I'm playing with an otherwise earthlike world where I have handwaved away the visible sun and moon: anytime you look at a cloudless sky, you see stars, and only stars. How can the people of this world use the stars to tell time?

Thanks to Artifexian's videos on the subject, I'm already familiar with constructing lunar, solar, and lunisolar calendars. I'm interested in finding similar information for what a stars-only calendar might look like. Info on smaller-scale demarcation of days and hours would also be useful. I know that certain constellations in the zodiac correspond with certain months, and that one can tell the hour of the night by looking at the position of the Big Dipper, but both of these methods require cross-referencing with solar and lunar timekeeping. Could one tell time in this way using only the stars? (I'm willing to include "wandering stars" or visible planets.)

Note that the people of my world are very low-tech but reasonably intelligent. They are restricted to naked-eye observations.

  • 1
    $\begingroup$ What is energy source? Removing star remove heat and light needed for plants. Cold starving death. Removing moon removes tectonic and tidals. You are left with nuclear and chemical sources but they are not enough to sustain life. That means magic - use magic to make time counting. $\endgroup$
    – k_z
    Commented Mar 1 at 14:53
  • $\begingroup$ the problem is that stars are insanely far away to produce retrograde effect, then planets that do retrograde might not show up in the sky for months or even years, so the moon is your most reliable extraterrestrial timekeeper yet. $\endgroup$
    – user6760
    Commented Mar 1 at 15:08
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    $\begingroup$ Does your planet still rotate? Does the surface of your planet experience alternating periods of light and dark, in some way equivalent to day and night? Does your planet revolve around something in the same way it would a star? Does your planet experience seasons? - Possible answers are going to depend on these things. $\endgroup$ Commented Mar 1 at 15:21
  • $\begingroup$ I'm not sure what time scale your world is working with but it is also worth noting that at least for a planet in a orbit like Earth, stars will eventually drift in a more permanent way as well (I don't know how fast that is off the top of my head though $\endgroup$
    – Informer
    Commented Mar 1 at 15:36
  • $\begingroup$ If there is no object bright enough to act as a visible sun for this planet, what's making the other planets visible to the naked eye? $\endgroup$
    – David K
    Commented Mar 2 at 17:05

2 Answers 2


It can be done, as long as you have clearly recognizable stars, and you will be using the period between recurring events related to those stars as time reference.

In ancient Egypt, for example, the coming of the flood season was determined by the raising of Sirius:

Sirius is recorded in some of the earliest astronomical records. Its displacement from the ecliptic causes its heliacal rising to be remarkably regular compared to other stars, with a period of almost exactly 365.25 days holding it constant relative to the solar year. This rising occurs at Cairo on 19 July (Julian), placing it just before the onset of the annual flooding of the Nile during antiquity.

Additionally, if I remember correctly, in the Hebrew timekeeping the sunset was determined by the moment when at least 3 stars were visible in the sky, not by the disappearing of the Sun.

  • $\begingroup$ Both of these examples depend on a sun that is visible (in the Egyptian case; there's no "morning star" without morning) and that makes stars invisible during the day (in the case of Hebrew timekeeping). Both these requirements are contradicted by the question. $\endgroup$
    – David K
    Commented Mar 2 at 0:17

Setting aside the big question of how such a planet could develop or sustain life, the absence of a visible sun makes many of the problems of timekeeping much simpler.

You don’t need to know how much time you have to get home before dark, because it’s dark all the time. There is no Sun that causes it to be summer sometimes and winter other times.

Using the Big Dipper (or other polar constellation) as a clock is simple. The constellation appears to rotate once every sidereal day, so just declare a particular orientation to be the start of a day and look at how far the constellation has rotated from that position. It’s more complicated on Earth because we generally are concerned with the time relative to the current mean solar day, which continually shifts relative to the sidereal day. So an orientation of the Dipper the occurs at 6 am in March occurs at 6 pm in September. No such correction is required to tell purely sidereal time.

For periods longer than a day you can group them any way you like. Have a year of 12 months of exactly 30 days each if you like. Or a 100-day month and a 1000-day year. What makes one better or worse than the other? You don't need to know when spring is coming because there is no Sun to bring on spring.


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