A properly designed pendulum (long and heavy) will keep swinging for a day if started in the morning. On an unmoving Earth, you would expect it to keep swinging in the direction you started it in, or maybe randomly deviate from that. What actually happens is that the direction of the swing rotates like clockwork in the same manner and at the same speed each time you repeat the experiment (which was each day in case of the physics building), so much so that you can actually paint a clock on the floor and measure time accurately by this rotation. The speed of that rotation depends on the latitude of the pendulum; at the North Pole, it would rotate ~360° in 24 hours as the globe rotates "below it"; at higher latitudes it becomes less until it won't rotate at all at the equator, and rotate the other way going further south.
There is no other explanation for the rotation of this pendulum (and no other way to predict it) other than to assume that we're on a globe that rotates once per day. Thus, Foucault's Pendulum proves that Earth rotates.
A good thread which confirms the unreliability of Foucault's pendulum as a way to prove earths rotation.
https://physics.stackexchange.com/q...cault-pendulum-really-prove-earth-is-rotating
"If Earth weren't rotating and a Foucault pendulum started in a state with zero velocity, it would keep swinging back and forth along the same line. If at its highest point, it has a tiny velocity in a direction perpendicular to the direction to the lowest point on the pendulum, then maybe it would have such a tiny deviation from moving exactly back and forth that a human can't see that tiny deviation with their own eyes, but that deviation would result in a slow precession of the pendulum and a day is so long that the direction it's swinging in would rotate a significant amount."
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About the construction of a Foucault pendulum:
I have read several accounts from teams that had constructed a Foucault pendulum. And indeed a recurring theme is that is very hard to get the parasitic effects down to a level where the Foucault effect dominates. Many a time a team saw with elation how their pendulum finally showed precession, only to realize that it was in the
wrong direction. Also it is common to add a driving mechanism, so that the swing doesn't decay. But it's very hard to eliminate a bias from the driving mechanism. I read an account that went something like this. "We tweaked our setup until we obtained the theoretical precession rate. To be honest, we can't be sure whether our pendulum is doing a true Foucault precession, or whether we've merely dialed in the precession rate."
So, yeah: from a purely scientific point of view a Foucault pendulum setup is not a particularly good way to demonstrate that the Earth is rotating. (However, Foucault's gyroscope
was, I'll get to that in a few paragraphs.)
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There is a 1969 paper by E. O. Schulz-DuBois titled, 'Foucault pendulum Experiment by Kamerlingh Onnes and Degenerate Perturbation Theory', discussing the research into pendulum swing by H. Kamerling Onnes
Schulz-DuBois writes about the solutions to the equation of motion for the asymmetrical case: "the first and second eigenfunctions describe ellipses with interchanged major and minor axes, and with opposite sense of circulation. [...] If the pendulum is excited to an orbit described by an eigenfunction if will continue to move in this orbit without change [...]. Any other excitation involves both eigenfunctions. Due to the frequency difference between them the pattern of pendulum motion changes with time."
So: the problem of imperfect construction is acute.
(For completeness:
Kamerling Onnes had designed his setup in such a way that it was adjustable. He devised procedures to systematically home in on adjustments to eliminate any asymmetry. Schulz-DuBois writes: 'When adjusted his pendulum performed as expected from unsophisticated theory.')
***I think the thread cited on pyhsics.stackexchange highlights the issues of the Foucault pendulum. The gyroscope Foucault later commissioned, and to this day seems a more credible way to measure rotation, like I've said earlier in the thread I need a better understanding of this, and how to apply to both models.