How does the Foucault pendulum work exactly, and would it work at all, if the Earth didn't rotate?

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    $\begingroup$ 'Didn't rotate' relative to what frame of reference? Tidally locked with the sun is one possibility, as is the possibility that it does not rotate w.r.t the milky way centre, or maybe you mean a particular galactic cluster? Even the distant stars and galaxies are moving, so there seems to be no "universal stationary frame of reference". $\endgroup$ – Jus12 Sep 14 '11 at 19:46
  • $\begingroup$ @Jus12 The answer is given to you by the pendulum itself. If it doesn't precess than it's surroundings are an inertial frame (i.e. one which is not rotating). $\endgroup$ – dmckee --- ex-moderator kitten Sep 19 '13 at 23:09
  • $\begingroup$ Typically the response to this question given by flat-Earthers nowadays is that every Foucault pendulum is fake, and the precession is accomplished through hidden motors. $\endgroup$ – probably_someone Mar 7 '19 at 9:51

Well, for the basics of the Foucault pendulum, this wikipedia page does an adequate job describing how it works (Specifically read up on the Precession as a form of parallel transport section).

This page also has a nice explanation of how they work:

A Foucault pendulum is just like any other pendulum, nothing more than a weight attached to a wire; but to work well it needs to have a very long wire, with a really heavy weight. They're often attached to the ceilings inside very tall buildings, such as museums and cathedrals, so that they can hang a great distance and swing impressively slowly. These sorts of locations also tend to have fairly constant temperatures, which avoids expansion and contraction of the pendulum which would cause complicated variations in its period.

What makes a Foucault pendulum different from a normal pendulum is that it is attached at the top to a universal joint which allows the pendulum to rotate freely around its fixing point as it swings. Once you set one in motion, its direction of swing will rotate at a rate of about 0.2 degrees every minute. But in fact, it isn't really the pendulum that's rotating: the pendulum is swinging back and forth in exactly the same direction. It's the Earth which is rotating underneath the pendulum, which makes it appear that the pendulum is in fact changing direction.

Now, since I can't even conceive of a way to make a planet (or any other large enough body to build any type of pendulum on) that would not rotate, this is a purely theoretical exercise. Bottom line:

The pendulum would swing back and forth, and never deviate from its original arc (assuming no wind or other disturbances) if it were set up on a non-rotating body.

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    $\begingroup$ TL;DR it wouldn't. $\endgroup$ – DilithiumMatrix Sep 20 '13 at 2:13
  • $\begingroup$ If a Foucault pendulum was set up on the Moon, the precession would be based upon the Moon's rotation around the Earth, yes? $\endgroup$ – Mark Hurd May 13 '14 at 2:22

I keep thinking the pivot would rotate with the earth and therefore the Foucualt Pendulum would not work. But upon further thought the pendulum must track in same line. The pendulum stays constant because gravity keeps it lined up with the center of the earth while the earth's crust rotates the center does little and holds the pendulum on plane while the earth rotates under it. So if the earth did not rotate then the Foucault Pendulum would track the same line since the earth would not be moving under it. What else but gravity could keep the Pendulum course constant?


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