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I've got a cave civilisation who have never seen the Sun, and don't have much of a concept of time. I was inspired by a Reddit post from someone with a similar situation, and like me, the OP was wondering how people can keep time underground. Someone suggested sand timers, water timers, etc.

What I want to know is: is it possible to build a timer, e.g. a water timer, which dripped from a top chamber to the bottom chamber, in such a way that it caused the whole thing to gradually rotate by itself? I.e. without a motor? Because I could say that the civilisation has tech and can create motors or electricity, but on what basis would they have to time the motors? How many "seconds" (for they won't have a concept like that,) would they decide is needed before the mechanism turns the timer? The only alternative I can think of is having one person who turns the timer every time it reaches the bottom, every day, for the rest of their lives.

So it would be cool if it were possible to build a timer which rotated by itself, simply using the flow of gravity. Is such a thing even conceivable, let alone possible?

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  • $\begingroup$ I think you should start by asking what need would they have for timing and then create a solution for that need. That is how the solutions would actually evolve and they'd likely be different from solutions that we have evolved for our needs. $\endgroup$
    – Ville Niemi
    Commented Jun 26, 2016 at 0:43
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    $\begingroup$ Why not a pendulum clock? $\endgroup$
    – Schwern
    Commented Jun 26, 2016 at 2:10
  • $\begingroup$ They need to know when to sleep, when to start working etc. $\endgroup$
    – Lou
    Commented Jun 26, 2016 at 17:03
  • $\begingroup$ A pendulum clock is a great idea! I didn't think about that as a non-electrical solution. $\endgroup$
    – Lou
    Commented Jun 26, 2016 at 17:06

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You need to have some energy input from somewhere. Without that, you're trying to build a perpetual motion machine, which is notoriously difficult.

A more conventional route would be to discover that pendulums move rather regularly, and then discover the rotation of their planet, via the Foucault Pendulum experiment. That should give them the motive to invent the pendulum clock.

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If you have a continuous source of water, e.g. an underground stream, you could construct any one of several known devices that result in turning a wheel of some form. Several ancient designs did exactly this. One of the simplest, is an overshot waterwheel that requires filling of buckets to turn the wheel.

Simple Water clocks are not very accurate, but if your underground environment is deep enough to have consistent temperature and humidity, you would have solved a common source of accuracy problems with water clocks. With considerable care, you might create a clock accurate to within 5 or 10 minutes per day.

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Shishi-odoshi could be used to gauge time if the rate of flow is constant: They have a natural waterfall/drip to run it: as long as the flow is constant, it should keep good time and auto resets. It wouldn't be difficult to add a ratchet to have it click an hour hand to the next hour or something either.

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Ancient Greeks constructed something vaguely like this - a water clock that dripped water from a small hole at a constant rate. If some clever engineer set up a water clock next to a river, got it to constantly fill itself, and rigged up a system below the clock that dumped itself as soon as the water level reached a certain height, then it'd dump itself on the hour every hour and allow you to time minutes. Combine that with a sundial, and you've got yourself a primitive clock.

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