My question is as stated above: if a planet is tidally locked to its sun, how would the people on the surface, assuming it's habitable, be able to tell time? What devices would they be able to use? Obviously a sun dial is out of the question.

I am also curious as to how they would develop a concept of time because they wouldn't be able to note the passage of time nearly as easily. I understand things would still grow and change, but this would take much longer.

So, assuming they evolved on this planet, what would a device look like to measure time on this planet, and how would this device have come about to begin with?

  • $\begingroup$ may be wrt other stars or planets moving visible to them.. $\endgroup$
    – Amruth A
    Commented Nov 2, 2017 at 6:04
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    $\begingroup$ A tidally locked planet without a moon (I guess or you would've chosen the moon) where intelligent life has developed is a fantasy concept. Could you introduce us to your world in greater detail because of that? I guess the sun is supposed to be as bright as our sun? (realistically it would be very far away (and only tidally locked by random chance) and one would be able to see stars in broad day light, but again, fantasy concept). Where do the people live geographically? What's the weather like there? How developed are they in general? $\endgroup$
    – Raditz_35
    Commented Nov 2, 2017 at 9:39
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    $\begingroup$ Since BobDylan specifies a primitive (developing) society, part of the question and/or answer is "How do they even gain the concept of time passing?", since that would then grow into a system for expressing the passage of time. In that context, this is most definitely not a duplicate question, but BobDylan, you will probably need to modify your question to make it clear as not a duplicate to get those flags taken off. $\endgroup$
    – Rissiepit
    Commented Nov 2, 2017 at 12:01
  • $\begingroup$ They evolved there. Yes, I am asking about the devices people could use, or ideas people would develop around time. It just seems that it would be very hard for them to do this as they couldn't note a visible change between night and day. $\endgroup$
    – BobDylan
    Commented Nov 2, 2017 at 20:44

4 Answers 4


Please remember that time is a somewhat arbitrary concept that has only been refined through mathematical expression (and the fact that so many "constants" are wacko numbers to the nth decimal place might mean that our "second" is still fairly arbitrary). Therefore, your people really only need to pick a quantity of divisions and use a divisible resource (see above) to mark those divisions and voilà, you can tell time.

  • $\begingroup$ I was very lost, and you made that very obvious to me. Thanks much! $\endgroup$
    – BobDylan
    Commented Nov 2, 2017 at 8:13

If the planet is habitable it is probably only habitable within a ring around the terminator where it's neither too cold nor too hot.

From there it should not be difficult to see stars and other planets and be able to tell years and months from their movement.

If the planet has one or more moons they can be used as well to measure timespans from months to days.

If the planet's orbit around the star is excentric enough there might be stronger variations in solar irradiation leading to periodically changing weather patterns, e.g. in the winds crossing the terminator and the overall temperatures, leading to some sort of seasons.

EDIT: About devices used, for the first two points that would just be astronomical devices or simply their own eyes, and in general they could just simply count e.g. the rise of a certain star or planet or moon, or the arrival of a certain warm wind or cold wind. All this could then be the basis for more artificial means of counting, e.g. by taking a moon period and cutting it into shorter timespans of equal lengths by using e.g. sand clocks or water clocks. Once certain shorter timespans are defined, they can again be shortened and with rising technology I can easily imagine mechanical clocks appearing, just like humans did on Earth. Once a basis for measuring timespans is established I don't see why they wouldn't invent the same kinds of devices as humans on Earth did. The main differences just lies in establishing the initial cycles everything else is based upon and those depend on the planetary circumstances.

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    $\begingroup$ Even minor eccentricity of orbit would also cause libration that would be noticeable by naked eye - sun would move a little. $\endgroup$
    – Vashu
    Commented Nov 3, 2017 at 9:08

They might think about time entirely differently from us--as something that comes from people. With no non-living examples of predictable change (assuming there are no cyclical weather patterns or seasons as well as no day/night cycle), these people might talk about time the way they talk about feelings e.g. "Gathering nuts takes a long time because it's boring." Standard time might be first tied to growing trees, which get obviously and predictably taller. Maybe every tribe has a "clock tree" it uses to make sure everyone has the same referent. If you want time cut into pieces smaller than "growing a tree," perhaps they could measure time by heart beats or eye blinks (depending on their anatomy) how long it takes to get hungry after eating again, and how long it takes to gestate a child (or incubate one or whatever). More accurate and precise water clocks or candle clocks (or indeed clockwork clocks) won't be technically any harder for them to produce than they were for us, but I bet the inhabitants of this world will take longer to develop accurate and precise timekeeping, since they'll start from the assumption that time governs only earthly things, rather than heavenly.



The US measures distance in feet. However, even if humans had not grown feet, the general concept of distance would still exist.

Similarly, if there are no solar days on this planet, they wouldn't be using a unit of time based on a solar day; but that doesn't mean that the locals are oblivious to the general concept of time progression.

How are people able to keep track of which day of the week it is, since a week is an artifical construct that does not occurr naturally?

The answer is that it is inherent to tracking time. When you track time, you are inherently aware of what time it is at a given point.

if a planet is tidally locked to its sun, how would the people on the surface, assuming its habitable, be able to tell time?

Even though our planet experiences sunrises and sunsets (and your tidally locked planet does not), our method of keeping time is no longer connected to the sun rising and setting.

We've created completely independent time tracking systems (atomic clock, quartz crystal, expressing time as a function of lightspeed, ...), and we've simply decided to retain the traditional time units that used to be relevant when we tracked time based on the position of the sun.

There is nothing that stops humans from changing the time units to something completely artificial and unrelated to our planet's movement relative to the sun. We could e.g. decide that a year is now 400 days long for the sake of mathematical simplicity (i.e. the length of a day is unchanged, but there are now more days in a year). Although a year (400 solar days) would then no longer accurately reflect Earth's orbit around the sun, there is no problem with this change from a time tracking perspective.

What devices would they be able to use? Obviously a sun dial is out of the question.

Sundials are passive clocks that do not need to be set (other than physically placing them). Which is nice, but much too inaccurate for our modern day standards anyway.

Your civilization can use any other clock that we use. Since they have no natural frame of reference (e.g. sunrise/sunset), they are free to choose whatever method they want. The only thing that's important is that the used time tracking method must independently yield consistent results. If you and me synchronize our watches, split up and meet again in the future, our watches should still be in sync (I'm omitting relativistic time dilation for the sake of simplicity).

So, assuming they evolved on this planet, what would a device look like to measure time on this planet, and how would this device have come about to begin with?

As I said, most human clocks nowadays do not observe the movement of the sun. They simply happen to be using a unit of time that coicides with the duration of a solar day.

Therefore, you can use any timekeeping system which does not rely on solar positioning:

  • Quartz crystals (the crystal's resonance defines the passage of time)
  • Atomic clocks (the atoms' resonance defines the passage of time)
  • Pendulum clocks (as long as gravity is constant everywhere on the planet at a given altitude, a pendulum can define the passage of time)
  • Hourglasses (as long as the material consistently takes the same amount of time to go through the hour glass)

I am also curious as to how they would develop a concept of time because they wouldn't be able to not the passage of time nearly as easily.

I don't quite agree with you. Even if our sun would not have moved, or would have moved erratically, animals would still have needed to rest from time to time.

Because we have a consistent sunrise/sunset, animals have historically adapted to setting their biological clocks to this rhythm, for the sake of simplicity (so that e.g. our eyes only really need to work in daylight, and less so in darker conditions). If we had evolved a sleeping pattern that was unrelated to the sun's position in the sky (e.g. awake for 30 hours, sleeping for 10 hours), then our bodies would simply have evolved to deal with life both at night and during the day.

Time is not inherently tied to planetary movement. Time is nothing more than a linear progression of events. It just so happens that we decided to use our planetary movement as a unit of time.

Analogously, the US measures distance in feet. However, even if humans had not grown feet, the general concept of distance would still exist! We would simply have used a different unit of distance, possibly either related to a different body part ("This wall is ten heads long and fifty heads wide") or using an artificially decided unit of distance.

Interestingly, the meter is such an artifically constructed unit of distance:

In 1799, [the meter] was redefined in terms of a prototype metre bar (the actual bar used was changed in 1889).

People literally just made a metal bar, and then said "the length of this bar is now called a meter."

A definition for a meter was then drafted, which may have tweaked the actual distance of a meter, since they used a rounded definition: one ten-millionth of the distance between the equator and the pole. However, before they decided to define a meter as such, they were already aware of roughly the unit of measurement they were looking for. If the outcome of this definition would have been 1mm or 1km, then they would have used a different definition for a meter, rather than define the meter as whatever the outcome of the definition is. Therefore, the decision came before the definition.

Why was this bar exactly that length? No reason. They just had to pick anything, really, so they picked this particular distance.
The only real consideration that was made is that the chosen distance was sensical to humans, e.g. it's impractical to define a unit of distance based on something a human cannot perceive, whether it's the width of an electron or the diameter of the sun. These are not intuitive because we have no way to easily see this distance or use it for comparative purposes.

As it is defines, the meter makes sense. It's roughly as long as a human leg, which means that we have a somewhat accurate representation of the meter on our body (just like the US foot).

  • $\begingroup$ The length of the meter wasn't picked arbitrarily. When they were defining the length of the meter, the initial definition was one ten-millionth of the distance between the equator and the pole (ie, a distance of 10,000 kilometers), and then changed that to the length on a bar a few years later. When they came up with this in 1793, they didn't realize the planet wasn't a perfect sphere, but they were close: that's why the circumference of the Earth is 40,007 km through the poles, and 40,075 km at the equator. $\endgroup$ Commented Nov 3, 2017 at 14:51
  • $\begingroup$ @KeithMorrison: You're putting the cart before the horse. The meter wasn't decided because it's one ten-millionth of the distance between [...] (what would be the significance of using such a distance? It's a meaningless distance to humans). A meter was decided as an approximate length, and only then did they try to express that length as something that is universally measurable and reproducable. You're right about the first official definition of the meter, but that only came after deciding what a meter should (roughly) be from a practical point of view. $\endgroup$
    – Flater
    Commented Nov 3, 2017 at 14:59
  • $\begingroup$ I was responding to the statement that people just marked up a metal bar and decided that was a meter. That statement is wrong. The bar was that length because the calculation they made told them the bar was to be that length. They were slightly off on the calculation, which is why a century later they decided to just go with the length on the bar. It says all this in very article you linked to. $\endgroup$ Commented Nov 4, 2017 at 2:18
  • $\begingroup$ @KeithMorrison: Before they decided on the definition of a meter, they first considered an estimate of what would be a practical unit. They didn't just calculate one ten-millionth of the distance between [...] and then decided to use that distance, regardless of whether the outcome was 1m, 1mm or 1km. They already knew that they were going to get a sensible unit of distance before they considered defining the exact distance. They first decided on a practical unit, and only then worked at creating an objective definition for that practical unit. $\endgroup$
    – Flater
    Commented Nov 6, 2017 at 8:52
  • $\begingroup$ @KeithMorrison: Note that your comments are not wrong, but you are referring to the definition of the meter, which is not the same as the initial rough estimate of what would be practical. I have updated my answer to disambiguate the two, I hope that settles the matter. $\endgroup$
    – Flater
    Commented Nov 6, 2017 at 8:55

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