telling time on a tidally-locked world

my setting is a tidally-locked planet. due to this, the sky is pretty much eternally dusk, with various moons passing by as they orbit. how would one tell time in such a setting? there's no real basis for days due to the sun never moving. similarly; would moons wax and wane on this planet?

If moons orbit they would still wax and wane, that depends on the relative positions of the moon, the planet and the observer. I suppose depending on where you stood you would see a moon or moons pass through their phases as they orbit... which would take a lot longer subjectively speaking since it's only based on the moon's orbital period.

As for time telling, its going to depend on circumstances. If this planet was colonised by people from Earth (for the sake of argument) it would simple enough to declare the entire planet a single time zone and synch their clocks to a standard 24 hour day. Those living more sunward would have to deal with a perpetual midnight sun and nightsiders a starlit noon, but it can be done. Just see how its done in extreme northern towns and cities in Scandinavia, Alaska and Canada. You might even have a triple shift thing going on because there's no difference between getting to work at midnight vs 8am when the sky is the same not matter what.

I assume you have solved (or are ignoring) the problem that a world that is tidally locked with respect to its primary star will have extreme heat on one side and extreme cold on the other. If it has an atmosphere, there will be a perpetual storm system transporting hot air to the cold side and vice versa, with winds of hundreds of miles per hour perpetually sanding the surface down to perfect smoothness. (You could also have an extremely dense atmosphere, like Venus, to buffer, but then everything on the ground is crushed, cooked, and dissolved.)

If anyone could possibly live on such a planet, it would be deep underground where telling time by the sun and stars would be a moot point anyway.

Let's assume we do ignore that. People tell time by rhythmic phenomena. The orbits of the moons would be exactly that. Also, on the night side, the stars would be visible and change positions as the planet moved in its orbit. These would both be time-telling mechanisms. For a tidally locked world, a year equals a day... sort of. It doesn't have seasons, but its orbit around its sun could serve as a way to track the passage of time.

Of course, you could only see the stars on the dark side, possibly the moons as well depending on how close you are to the sun and their relative sizes and distances. It's possible that cultures could develop in those places who strongly disagree about the passage of time. "What is this thing you call a 'year'? Everything is eternal and unchanging." "Come to the dark and see things change." "I will not leave the light, heretic!"

FYI, about moons. A lot depends on the relative masses of the moons to the planet, and how near the planet is to its sun, because these determine the relative strength of the tides from each, and tides are what cause a body to become tidally locked. The Earth feels our Moon's gravity much more strongly than the Sun's in this regard, and were it not for the Sun's inevitable death, over time our day would become exactly one month long.

A planet with multiple large moons would probably not become tidally locked due to the competing influences, but would enter into a stable pattern, like 3 rotations for every 2 orbits or something. A planet that is tidally locked to its parent star and has tidal moons would be... unstable. I'm not sure how that could happen realistically.

Edited by Fighteer on Apr 29th 2020 at 10:11:49 AM

There's always water-clocks.

Going by this, a three body system where most bodies are at tidal locking distance is not stable - one of the bodies would tend to be ejected. In your case that'd be the moons. Also, tidal locking produces a lot of heat that can sterilize the body ("Tidal Venus"). Mind you, in a system like the Solar one the time it takes for the moons to be ejected can be much longer than the lifespan of the star. I am not sure how this all works for four or more bodies.

I have to assume that the moons in this hypothetical system are not tidally significant. Of course, if that's the case, they are probably also very small and thus not visible from the lit side of the planet.

The problem is that the sun in this scenario would also exert tidal forces on those moons, tugging them harder on one side of their orbits than the other, and that gravity could (should, even) disrupt their orbits over time.

Edited by Fighteer on Apr 29th 2020 at 10:17:40 AM

these are all great replies, thanks! im okay with getting rid of moons if they're only gonna cause more problems. in terms of civilization, they live on the strip between the hot side and cold side (i know its not totally realistic), and originate on the planet rather than coming from earth. an important part is that the planet has an abundance of giant mushrooms due to exceedingly fertile ground. is there a way they could be used for time-telling?

You mean plant growth in general? Maybe. There would be no day or night, and no meaningful seasons, so it'd just be a steady process.

I know you're ignoring the actual physics of a tidally-locked planet: that border region would be continually beset by hurricanes hundreds of times more powerful than anything ever seen on Earth, so that any organic life would have to be semi-porous. I also wouldn't worry about the moons too much if you're already taking this kind of liberty.

Given that that's not an issue, I'm not clear how visible things like moons and stars would be: you might have treks into the Dark Lands to gaze at the stars and watch their stately, annual march across the perpetual night sky.

There could be cyclical events on a less macroscopic scale, such as rains, storms, floods, etc., but from an astronomical perspective, not much else. You could also have the planet's orbit be slightly irregular so that it does experience seasons of a sort.

Edited by Fighteer on Apr 29th 2020 at 1:51:44 PM

Am i weird for my first thought being "Clocks"?

But given how much of our perception of time comes from day cycles, moon cycles and seasonal cycles, a species on a tidally locked world would have a very different idea of time. Still, other cycles could still exist, such as stable weather cycles due to small variations in the orbit, or cycles in the star itself.

i was thinking clocks too, to be honest. this thread has helped to realize that...theres not really anything solid enough to base time on, so i thought itd work if they instead use time to split things up.

so far, i have a clock with days split into 20 hours, 10 for activity and 10 for rest. public resources run off this clock. similarly, a month is 20 days and a year is 20 months (not for any specific reason other than sticking with the number 20).

Unless you have a global monoculture, I'd expect different societies to develop completely different timekeeping methods, since there's nothing to use as a universal referent. The Jorfans would ask the Boltoks to meet them again in seven septoons, and the Boltoks wouldn't have the foggiest idea what they're talking about.

The idea of a "day" wouldn't really exist. Animals wouldn't evolve any consistent circadian rhythms. If sleeping is a thing in their biology, there wouldn't be any regularity to it; either every species or group would develop its own cycle or each individual would rest whenever it felt like.

Heck, sleep might not even evolve at a biological level.

Edited by Fighteer on Apr 29th 2020 at 5:32:09 AM

thats fair. the story would only focus on 1 civilization, so im not developing more than one

also, the species is human-like, enough that they cant go forever without rest. the clock is just a way of organizing society (for ex. how can you hold public meetings when half the towns asleep?)

That's completely reasonable. Some method of timekeeping would be necessary for a society to organize its activities, but that method may not be linked to any natural units, as are our days, months, and years.

Edited by Fighteer on Apr 30th 2020 at 9:42:05 AM

Well, it's not like our hours and minutes and seconds are natural units of time either.

Also, i disagree that there would be many different time keeping systems. Much like in the real world, different systems would start to align and there would be much benefit in a planetary standard time. Except that since time zones would not be a thing, and any kind of time would be arbitrary, it might even be easier to accomplish a planetary calendar.

So maybe in a pre-modern civilization there would be many calendars and clocks (like on earth), but this would rather rapidly fade as the planet advanced and much like (and faster than earth), they would make a universal time. Standardization of units and measuring systems was an essential step in the creation of the scientific method.

I also think that biological functions such as sleep would still be needed. However since choice of cycle would be arbirary, you might have ecosystems with very complicated sleep cycles, some animals with very long and some with very short cycles living side by side. One may sleep once per unit of time, others 2,3 10 times.

Edited by devak on Apr 30th 2020 at 8:38:14 PM

The division of days into hours, minutes, and seconds may be arbitrary, but the day itself is a natural unit for people living on Earth, as are the month and the year. Any two people with no knowledge of each other's culture or language could easily learn to communicate about time using those references.

By the way, there are organisms on Earth that have adapted to fast moving currents and tides (they anchor themselves on convenient rocks), and other species that prey on them. Point being that massive hurricanes are just another set of natural conditions for life to adapt to.

We've stipulated that there are no massive hurricanes on this world, but the big problem there wouldn't be the ability of life to adapt per se, but the fact that the constant, extreme winds would erode any exposed land down to sea level. The surface would be sanded smooth.

Edited by Fighteer on May 1st 2020 at 9:18:16 AM

Inspired by but not a fixed length of time. There are several definitions of "year" astronomically speaking be it tropical year (equinox to equinox or solstice to solstice), sidereal year (time it takes for the Sun to reach a "fixed" point amongst the backdrop of stars), or orbital year (time it takes for Earth to finish one complete orbit in terms of distance).

The concept of "days" also has no fixed length or size. The concept of day measured from solar noon to solar noon can vary wildly over the course of the year, near the solstices this time is actually longer than 24 hours. The concept of day measured in terms of a star reaching a specific point in the sky (say zenith) is a fixed measure (23 hours, 56 minutes) but over time is unreliable for purposes of timekeeping.

Months are just conglomerations as are weeks. The only natural analogue to a "month" is basically Lunar Month, the time from Full Moon to Full Moon and even that varies. (Lunar months are why the oldest calendars in use today are lunar or lunisolar.)

i appreciate the world-building help, but i'd prefer to not include it on this thread, since i already have an idea for the world set

For a "primitive" culture, they worry about the position of the sun and moon in the sky and the changing of the seasons. Those are natural units for all purposes that count. Once they start keeping time in a more formal way, those problems start to develop, leading to more rigorous systems. But our "month" is originally based on the cycles of the moon, so it counts.

The point here is not to debate those things, but to think about what sort of natural time units a tidally locked planet would have, and the answer is: neither a solar day nor a solar year.

• Lunar months might be a possibility depending on how visible the moon(s) are. A moon large enough and close enough to be visible during the day would have significant tidal effects, which would probably prevent the planet from becoming stably tidally locked to its sun.
• Sidereal years are also possible, but would require traveling away from the terminator into the permanently dark and frozen lands to view the stars.

Edited by Fighteer on May 1st 2020 at 12:34:42 PM

I still don't think winds would necessarily be a huge problem on a habitable world. The combination of right size, right atmosphere (composition and density) and the planet itself could lead to properties that vastly reduce the effect of the large temperature swings. So between the hottest (substellar) and the coldest (antistellar) points could have a difference of around 50 to 100 degrees. The water cycle would ensure that the substellar is essentially covered in thick clouds with high albedo, while the winds would serve to distribute heat across the planet (and thus also in turn reduce the driving force behind said winds).

The downside is of course that a very cloudy hot side would be hard to tell time under.

There may be tremendous storms, but I would imagine that they would follow the same rules as storm everywhere, on Earth or Jupiter. One thing is that they would likely be cyclical in nature, rising and falling in response to the more or less regular process of large scale heat transfer. Any periodic natural phenomena will give rise to the cultural concept of "time" and change over time, even if they are not perfectly regular. After all, the seasons are not perfectly regular here on Earth, and they are an important meme of change.

It occurs to me that what this culture might develop is a concept of "irregular change". That is, in contrast to most humans, time passes and things change but not in easily measured identical units. Temporal change might be seen as a more organic, unpredictable force in affairs. Ultimately nothing is absolutely predictable, one cannot control the future, but time must be ridden like local flying aviary equivalents on the never-ending wind.

For a human-historical analog, see the philosophy of Heraclitus.

Edited by DeMarquis on May 2nd 2020 at 12:33:17 PM

While they follow the same rules, the outcomes are vastly different. There would be no hurricanes like here on Earth, as the planet would not have any meaningful Coriolis effect. The clear temperature gradient and the unidirectional flow of air would ensure that storms look very, very different from anything we see on Earth. It would heavily depend on the planet's oceans and seas (if any), but generally speaking storms would either be fairly small due to local variations, or gargantuan horizon-spanning ones due to large variations. They wouldn't rotate (which normally allows a massive buildup of energy), but could last days or more.

But yea, if the star has a dimming/brightening cycle like our sun, it would probably have a much higher influence, and this could lead to time keeping based on such solar cycles (albeit indirectly)

A tidally locked planet is still rotating and still has a Coriolis effect. What might prohibit the development of hurricanes on such a planet is wind shear: There'd be a monsoon-like wind system between the "day" and "night" sides of the planet, with air at low elevation flowing towards "day" and returning to the "night" side aloft. These intense winds blowing in different directions at different elevations will prevent the development of tropical-like storms.

That said, one thing to consider is that the planet will most likely not be "perfectly" tidally locked. It's called "libration" for Earth's Moon and on a tidally locked planet it will result in day and night like fluctuations of stellar brightness at the terminator.

What if there was a Hot Jupiter in an orbit in between the sun and the tidally locked world, making periodic eclipses?