A sundial uses more than just angular rotation and orbits, though. You can't measure time with angular rotation alone. Using a sundial as a clock (by registerin

A sundial uses more than just angular rotation and orbits, though. You can't measure time with angular rotation alone. Using a sundial as a clock (by registering its cycles or by looking at it and remembering how many cycles it went through) inevitably increases the system's entropy. You don't need a second repeating phenomenon, just as you don't need a second pendulum in the pendulum clock. The pendulum might repeat its motion at a short interval, but is able to measure intervals longer than its period. If you have a sundial that works without increasing entropy, it doesn't work as a clock. Just as a perpetual motion pendulum alone wouldn't work as a clock. Using other orbits doesn't change that. You need the entropic flow to register the passage of time. In the case of the sundial, it can be done by recording its cycles. A sundial on a rotating planet isn't able to measure time within its cycle without increasing entropy any more than a pendulum is able to measure time within its period without increasing entropy. They're not able to do that. A Sundial without light and a recording device isn't a clock, just as a lone pendulum with no energy losses and no recording device isn't a clock.