It's always ~7:30 PM.

Gravity and air are normal, and clouds and winds and storms exist, but there's no moon, and the sun is always just touching the horizon, without ever moving in the sky. The weather doesn't vary much. Temperatures never stray from 65°–75° Fahrenheit.

(Assume that the exact circumstances that cause this state are irrelevant. It might just be magic.)

The civilization in question has achieved roughly iron-age technology. They're peaceful and agrarian. They have a rudimentary writing system. They have abundant freshwater from lakes and streams. They have some neighbors they can trade with.

  • How would people measure time, either on a day-to-day scale or year-to-year scale?
  • What would their sleep cycles look like?
  • 1
    $\begingroup$ "It's always ~7:30 PM." Bright sunshine here at this time of the year. $\endgroup$
    – his
    Aug 22, 2015 at 7:50
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    $\begingroup$ A relevant comment. And it is pitch black at the equator at that time of the day. $\endgroup$
    – fantasia
    Aug 22, 2015 at 8:14
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    $\begingroup$ Relevant: Sleeping on planets with very long days. $\endgroup$
    – Frostfyre
    Aug 22, 2015 at 13:13
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    $\begingroup$ While I like this question, I feel like it's too many questions in one. I've edited it down to just your first question but please feel free to post the others as separate questions. $\endgroup$
    – Tim B
    Aug 22, 2015 at 17:35
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    $\begingroup$ If the situation is caused because the planet is tidally locked, then people will see different aspects of the sky as they travel. Going towards the hot pole brings you towards high noon, while travelling past the equator towards the cold pole brings you to night. Is there a reason anyone would want to stay o perpetual twilight? $\endgroup$
    – Thucydides
    Aug 22, 2015 at 18:21

5 Answers 5


It's entirely possible that a culture like this would not really measure time at all. What is time relevant for?

Calendars - used for planting crops, predicting weather, planning for the year.

There is no need for any of these things.

Time - used for meeting up with people, making plans, etc.

In a small settlement again there is no need for these things. You don't arrange a meeting with someone, you just go and see them.

There are plenty of precedents in tropical societies on earth, where time is less important than it is in seasonal areas. For example:


"Amondawa people, like any other people, can talk about events and sequences of events," he told BBC News. "What we don't find is a notion of time as being independent of the events which are occuring; they don't have a notion of time which is something the events occur in." The Amondawa language has no word for "time", or indeed of time periods such as "month" or "year".

As to sleep, people would most likely just sleep when they got tired. It's entirely possible that the head of a family or village would go to sleep and everyone else would sleep at the same time, or that they would deliberately sleep at different times so there were always people able to look out for trouble.


Farming would take a lot more land and energy. Without the influx of mid-day solar energy, plants would grow much more slowly. Farmers might be somewhat nomadic. They may have several "farms with farm houses" and travel from one to the other in sequence as they harvest and then plant.

However, it magic is involved, that might not be much of an issue.


Use a sand timer

It doesn't have to be a glass sand timer. It could be a metal filled sandtimer with a section of air in the middle that lets you see if the sand is flowing - if it's not flowing, the top is empty.

Make sand timers representing x time per flip, and you can measure however long you want your days to be using those flips. For example: If you want a 20 hr day in your story, make it so that there must be 19 flips before the day ends.

Sleep cycle: Completely up to you. If your flips represent half a day each, perhaps you could consider having a half day sleep cycle. 24 hour days? Keep the cycle the same. Less hours in a day? Less flips during sleep.

Just make sure the guy flipping the timer doesn't fall asleep; hire people in shifts for that.


Question Time

The biggest question is why do these people need to measure time?

Farming is the cornerstone of their civilization, so they will need to know when to plant and when to harvest. The crops will be harvested when they are ready and the new crops planted when the soil is ready for them. Side note: Given the consistant temperature and lighting, I would expect crop growth times to be reasonably consitant. While not a year by our definiton, I can see using a crop cycle for a time period reference.

There should still be the circadian rhythm of the denizens of the world that personally define a day. Since they live in a constant twilight, there is little incentive for everyone (or even most) to sleep at the same time. They might if they descended from a nomadic species. This would be personal days though -- not everyone has a perfect 24 hour circadian clock to use us for an example.

If there is a wet/dry season with any amount of regularity, then they would be used as the basis for a clock, if only to make sure that everything is either planted or harvested in time to dodge the inclement weather. Likewise if a herd of nomadic animals travel through the lands, expect that to be used as a period.

Personally, I think they might have the concept of a "year" as it relates to the crops that they tend, or though their own biologic cycles. But at this point in their societal development and with no astrological cues to take such times from, I don't really see anything like a defined calendar like we have coming up as anything beyond a curiosity.

Planting Time

My hypothesis is that if they would have a concept of a year, I would still expect it to revolve around crops. Given a fairly consistant temperature and lighting, my hypothesis would be that the growing time for crops would be fairly consistant. Only extreme precipitation events, such as floods and droughts, should this time period differ.

There is also little reason why some pseudo-seasons would not still happen. Trees could store excess energy for a set period and then transition into a reproductive mode before reverting back to a growth phase.

These plant-based seasons will not necessarily be set lengths, but they will still present a way to keep track of certain things. But really, they will harvest crops when they are ready and plant them when it is time to replant them. Given that there is no winter to have to prepare for, one could argue that all that is needed is to compost the parts of the crops that aren't eaten.

The main issue about using crops is that different crops will have different growth cycles. Could make for an interesting premise.

The Sleep Cycle

I see no reason why a sleep cycle wouldn't exist. We sleep even though we can be in perpetual lighting via artificial means. They will have evolved that way so will be quite used to the idea of perpetual light.

The biggest queston here is if most people share the same pattern of awake and asleep or if it is a personal thing. That feels more like it is dependent on the species -- a nomadic species probably rest together while solitary species probably have their own clocks and only sync up to mate or when they start gathering into larger groups.

Since your society is agrarian, I would think that there are probably groups that are up at certain time, almost like shiftwork. There would be overlap in waking times for most, and without a handy way to know when to get up, I can see people sleeping when they are tired and working when awake.

It may lead to people knowing something is wrong if their sleep cycle is disturbed by illness.


Sorting out the ecosystem will give you answers.

This setting has a fairly large ecosystem problem that needs addressing. Working it out will give additional details that will likely shed some light (pun not intended) on some things you're probably wondering about.

In our world, the sun gives life. All life, except for chemosynthetics living in deep-sea vents and the like, ultimately gets its energy from sunlight. The light makes plants grow, and everything else is supported off plant biomass.

But twilight is so much dimmer. Here is a rough intensity chart. Note even a cloudy day is close to two orders of magnitude brighter. Very roughly this translates to two orders of magnitude less energy for living things.

The problem is this corresponds (very roughly, mind) to two fewer trophic levels, since the rule of thumb is there's an order of magnitude loss going from level to level (i.e. herbivores get 10% of the usable energy out of the plants they eat).

In other words, if your world has nothing but unending constant twilight, that's not enough energy for the kind of energy dense plants that form the base of an ecosystem supporting animals on top of it. It might support plant life, but not things like fruit trees or flowers. And none of it would be fast growing.

To avoid an empty world, something else must be going on

A world empty of everything but slow growing and stilted plants isn't very interesting to read about. You're going to need something else that makes plants grow. Seasons will likely be marked off, and 'years' pass, in relation to whatever that turns out to be. Here are a few possibilities.

  1. Rivers of ambrosia

These rivers have an unknown source (the sap of Yggdrasil, thermosynthetic life forms living atop a mountain that's an active volcano, gushing out of the ground in geysers, whatever you want) and they fertilize the soil.

To be agrarian, your people have to use ambrosia to fertilize the land. They mark time by the Barrel. Giant barrels, big enough to make the land fertile for a season of crops. Different epochs will be marked by different sources of ambrosia; as large amounts of time pass, this geyser dries up, another one takes its place, and thus Yosemite 217 is followed by Velikan 1.

  1. Manna from heaven

Like the old story, periodically the land is fertilized by literal manna raining down. But this doesn't happen often; your society marks the passage of time by Breadfall - each time manna falls marks what we consider something like a year.

  1. Supernatural creatures

Phoenixes turn out to be migratory birds that fly in flocks. Every once in awhile, they'll pack up and move. Wherever they roam, their very presence brings life to all around them. When one dies and is reborn, a circle about 1.4 miles in diameter will bear fruit for what we would call ten years.

Or perhaps there are clouds of frost-mites, insects bearing supernatural cold from the seemingly-endless glaciers. They move through an area like locusts, but after they've been through, mysteriously, the plants grow back twice as verdant.

The "seasons" are then based around the lifecycle and migration patterns and so on of the creatures forming the backbone of the ecosystem.


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