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Very nice, but the description of Focault's pendulum, near the end, is incorrect. > ... though the pendulum appeared to change direction as it swung, in fact t

Very nice, but the description of Focault's pendulum, near the end, is incorrect. > ... though the pendulum appeared to change direction as it swung, in fact the plane of its swing remained constant .... Nope. Unless you're at one of the poles, the period of the rotation of the plane is not 24 hours; the plane can hardly be fixed. Wikipedia says a bit about this: https://secure.wikimedia.org/wikipedia/en/wiki/Focault_pendulum https://secure.wikimedia.org/wikipedia/en/wiki/Focault_pendu... And there was a nice article in the American Mathematical Monthly some years back, which I found here: http://geomsymm.cnsm.csulb.edu/courses/451/foucault_pend.pdf http://geomsymm.cnsm.csulb.edu/courses/451/foucault_pend.pdf But the basic idea is something like this: Say you're in the Northern Hemisphere. As the Earth rotates, your path essentially pushes you to the right (where "forward" is west). If you have a swinging pendulum with you, then the plane of its swing will rotate just as if you were on a flat surface, walking in a very large circle. The closer you are to the equator, the less you are moving to the right, the larger is this theoretical circle, and the slower the plane turns.

mechanical_fish 16y ago
Sure, but this is just the Science Writing Uncertainty Principle: You can be brief, or you can be precise, but rarely can you be both brief and precise. Langewiesche doesn't have time for a discourse on the physics of the pendulum -- which is basically just another kind of gyro. He has just a couple of sentences for his aside, and his readers are not physicists, so he grabs for the example of the Foucault pendulum, because that's a pendulum experiencing gyroscopic forces that every science museum visitor has seen.
- ggchappell 16y ago
  Ah, but the statement "the plane of its swing remained constant" is not imprecise; it is false. And I don't mean false-but-kinda-true like "pi = 3.14" is false-but-kinda-true; I mean just plain wrong. The fixed-plane idea seems to be a common misunderstanding, even among intelligent people, but it ain't so. The plane turns, even with respect to a "fixed" frame of reference that does not rotate with the Earth. I would have been better just to say that the plane changes, showing that the Earth rotates, and leave the explanations for another time.

mechanical_fish 16y ago

Sure, but this is just the Science Writing Uncertainty Principle: You can be brief, or you can be precise, but rarely can you be both brief and precise. Langewiesche doesn't have time for a discourse on the physics of the pendulum -- which is basically just another kind of gyro. He has just a couple of sentences for his aside, and his readers are not physicists, so he grabs for the example of the Foucault pendulum, because that's a pendulum experiencing gyroscopic forces that every science museum visitor has seen.

ggchappell 16y ago
Ah, but the statement "the plane of its swing remained constant" is not imprecise; it is false. And I don't mean false-but-kinda-true like "pi = 3.14" is false-but-kinda-true; I mean just plain wrong. The fixed-plane idea seems to be a common misunderstanding, even among intelligent people, but it ain't so. The plane turns, even with respect to a "fixed" frame of reference that does not rotate with the Earth. I would have been better just to say that the plane changes, showing that the Earth rotates, and leave the explanations for another time.