If a species were to evolve on a rogue planet, how would they track the passage of time? Without day/night cycles or revolutions around a star, they couldn't use the typical methods that civilizations on a planet orbiting a star would.

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    $\begingroup$ I know it's not the question you asked but you might want to explain how they exist/evolved (considering they would be on an ice ball a few degrees above absolute zero) as it may shape the answer. $\endgroup$
    – Tim B
    Oct 14, 2014 at 19:01
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    $\begingroup$ Especially the condition of the planetwhen the species became sentient would be important. $\endgroup$
    – overactor
    Oct 14, 2014 at 19:03
  • $\begingroup$ And you might need to narrow this down a bit - it seems likely to get closed for being too broad. $\endgroup$
    – mechalynx
    Oct 14, 2014 at 19:15
  • $\begingroup$ It could end up being closed as either, actually; I voted to close as unclear, because in my opinion this question definitely needs a bit more context to be properly answerable. @TimB pretty much nailed it with his comment. $\endgroup$
    – user
    Oct 14, 2014 at 20:16
  • $\begingroup$ @TimB Vsauce actually did a video on this, you can find it here. In his video, he mentions that life could still exist near geothermal locations, such as iceland, or volcanic vents near the bottom of the ocean. $\endgroup$ Oct 15, 2014 at 12:42

7 Answers 7

  1. Stars...either a constellation that 'rises and sets' just like the sun would, or a single star that is nearby but still quite a ways away. Remember our sun drowns out much of the star scape to us, a rogue planet would have a very bright star sky. The passage of the milky way across the sky could be one usable example.

  2. Life spans. This gets into the 'foot' being the kings foot for a unit of measure. A ruler's lifetime could be referred to as a unit of measure. Of course this means the unit of measure is inexact (so was 'foot'), but it is usable.

  3. Moons. If they are available, a moons cycle is a great time tracker.

  4. Arbitrary. 1 second is a relatively abstract concept (same as '1 meter'), as we've set that this is what a second is and we're using it (our second finds it's roots in early Sumerian culture where they divided things in to 6 sets of gods (one male one female) * 5 (number of fingers or number of man) = 6*2*5 = 60 seconds. Same root as 360 degrees and 12 * 2 hours in a day.

I guess the question of 'what does a civilization measure with time' becomes necessary to ask. Is time on a timeless planet really necessary? Is time nothing but arbitrary? If it's a 'slow' species (sloth?) it's possible the arbitrary unit they come up with is significantly longer for a second than a much more hyper race.

  • $\begingroup$ A planet like that would likely have to have moons or something similar to generate heat. Moons would likely be the answer here. $\endgroup$
    – House
    Oct 14, 2014 at 22:23
  • $\begingroup$ 1 second isn't completely arbitrary when you consider the origin in ancestry using base-12 numbering (which is where we get 24[12*2] hours, 360[12*30]ish days, 12 months, 60[12*5] hours, etc). Of course, the modern second is now either defined by so many oscillations of a particular energy state of some crystal, or the time for light to travel a set distance, I forget which. $\endgroup$
    – user2051
    Oct 14, 2014 at 22:42
  • $\begingroup$ @Byte56 - potentially...though rogue planets are usually formed in a system and then ejected through gravitational forces...not something that would be likely have moons at the start. Might pick up asteroids along the way though. $\endgroup$
    – Twelfth
    Oct 14, 2014 at 23:44
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    $\begingroup$ With no star producing light to reflect, moons would be just dark shades on starlit sky. The starlight would be somewhat brighter, but tracking this shade would be too difficult to be likely to be used as a calendar. However, astronomers might study the moon's movement well enough to be able to derive some abstract units (hours, seconds etc.) from it. $\endgroup$
    – Pavel V.
    Oct 15, 2014 at 7:08
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    $\begingroup$ LifeCycles - Maybe Crops life cycle could be a big player. After all, our own perception of time is heavily influenced by the behaviours of plants (e.g. the 4 seasons). Might be more consistent, and far more granular than leaders life. $\endgroup$
    – NPSF3000
    Oct 15, 2014 at 9:12

The conservation of the planet's angular momentum means that your rogue planet will spin whether the planet was flung from a star system or developed on its own. Without a sun, the stars will be clearly visible at all times. The planet will have a day based on its rotation to the distant stars, a sidereal day (which Earth also has, in addition to a solar day) that is nearly as obvious to any developing civilization as the day and night are. They can divide this day into smaller units like our seconds and larger units like our years.

However, instead of the larger units being based on the Moon (months) and Sun (years), years are more likely to be based on simple multiples of whatever base-counting system is common there (e.g. 100 or 360 days). Without the Moon, Sun, and seasons to keep things semi-consistent, different developing civilizations will likely have widely varying customs here: e.g. some early cultures may count generations, life spans, pregnancy terms, or plant seed to harvest times, or not have a year-like system at all.

Further, the species will likely not naturally have a daily circadian rhythm like we do. While observing stars is something any intelligent being can do, it's not going to be something important to the evolution of life on the planet. They may not sleep at all, and only rest in intervals that would seem strange to us. However, in order to run an efficient advanced society (anything after, say, an Industrial Revolution), it seems likely that they'd have standard working hours and resting/personal hours, synced to the planet's day.

Depending on how developed and globalized the culture(s) on your planet are, this may have long since been standardized to one or two systems (e.g. our methods of measuring are mostly Imperial and Metric now) or still have many coexisting systems (e.g. our currency).

This assumes that space can be seen from the planet. If the planet is under total cloud cover (as may be needed to keep a rogue planet warm enough for life-as-we-know-it), everything about a sidereal day and stars can go out the window and it's only after the advent of cloud-penetrating technology that any day-like structure can be easily observed. By this time, they'll likely have timepieces that use arbitrary units.


While a species on a rogue planet wouldn't have the usual reference points for time, there are still many ways to track time and many reference points for it and all it requires is having a sense of the passage of time.

Environmental indicators

Assuming the planet is kept warm through internal processes, if these have a periodicity, they could serve as a reference. For example, if the planet has a liquid mantle that causes volcanic activity, they could use eruptions, earthquakes and the like as reference points, provided their is periodicity to them.

The same can happen with the biosphere as it forms a complex system. In systems and control theory, we study the behaviors of simpler systems and how they react mathematically:

step response

This is the response of a lower order system to the step function (which is a fancy way of saying a change of input between two values and remaining at the second one). In higher order systems, with more complex input, the oscillation can be sustained, instead of diminishing as you see in the diagram. This is especially common when a system has a positive feedback loop (which is a fancy way of saying that energy is added to the system proportionally to its output - you get 5 tigers, so then you add 3 - now you have 8, so then you add 5 more).

Considering that on a rogue planet certain dampening factors or external influences may be missing, such biosphere oscillations may remain stable for long enough for this civilization to observe them and notice their periodicity and how long each cycle lasts. After all, even the celestial objects and day-night periods we used for timekeeping are not always of the same length and their average length changes with time (just like the Moon flies away from us a couple of centimeters per year, but that doesn't prevent us from using it for accurate enough time measurements).

Everyday life

Just like we do today, this civilization may develop some standard for time. This would be necessary for trade, which is why we developed many sciences in the first place. A simple time measurement would be to define, for example, the time it takes for a fuse of specific dimensions and material to completely consume itself as it burns. Such standardization would initially depend on such everyday needs but could also include periods such as the incubation time for eggs, the time to carry a child to birth, the time it usually takes people to sleep and wake up, the time it takes to walk to places etc. - these could serve as a base for longer time measurements.


Of course, eventually they would develop complex physical understanding of the world and require precise timekeeping. There is no reason technology wouldn't serve them as well as it has served us in this case. There would already be agreement on some standard measurement of time within cultures and probably globally wherever trade has spread.


The key thing about tracking time really boils down to a type of pattern recognition, we had sunrise/sunset as early time "markers" but a rogue planet will lack the larger astronomical patterns to base time off of; instead they will likely use something more akin to our early measures of weight or distance (the cubit was length from elbow to the tip of the middle finger) these varied but were good enough based on the average at the time.

Initially their understanding of time may be based off of sleep/awake cycles for something like a day and a year might be based off of a "yearly" reproductive cycle or hibernation. In general the time they keep will be related to things they can initially "track" with their own senses (another "year" passes when they lay eggs again).

As time goes on and they improve in terms of science they may then base it off other principles like the half-life of certain radioactive elements or the frequency of vibrating crystals (like quartz at 32,768 Hz http://en.wikipedia.org/wiki/Quartz_clock)

  • $\begingroup$ That isn't an inherent property of quartz crystals. Those crystals are "tuned" by cutting them to a size where the resonant frequency is 32.768 kHz because it is convenient to build a binary divider circuit dividing by 2^15 to get a 1 Hz signal. It can't be used to define a time unit, unless they were to pick a single specific crystal (like the prototype kilogram). $\endgroup$ Oct 15, 2014 at 1:45
  • $\begingroup$ Oh, well thanks for the clarification; learn something new every day =D $\endgroup$
    – Culyx
    Oct 15, 2014 at 13:50

First of all you should not ask how, but why?

You mean a rogue planet without the sun, where the only source of energy for living beings is the geothermal energy from beneath the planet's surface? If the energy flow is relatively stable, there would be no need for any calendar measurement similar to ours. Days, months and years arises from the need to adapt the life to the cycles of the nature.

It's quite unlikely that the beings on your planet would have eyes. Eyes are not very useful on the planet with minimal amount of light, there's more use in developing other senses. Without day and nights, sleep cycles like ours are unlikely to develop.

If there are regular cycles in the geothermal energy fluctuations, your folks would develop the calendar based on those cycles.

And when they develop technical civilization, they would adapt time measurement techniques adapted specifically to the timeline of those processes.


Candles, water clocks, sand timers - 'hour glass', migration patterns, growth of vegetation - a 'clock plant' if you will, all of this could be highly abstract at the stage of development of your civilization - in that the smallest unit of time might originally be the length of time it takes a river to fill a vessel with a specific size of hole in it but by the era of mechanical technology might be only referenced by name - having been replicated by gears and springs.


The planet will still spin on it's axis, so you can still define the sidereal day (relative to distant stars). The planet might be orbited by naked-eye visible moons or other debris which you can use to define longer units of measurement, but there might just as well be no obvious longer cycles to calibrate time against. However there are some other astrophysical phenomena you might use.

The most obvious would be things like Cepheid variable stars. Because the maximum brightness of a Cepheid is directly proportional to it's pulsation period, choosing say "the dimmest Cepheid visible to the naked eye" could be one way of defining a week. Of course that uses relative rather than absolute magnitude, which would be a source of error, but it is possible to determine absolute magnitude. Cepheids have periods ranging from a couple of days to a couple of months, and are very consistent.

Similarly there are Mira variables, which have periods measured in hundreds of days. Instant year.

Whether a long unit of measurement, like the year, is actually useful in an environment which has no seasons is of course another question entirely.


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