I'm in the early stages of a project involving a habitable tidally locked world, this one a fairly Earth-like (large ocean) around a K-class orange dwarf. And using this study for reference, where the planet in question has relatively mild hurricanes.

But while researching the complicated climate and orbital questions, I ran into a much more mundane one -- city planning when the sun is essentially fixed in the sky.

It would be extremely easy for say, a downtown filled with skyscrapers to leave large portions of a city in permanent shadow. Especially close the edge of the day/night side, when the sun is essentially on the horizon. Much less so if a city is built centered under the substellar point and the sun at its zenith (but then you need to worry about weather concerns).

I'm trying to figure out how city designs could account for this, while still avoiding inefficient sprawl as much as possible.

For a few examples of what I mean, a couple of ideas I had so far were:

  • Cities/towns in the 'twilight zone' could generally be built on steep slopes facing the sun, hobbit style. Such that no building overshadows the next one up. This sharply reduces how densely one could build though.
  • Cities very close to the substellar point would have the sun directly overhead, and so could build more densely with less concern. (Again potential weather issues)

Obviously this is all under the assumption that residents would prefer sunlight to permanent darkness.

Edit: Whether -> Weather, typo

  • $\begingroup$ I have never understood what is "inefficient" sprawl. How do you define efficiency and inefficiency? At what point does the "inefficiency" of land use for human habitation begin to have any real importance? $\endgroup$
    – AlexP
    Mar 15, 2023 at 17:53
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    $\begingroup$ About the time you get a housing crisis or cities go broke maintaining the equally sprawling infrastructure (sewage, electrical, emergency services, etc.). For some quick charts though, Urban3/Strong Towns make some interesting charts: strongtowns.org/journal/2022/12/15/…. This is a broad topic with many facets, but Strong Towns, Not Just Bikes, and Vox for instance all have good rundowns. But covering that kind of distance does have a significant infrastructure cost to maintain it. $\endgroup$ Mar 15, 2023 at 18:08
  • $\begingroup$ It will depend on where you CAN build a city. Your planet might be way too hot under the vertical sun. MAybe, the day/night edge might be the only habitable zone. In a 30..60 degree zone, cities could look like real ones. Just more windows toward the sun. $\endgroup$ Mar 20, 2023 at 13:45

3 Answers 3


You might be overthinking this. Sometimes simple economy overrides massive architectural behaviors. For example:

Total Blackout Shutters

Years ago while traveling on business to Japan, I pointed to a bulky looking contraption that appeared on the windows of a number of apartments that we passed and asked my coworker what they were.

He explained they were total blackout shutters, used to block every photon of light for people who worked graveyard shifts and needed to shift their natural diurnal rhythm to accommodate their work schedule. Being someone who suffers from light sensitive eyes, I get it!

You don't need to change the overall nature of your city's construction. You simply need to incorporate total blackout shutters as a normal part of construction, allowing inhabitants to not only create the diurnal cycle they need, cut a cycle of any length.

In other words, this would be a great world for tourism because by design the entire planet can basically accommodate the natural rhythm of any visitor.

What your planet won't have is a "night life," because there won't ever be a night that isn't indoors in one way or another. No big deal! The economic value of 24/7 beaches isn't to be ignored — and habits change. The world simply won't have as much need for neon.

In other words, those constructions that are trying to take advantage of (e.g.) the twilight region of the habitable band become points of interest and don't reflect the overall construction of the world — which will always be bound by the Law of the Lowest Bidder.

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    $\begingroup$ Sunlight is a bit different, and has a number of health impacts. Most notably on Vitamin D. Some are more debated than others but are in the literature (see en.wikipedia.org/wiki/Seasonal_affective_disorder), however it is very much considered to have health impacts ncbi.nlm.nih.gov/pmc/articles/PMC7400257. And likewise there's the mental impacts and personal/cultural preferences to not want to live in the dark. $\endgroup$ Mar 15, 2023 at 18:31
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    $\begingroup$ Sounds like large cities will have shadowed slums... And the rich will live sunward. $\endgroup$
    – Questor
    Mar 15, 2023 at 18:36
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    $\begingroup$ @ConnorHiggins, you may consider the use of mirrors and other methods of directing sunlight where it's needed. $\endgroup$ Mar 16, 2023 at 7:03
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    $\begingroup$ That is a good suggestion as well. Subtle incorporation of mirrors and reflective building materials would be a low-tech and low-energy way to help mitigate the problem. Having buildings incorporate sloping reflective overhangs to help illuminate the street might even create a unique and aesthetically pleasing architectural style. $\endgroup$ Mar 16, 2023 at 16:38
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    $\begingroup$ @ConnorHiggins Especially if you hang disco balls now and again. 😁 $\endgroup$
    – JBH
    Mar 19, 2023 at 18:14

So while I am still interested in finding better answers, I want to share what I ended up settling on for my own world and everyone else's reference. Overthinking? Maybe. But nice cities are my kind of worldbuilding.

tldr; cities would choose to build in the "iris" of the eyeball world.

Quick definitions and an (approximate) equation

Here's our situation, assuming a spherical planet for simplicity:

enter image description here

The sun being fixed is inconvenient in that we need to plan around it, but convenient in that it makes calculations easy. At zenith (or the substellar point directly under it), the angle between the sun and ground is 90 degrees. At the terminator or day/night boundary, it is on the horizon and the angle is 0 degrees.

The size of a shadow is just: $H/\tan({\theta}) = L$. Where H is the height of the building, $\theta$ is the angle of the sun, and L the length of the shadow.

Rejiggling slightly we can get an equation we can use for city planning:

$$(AverageFloorHeight*NumberOfFloors)/\tan({\theta}) = AcceptableShadowLength$$

Obviously your city needs to make some judgement calls on what is acceptable. And may require/provide forms of illumination to cover the shadows at street level.

Architecturally perhaps, building on @Krišjānis Liepiņš's suggestion, buildings are intentionally build to reflect light onto street level. On the flipside, building off @JBH, the sunward sides of builds may feature blackout curtains for when people want sleep.

However regardless of what you pick the overall trends are the same unless you don't care about this at all.

Also you might not want to have roads running directly into the sun, especially at low angles of elevation. Just so you don't blind people going in one direction. Less of an issue with trains though.

At the extremes

At the zenith/substellar point

Directly under the zenith the shadow is basically zero, so on these grounds you can build as tall as you want. However, the substellar point is ... a point. Whatever geography is or isn't there, is what you get. Also the interiors of the buildings would receive little sunlight. Overall this may not be ideal.

At the terminator line

You can't build anything on flat ground without casting an impractically long shadow. Anything built here would likely be picturesque, but low density, "twilight towns" built on/into things like mountain slopes. But not material for large cities, these would be restrained by geography. It would be a potential boon to nature though if large portions of the planet are considered impractical for urban development.

So if neither are ideal, instead consider...

"Iris cities"

Which is what I'm terming them. You can develop within the radius determined by how tall you can build practically while keeping the shadows cast at an acceptable length. For instance, say you decide your floors are 4.5 meters tall at most, you find a shadow length of 3 meters acceptable (maybe your streets are 9 meters wide, and 33% shadow is acceptable), and you want to be able at least build 5 story tall buildings. Well you need to build between an angle of elevation of 82.41 minimum to 90 maximum. Which means very close but not necessarily on, the substellar point.

So you get,

  1. Low-rise cities only! Which we do have on Earth. Skyscrapers would be even more obnoxious and impractical. Luckily low-rises are pretty efficient from an infrastructure standpoint, even here on Earth.
  2. High population. Urban development elsewhere is far less practical. So there would be a lot less cities to "compete" with for residents. And for that reason, people would find housing and other amenities much easier to come by within the "Iris".
  3. A lot of free solar and wind energy. You won't have issues with energy generation.
  4. Agricultural production may also be able to take advantage of the constant sunlight. Plants need time out of the sun as well, at least on Earth.
  5. Storms: The worst weather the planet can throw at you is likely to occur here. Cities are going to want to take advantage of geographically sheltered areas, and have elaborate defenses against extreme weather. They can expect to take a hit from time to time.


The sun may actually have significant effects on development. Barring everything becoming extreme low density housing or a dystopian shadowed cities, there would actually be significant factors pushing towards concentrating development/urbanizing a relatively small area.

So you may end up with low-rise mega-cities being the norm. Considering that kind of development is both efficient for the city, can lead to affordable housing with an ample housing supply, and leaves the rest of the planet to the wildlife ... it could actually become quite nice indeed.


How much time does one spend in the sun really honestly truly? As a software developer, most of my sunshine is on my morning cycle to work.

I expect to see the city strongly divided into sunshine activities and non-sunshine activities. The tops of building and sun- facing sides will be the prime locations for cafe's, restaurants, gardens, elevated walkways and maybe a few very expensive penthouses. The shady side? That's where the apartments, office blocks and heavy industry is. Those sorts of places are often often artificially lit anyway, so being permanently shaded isn't too bad.

This will be a normal part of construction for the inhabitants: You will go to a new city and just know that car parks will always be in the shade, that food is found on the top floor of a mall, and you'll never need a compass to have a sense of direction.

On flat ground roads would probably run predominantly East/West on with regular spacing to maximize sun exposure. The buildings may be reasonably continuous walls that line a street side, with occasional alleyways to allow traffic through. There may be few long north/south roads as they could rapidly achieve incredible surface temperatures. Pure sun is probably just as dangerous as pure shade.

Structural engineering will also accommodate the constant thermal difference - a poorly designed building a few degrees hotter on one side will lean away from the sun due to thermal expansion on the sunward side. Buildings may be built with large thermal management systems to ensure the heat from the sun-side is distributed properly to the shaded side without needing to run heaters at the same time as AC.

Mold/damp would be a real issue in the shadows near ground level (or areas of low wind), and in winter you may find snow and ice accumulates in some parts of the cities for far longer than expected - it simply never sees the sun. There could be large scale engineering to solve these issues, or just more snow plows that move the snow to the sunny side of the road and street sweepers to keep the place hygienic.


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