I'm making a planet where a day last 28 Earth days and I'm trying to figure out what could be the units of measure of time. The planet has these features:

  • A day last 28 Earth days.
  • There is no moon.
  • A year last 0.449 Earth years; however the axial tilt and the eccentricity are neglible, so there no significant "stagional" change.
  • The inhabitants' technological level is quite diconnected, but the standard is the Ancient Greeks' level.
  • Not sure it's useful, but the star that the planet orbits has a companion with a period of 483.66 years

These are the limits/requests:

  • They need to be practical. Everything should be based on something understandable by normal people, not scientists.
  • They should require the less technology that it's possible.
  • They don't have to be very precise. An error less than 5/10 % is acceptable.

The question:

what measures of time could be used?

I'm especially interested about measures of time that last less than a local day.

  • $\begingroup$ If it may be useful, there are additional information about the weather of the planet there: worldbuilding.stackexchange.com/questions/31915/… $\endgroup$
    – Eithne
    Mar 14, 2016 at 14:07
  • $\begingroup$ What light sources are used during the nights? With 14 earth days to a night, sleeping through is probably not feasible, so some light source will be vital and is also the likely timekeeping mechanic. $\endgroup$
    – Cyrus
    Mar 14, 2016 at 14:11
  • $\begingroup$ For light and heat, fire. But for light only, the most of the year the companion star shines with an apparent magnitude of -17.86 to -15.28 (the system is P Eridani). In some places there is also some energy for electricity avaible, but it's not an important source of light (in fact, it's rarely avaible). $\endgroup$
    – Eithne
    Mar 14, 2016 at 14:15
  • 1
    $\begingroup$ A statement like "a day last 28 days" can be rather confusing. Do you mean that a sidereal day on the planet lasts 28*24 = 672 hours (based on the Earth definition of an hour)? (I really recommend looking over the Wikipedia article on sidereal time; the concept is very useful when trying to come up with a timekeeping system for one's world.) $\endgroup$
    – user
    Mar 14, 2016 at 14:50
  • $\begingroup$ With "day" I mean the time that pass from a dawn to the he next dawn. $\endgroup$
    – Eithne
    Mar 14, 2016 at 15:09

3 Answers 3


Candles and star positions

Candles have been used as timekeeping devices for a long time. Experienced candle makers could create them to burn at a specific rate and mark the subdivisions with lines along the length of the candle. Candles would be available that burn for a whole day, half day, tenth day etc. Our month might be the white "day" candle with an arbitrary number of subdivisions that mark the equivalent of our days. 10, 20, 24, number-of-hands * number-of-digits, pick your favorite. Within each subdivision, another (red) candle would mark the hour-equivalent. So each of the 20 "days" may have 20 "hours". You might work on every day except tenth and twentieth on the white, from 2 o' red 2 until 12 o' red.

To synchronize time, towns have a timekeeper who watches the stars' positions, lights a new candle at the correct time and tolls a big bell telling all to light their new candle at home.

Additionally, other colored candles might be available to mark short periods of time. Green might last one "hour", a blue one 10 "minutes". These would be lit at the start of some event to measure how long it's taken. The plumber might charge you "by the candle" and light a green candle with the official plumbers guild seal when he starts, snuff it out when he's done, then make you pay for the difference.

Alternative: the Water Clock

If candles are too messy for your taste, water clocks are the best alternative of "Ancient Greek" tech level.

Rather than a slowly burning candle, these feature a slowly emptying (or filling) container with lines indicating the "hours". They need to be refilled (or emptied) to reset them. The Greeks used sundials to synchronize their clocks at refill time.

Compared to candles, water clocks have some disadvantages:

  • They are not nearly as portable.
  • They are vulnerable to freezing at night, meaning they can't be used outside during the night or left in a house that's not being heated.
  • You'd need a candle or fire anyway to be able to see what time it is during the night.
  • The extremes of heat and cold, while mitigated inside houses, would affect the accuracy of the clock as water expands/shrinks and evaporates at different rates.

I rather would try an historical approach. Our time system is very related to our ancient activities, and our biological ones: we work mostly at day, the seasons affects agriculture, the stars (related with this things) affected our religious culture, etc.

So, before design a realistic system, one must ask "what made it practical in the primitive cultures?".

  • Was there at least one agriculture cycle, which the calendar must be adapted to?
  • Wich was the biological rithm and activities, which the calendar must be adapted to? They are sleep-awake 8-16 hours, or any other range?
  • Was there any other cyclic phenomena that highly affected human life? Predators, climate, mating, etc.?

    Example: The egyptians have several calendars, the civic calendar was not very adjusted to reality, the religious calendar one was adjusted moire tightly because when the Nile river rise and overflow, it inundates the fields.

    There are speculations that the ancient priests works as a de facto division of power, using their knowledge as a power to control and limit pharaons.

Once answered, we can ask:

  1. In which way that affect the religion and culture, and then the original calendar?
  2. What historical events change the calendar?

I recommend you to read this articles:


Optical Clocks and Stars

I'm not entirely sure about the technological requirements, but I'm pretty sure that the Greeks had access to optics, (At least they get it pretty quickly when I play Civ). That said, I think that having a way to use a magnifying glass in a similar method to a sundial might work. Probably cheaper than all the candles, which was my first thought. But the magnifying glass would be reusable, additionally, because you could concentrate the light into a really small focal point you would be able to have much smaller marks on your "sundial." So for example, every inch could have a mark that is a half an Earth day. And you could have smaller marks for more precise units, like quarter days, hours, etc.

I would hope that the path of the sun was more or less consistent, but even if it wasn't it could probably be accounted for. At night, they would likely use the stars positions, as denoted in Cyrus' answer. Issues would of course be things like overcast and inclement whether. Also, I'm not really up and up on the physics of optics, so I'm not entirely sure if you would need a really large magnifying glass to capture and focus the light into a small enough point for it to be useful, and it could get quite hot, so you might need to make sure there's a nonflammable stone or metal to use for the marks. This would also need to be outside in a relatively safe spot where it can be guaranteed not to be tampered with (glass and all that).

If all that seems workable, I think Magnifying and star positions could be a good choice.

As a second choice:

Foucault Pendulum Clocks


Perhaps your people have the technology necessary to create one of those. Since you have a day night cycle, you are likely to have a rotating planet. This can be used both day and night, but it would require some way to replace the energy lost each swing without disturbing the path which is tricky given limited technology. Also, this would likely need to be HUGE to have enough meaningful markers on it so you might just have one in the center of the village or city. Additionally, you will almost certainly have to hire guards to make certain no one messes with it (because you just know some delinquent trying to skip class or work is going to try and speed up time). This one is certainly more of a stretch, but if they could make it work, the clocks could be become something of an interesting city center for tourists.

But yeah, give those a shot and see how they work for you.

  • $\begingroup$ I just reallized that there is a companion star in this system. Ugh. I'm not certain my optic clock idea will work now. I believe it could be possible to account for it, but, it would require more science that I believe they would not have. But maybe not. Can someone smarter than I note how difficult that would be? $\endgroup$
    – JGaines
    Mar 14, 2016 at 19:22

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .