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Foucault's Pendulum in
the Panthéon, Paris
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I stood and tilted my head and furrowed my brow and tried to figure out how THIS proved THAT - Only to, in the end, derive some small comfort from the fact that such things were not, are not, and never will be left for me to prove to the world.
(My brain tends to erect a comprehension barrier against any and all scientific concepts straying too far from my comfort zone of biology.)
At either the North Pole or South Pole, the plane of oscillation of a pendulum remains fixed while Earth rotates underneath it, taking ~24 hours to complete a rotation. So, relative to Earth, the plane of oscillation of a pendulum at the North Pole undergoes a full clockwise rotation during one day; a pendulum at the South Pole rotates counterclockwise.[1]
When you go to latitudes lower than the North Pole, the time it takes the plane of the pendulum to rotate lengthens; and at the equator, the plane of oscillation remains fixed relative to Earth.[1]
At the latitude of Paris, a full pendulum day* takes ~32 hours.[1]
***
Okay, so I've now done some reading, and some watching, and it turns out it's not that difficult a concept after all (and now I feel rather silly). If I had just stood there a little bit, or rather, a lot longer, I would have eventually figured it out. Sadly, I'm one of those thinkers who must visualize concepts clearly in my head before the logic behind them seeps into my brain. But I think I got it. I hope I got it. And in any event, since I've started this, I will stay the course. Perhaps, this information may be of benefit to anyone else who finds themselves staring at this swaying pendulum inside the Panthéon, and wondering what it's all about.
Animation of a Foucault pendulum at the Pantheon in Paris (48°52' North), with the Earth's rotation rate greatly exaggerated. The green trace shows the path of the pendulum bob over the ground (a rotating reference frame), while the blue trace shows the path in a frame of reference rotating with the plane of the pendulum.[1]
The Foucault pendulum, named after the French physicist Léon Foucault, is a simple device conceived as an experiment to demonstrate the earth’s rotation.[1]
While it had long been known that the Earth rotates, the introduction of the Foucault pendulum in 1851 was the first simple proof of the rotation in an easy-to-see experiment.[1]
The experimental apparatus consists of a tall pendulum, free to swing in any vertical plane. The wire needs to be as long as possible—lengths of 12–30 m (40–100 ft) are common.[2]
While it had long been known that the Earth rotates, the introduction of the Foucault pendulum in 1851 was the first simple proof of the rotation in an easy-to-see experiment.[1]
The experimental apparatus consists of a tall pendulum, free to swing in any vertical plane. The wire needs to be as long as possible—lengths of 12–30 m (40–100 ft) are common.[2]
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A Foucault pendulum at the north pole.
The pendulum swings in the same
plane as the Earth rotates beneath it.[1]
|
At either the North Pole or South Pole, the plane of oscillation of a pendulum remains fixed while Earth rotates underneath it, taking ~24 hours to complete a rotation. So, relative to Earth, the plane of oscillation of a pendulum at the North Pole undergoes a full clockwise rotation during one day; a pendulum at the South Pole rotates counterclockwise.[1]
When you go to latitudes lower than the North Pole, the time it takes the plane of the pendulum to rotate lengthens; and at the equator, the plane of oscillation remains fixed relative to Earth.[1]
At the latitude of Paris, a full pendulum day* takes ~32 hours.[1]
*A pendulum day, is the time needed for the plane of a freely suspended Foucault pendulum to complete an apparent rotation about the local vertical.[3]
A Foucault pendulum requires care to set up because imprecise construction can cause additional veering which masks the terrestrial effect. The initial launch of the pendulum is critical; the traditional way to do this is to use a flame to burn through a thread which temporarily holds the bob in its starting position, thus avoiding unwanted sideways motion.[1]
However, if someone was at all curious (not that I'm encouraging anyone try this), all one would need is a watch, a pendulum, and some height. It would need to be suspended so that it can swing back and forth in any direction along a vertical path. One could line up dominoes, for instance, in the circle around the pendulum. The Earth's rotation will cause the trajectory of the pendulum to change over time, knocking down dominoes at different positions as time elapses and the Earth rotates.
The South Pole Pendulum Project (as discussed in The New York Times[4] and excerpted from Seven Tales of the Pendulum[5]) was constructed and tested by John Bird, Jennifer McCallum, Michael Town, and Alan Baker at the Amundsen-Scott South Pole Station.[1] Their measurement is probably the closest ever made to one of the earth's poles.[1] The pendulum was erected in a six-story staircase of a new station that was under construction near the pole.[1]
However, if someone was at all curious (not that I'm encouraging anyone try this), all one would need is a watch, a pendulum, and some height. It would need to be suspended so that it can swing back and forth in any direction along a vertical path. One could line up dominoes, for instance, in the circle around the pendulum. The Earth's rotation will cause the trajectory of the pendulum to change over time, knocking down dominoes at different positions as time elapses and the Earth rotates.
The South Pole Pendulum Project (as discussed in The New York Times[4] and excerpted from Seven Tales of the Pendulum[5]) was constructed and tested by John Bird, Jennifer McCallum, Michael Town, and Alan Baker at the Amundsen-Scott South Pole Station.[1] Their measurement is probably the closest ever made to one of the earth's poles.[1] The pendulum was erected in a six-story staircase of a new station that was under construction near the pole.[1]
The altitude was about 3,300 metres (atmospheric pressure only about 65% that at sea level), and the temperature in the unheated staircase was about −68 °C (−90 °F).[1]
The pendulum had a length of 33 meters and a 25 kilogram bob. The new station offered an ideal venue for the Foucault pendulum; its height ensured an accurate result, no moving air could disturb it, and low air pressure reduced air resistance.[1] The researchers confirmed about 24 hours as the rotation period of the plane of oscillation.[1]
References:
[1] http://en.wikipedia.org/wiki/Foucault_pendulum
[2] "Foucault Pendulum". Smithsonian Encyclopedia.
[3] "Pendulum day". Glossary of Meteorology. American Meteorological Society.
[4] Johnson, George (September 24, 2002). "Here They Are, Science's 10 Most Beautiful Experiments". The New York Times. Retrieved September 20, 2012.
[5] Baker, G. P. (2011). Seven Tales of the Pendulum. Oxford University Press. p. 388. ISBN 978-0-19-958951-7.
https://www.youtube.com/watch?v=aMxLVDuf4VY
https://www.youtube.com/watch?v=M8rrWUUlZ_U
Image Credits:
"Pendule de Foucault" by Arnaud 25 - Own work. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Pendule_de_Foucault.jpg#mediaviewer/File:Pendule_de_Foucault.jpg
"Foucault-rotz" by Nbrouard - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Foucault-rotz.gif#mediaviewer/File:Foucault-rotz.gif
"Foucault pendulum at north pole accurate" by en:User:Krallja - http://en.wikipedia.org/wiki/Image:Foucault_pendulum_at_north_pole_accurate.PNG. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Foucault_pendulum_at_north_pole_accurate.PNG#mediaviewer/File:Foucault_pendulum_at_north_pole_accurate.PNG
References:
[1] http://en.wikipedia.org/wiki/Foucault_pendulum
[2] "Foucault Pendulum". Smithsonian Encyclopedia.
[3] "Pendulum day". Glossary of Meteorology. American Meteorological Society.
[4] Johnson, George (September 24, 2002). "Here They Are, Science's 10 Most Beautiful Experiments". The New York Times. Retrieved September 20, 2012.
[5] Baker, G. P. (2011). Seven Tales of the Pendulum. Oxford University Press. p. 388. ISBN 978-0-19-958951-7.
https://www.youtube.com/watch?v=aMxLVDuf4VY
https://www.youtube.com/watch?v=M8rrWUUlZ_U
Image Credits:
"Pendule de Foucault" by Arnaud 25 - Own work. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Pendule_de_Foucault.jpg#mediaviewer/File:Pendule_de_Foucault.jpg
"Foucault-rotz" by Nbrouard - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Foucault-rotz.gif#mediaviewer/File:Foucault-rotz.gif
"Foucault pendulum at north pole accurate" by en:User:Krallja - http://en.wikipedia.org/wiki/Image:Foucault_pendulum_at_north_pole_accurate.PNG. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Foucault_pendulum_at_north_pole_accurate.PNG#mediaviewer/File:Foucault_pendulum_at_north_pole_accurate.PNG
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