to live (in the case a 9 year day), would it be realistic to build and create hovering buildings/cities so people do not have to walk the whole day? If it is realistic how would they go about doing it?

I am NOT saying they would hover hundreds of feet above the ground, all they would need to do is move above the ground. Below is some information about what they would have.

Some Background

  • The 'cities' would contain around 70 to 130 people at the most 200
  • Their most common resources is rock and wood
  • They have very basic energy based technology
  • They use primarily solar power
  • The do not have access to radioactive materials
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    $\begingroup$ Seems it would be a lot easier to move everything but the buildings using regular motorized transport. You just have two houses, and you pack up most of your stuff for the journey. $\endgroup$ – Oldcat Jul 17 '15 at 17:11
  • $\begingroup$ But that isn't realistic on a larger scale $\endgroup$ – TrEs-2b Jul 17 '15 at 17:15
  • $\begingroup$ What's the technology/magic level? The answer would drastically differ between worlds with working anti-gravity, advanced superconductors with plentiful energy, or a society where hot air is the only lifting power. $\endgroup$ – user4239 Jul 17 '15 at 17:18
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    $\begingroup$ hover cities are hardly realistic on any scale. If you can't have a permanent settlement you move into then you'd just do without and live in yurts or something lightweight, or houseboats. $\endgroup$ – Oldcat Jul 17 '15 at 17:20
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    $\begingroup$ Inadvertent self-promotion, but worldbuilding.stackexchange.com/questions/14421/… is relevant. $\endgroup$ – HDE 226868 Jul 17 '15 at 20:01
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I would expect them to be more like traveling caravans, kind of like the wagons traveling across the prairies. It would be a slow grinding pace though, they just need to keep moving but they don't have to move fast.

How big these get would be dependent on the terrain they have to travel over and any large bodies of water they might have to float across.

Even if you got to the point of having a 'hover' city you still have the problems of mountain ranges. So as a species, they would have to make huge passes through the mountain ranges because it doesn't do any good to create even a wagon that can't stay with you, unless it is easy to disassemble and pack over the difficult terrain and reassemble on the other side.

The major pointed out hot air and blimps. (should have thought of it myself). This might work, at least to increase the size of the vehicles that are built and moved. It would allow much larger vehicles to be transported through harsh terrain, including swamps and marshes and would allow more direct travel around the globe. Resources to keep these balloons up and filled could be difficult. and keeping the buoyancy right so the 'city' doesn't get to high or drag on the ground would take some practice.

Hydrogen is easy to get for a zeppelin but that is dangerous and helium would be extra difficult, especially since the whole culture is on the move. Any mining endeavors would only be available for a couple years out of 9. One step before that might be having stationary 'cities' located every so often that are abandoned when everyone moves on to the next one until the next turn. These could also act as way stations for the floating 'cities'.

Hot air balloons have the issue of keeping the air hotter inside in order to provide lift. This requires controlled burns and lots of fuel. If they reached this level of tech, I think zeppelins to carry the majority of the weight would be balanced by hot air balloons to help dial in the elevation more easily.

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  • $\begingroup$ Thank you for the comment. also what about hot air balloons and blimps $\endgroup$ – TrEs-2b Jul 17 '15 at 17:55
  • $\begingroup$ hhmm. good thought. I'll add something about them. $\endgroup$ – bowlturner Jul 17 '15 at 17:56
  • $\begingroup$ This is very good but the whole day, night included is 9 years not just the day. I absolutely love the idea of abandoned and returned permenent structures $\endgroup$ – TrEs-2b Jul 17 '15 at 18:10
  • $\begingroup$ the year is actually 7 thousand earth day. give or take I don't have the full number on me $\endgroup$ – TrEs-2b Jul 17 '15 at 18:27
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It's unlikely that sentient species on that world would ever need to kludge some very stationary concept like a city into their migratory nature.

They would more likely live more like hunter/gatherers. Having mostly mobile communities with a few permanent structures that they return to Tomorrow (nine years later).

If they ever developed the technology to manufacture things like motor vehicles, then they probably just all live in RVs and double-wides.

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  • $\begingroup$ I like this answer because of the 'Returning' aspect. the idea of a fixed route that they return to is Interesting.... $\endgroup$ – TrEs-2b Jul 17 '15 at 17:32
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Hovering cities? Not really for several reasons.

Migrating cities? Absolutely possible although they may not be necessary for this species since all life on this planet will either migrate or function as extremophiles surviving extreme heat and cold. If the people in the OP are from this planet, they will be intensely migratory and may not need any kind of structure beyond their own skeleton.

Migration Speed Required

Since one day is 9 years, that gives us 4.5 years of continuous day and 4.5 years of continuous night. The cloud cover on a planet with slow rotation like this might be quite extensive.

The non-rotating planet is a staple of sci-fi, and generally it’s declared that life could survive in the “twilight ring” between the day and night sides. In fact, in the twilight ring the temperature would be colder than our poles are today (which are kinda like “twilight points”). You find the “comfortable zone” about 20° into the day side from the ring.

Let's go with the 20 degrees away +/- 1.5 degrees as the comfortable zone. On an earth sized planet, at the equator, this will give us a zone about 300km (186 miles) wide. The two habitable zones on the planet are 27 days wide, though the cities would probably want to move every two or three weeks to stay in a comfortable temperature band or to reach new resources. No walking all day!

Why flying isn't great

The prevailing wind systems on this planet will be huge with large amounts of cloud cover that look something like this:

Clouds on a tidally locked planet.

with Hadley cells that extend across the entire planet:

Hadley Cells on Earth

The atmosphere is difficult to model as we just don't know a lot about how atmospheres on near tidally locked planets work. It could be an incredibly violent place to live.

Flying takes a lot of energy. Hovering especially so because no lift comes from static lift structures such as wings. (Compare a condor to a hummingbird in flight to get an idea of the difference in energy requirements for the two flight modes.)

Technology on the level of 2030 for these people is unreasonable in my opinion as these people will be constantly nomadic. This limits the amount of stuff they can carry. On Earth, technology didn't get past tents, bows and spears till humanity settled down to start working on agriculture. Remember that current modern technology depends on a vast economy of stationary buildings and high precision emplacements. Look at Intel's clean rooms to give you an idea of what's needed to make modern technology. It's possible to build a steel refinery that can be moved but that doesn't make any sense. Building lots of them spread around the world doesn't make any sense either because the people only have a week or two to do it. Why spend a week building something you may not see again for 9 years when you could be gathering food?

Cheating on the definition of 'people'

Define 'people' as some kind of avian species that naturally migrates instead of a bipedal simian species. This solves the floating problem and how to cross any seas that might appear.

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Using modern materials, Buckminister Fuller calculated that a geodesic dome of a certain size would contain so much air that the mass of the air would vastly outweigh the mass of the materials making the dome. If the sun's heat were to increase the temperature of such a dome by 1 Fahrenheit degree, then the dome would become a hot air balloon and rise into the air.

Of course, the domes Fuller did his calculations on would be over a half mile in diameter as a minimum, so well beyond the technical abilities of your setting (for now). I suspect that since Fuller did his work on "Cloud 9's" in the 1950's, the evolution of technology and material science wold mean that a modern dome made of carbon fibre struts and very thin Mylar covering would be able to take flight with a somewhat smaller size. The important thing for a Cloud 9 to work would be the relationship between the surface area of the dome and the entrained volume of air within, so we are going to be talking about very large domes regardless.

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This doesn't directly answer your question but I thought I'd throw it into the ring because it might provide some useful insights.

Several novels have been written about migrating cities. The most recent one I read had to do with cities migrating on the planet Mercury.

In this novel and another I've read, the cities move along a developed path or track that encircles the planet. It's only safe for the residents to live along the night or terminator of the planet as the solar radiation during daytime is enough to incinerate the contents of the city (the mechanical parts of the city can withstand the heat).

These cities do not hover, unless you consider magnetic levitation on a developed track "hovering."

My answer does not fit your bounding of the problem, but it fits some of the bounding (bolded meet your criteria):
- The cities would contain around 70 to 130 people at the most 200
- Their most common resources is rock and wood
- They have very basic energy based technology
- They would be solar powered
- The would not use radioactive materials

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I suppose if you're willing to accept some sort of 'mega-fauna' which would also have evolved. I'm thinking of plant which grows an enlarged pod-like structure, fills it with gas, and then rides on the edge of super massive high-pressure systems (which such a planet is likely to have). People could lash several of these to make primitive zeppelins, and later cultivate their own and select for the biggest, most buoyant, and strongest husks. Plants can also be bonsai'd for custom shapes.

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sure. I've been on air planes with more than 200 people.

Suppose that they suspended the structure from a very large hot air balloon. Multiple Air ducts rout hot air from the city's many coke furnaces into the balloon, while the relatively cool air drops out the bottom. (coke is a coal like fuel that is created by charring wood under controlled conditions) The city can then be toed by a large team of bullocks on the ground. Some people just need to travel a week ahead of the city to create piles of coke from the forest for the city to pick up as it goes. (There could be a limit to how much fuel it could carry before it is too heavy.)

Certainly, it's possible. I'm actually surprised by some of the answers saying that it is physically impossible.

However, how feasible would it be? Providing the fuel seems like allot of physical work and logistical planning. Compare this to asking everyone to pitch a tent each night... Caravans seem like a more sensible mode of migration. I mean, you need people on the ground to provide fuel and other resources anyway, right? What exactly is the advantage to floating? Perhaps this mode of transportation could be only for the royal family and advisors, who need to be stretching their legs and about their business through-out the day, rather than confined to a small carriage.

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Hovering cities are too impractical, as my own answer to the link post goes into detail about. If your low to the ground you run into trees, mountains, etc. If your higher up you run into heavy winds. More importantly, the energy expenditure is just not realistic.

However, one very similar idea is quite simple to do. Boat cities. A city on a boat can easily travel and migrate without difficulty. It's energy output is far smaller, moving it is easier, and it's just far more maintainable.

Boat cities still aren't easy mind you. The biggest problem would be storms and the risk of your boats being destroyed by them.

The other issue would be growing food without traditional soil. With modern hydroponics that is doable, but how a culture evolved to the point where they had such techniques is harder to explain. If your having a species that evolved on your planet making them capable of living off of fish alone would fix the plant growing issue; humans can't do that but your species easily could have evolved in such a way.

I think the planet I assume is the basis of this question would also be prone to more extreme weather due to the difference in temperature between the two halves of the planet; which could be a complication on boat living and storms. I'm not quite as certain of the weather effects this would have though; not my area of expertise.

I won't go into more detail yet, boat-cities could be their own separate question on here if you wanted more detail.

Beyond the boats, it would be easier to stick to migratory technique that humans have used. Create temporary towns and just pack them up and move then via horse, mule, or eventually tank (not car, you need treads not wheels). Once technology was sufficient create a few towns designed to survive being abandoned for 9 years and migrate between your 3 towns every 3 years.

Of course, once your technology is sufficient you don't need to migrate. Long before you reached the point of floating city technology you would have sufficient means to survive in the dark. You could store energy captured via solar power or other sources during the 'day', and then use it to power your city during the 'night', including growing food via indoor green-house/hydroponics. Admittedly your need to store up allot of energy during the day to manage this, enough that migration would likely be the easier option for quite awhile, but eventually you could manage this approach.

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    $\begingroup$ the weather would be kid of like our but with very few storms except for the dusk and dawn regions which would suffer major storms $\endgroup$ – TrEs-2b Jul 17 '15 at 22:22
  • $\begingroup$ To the last paragraph, being in the dark would be kinda depressing, though I guess you would get a real appreciation for the stars. Boat cities should easily be able to attach themselves to the coast, giving them easy access to the soil on the land. Well, unless they are too slow to far ahead of the rotation, then I guess they would always be moving. $\endgroup$ – Necessity Jul 17 '15 at 23:34

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