Wikipedia is an odd sort of thing. People I know and respect use it heavily though often with a bit of shame. It's a grass roots encyclopedia, and of course if you're reading a blog online ... you know that anyone can contribute to Wikipedia, even if they sign in as "Mr. FooFoo." How good can it be, seriously.
Despite my reservations, I do check in fairly often, and did so after a visit to Chicago's Museum of Science and Industry. There's a very long pendulum there that has been swinging, as far as I can tell, since the beginning of time. It hasn't of course, they start it up daily, but it does mark out time nicely, rotating clockwise in a slow, steady rate through the day. This is a Foucalt Pendulum. My problem was that I know the Coriolis Effect which also causes things to turn to the right in the northern hemisphere, due to the curvature of the Earth, and in my mind I couldn't quite figure, once the pendulum moved into a due east/west swing, why would it continue turning? But it does.
A little lesson on Coriolis may be in order, it's a fun one to turn an auditorium on to. So much illumination with so little effort. But skip four paragraphs if you want to miss the lecture.
The Earth's weather all occurs in the zone called the troposphere. It is bounded by gravity but you can imagine a plexiglass shell around the planet, about 7 miles above the surface. Because the Earth spins, it drags the air with it, and obviously the Earth and air is moving through space east faster near the equator and not at all near the poles. To be more accurate, it moves 1,037 mph eastward near Ecuador and 0 mph at the poles. Ecuadorians don't notice, of course, because they are moving too.
The sun hits the equator more directly than other areas, heats the air at the surface there, and of course hot air rises. So there's a band of air at the equator that is constantly rising, but it runs into this plexiglass ceiling and is deflected north and south. We'll follow the northern part but the southern hemisphere is just a mirror image. It's going merrily along, toward the North Pole where -- because its colder, you'd think the air should drop back to Earth. The problem is, as that air moves north, it's starting to shift east (from the land's perspective), because the circumference of higher parallels is shorter as we have already noted so the land slows down. The air at 30 degrees north or so is now slipping east much faster than the ground below. The band of rising air at the equator has now become a river of air moving east, many miles above the surface. That's the Coriolis Effect. But let's continue. That air, too, must go somewhere. Much of it, cold now, goes down.
As it rose it had cooled and rained, so as it falls it warms and dries; therefore we get the wet tropics near the equator, and at roughly 30 degrees north and south the world's great deserts. Hitting the earth, the air divides again. The part going south merges at the equator with the mirror-image winds in the southern hemisphere. You may have noticed that the winds have all turned to the right in the northern hemisphere and to the left in the south, so when they merge they are all drifting westward. These are called the trade winds, as they are steady and reliable. At the equator we find the doldrums, the intertropical convergence zone where wind, if you want to call it that, goes up.
It's worth completing the system. Back at 30 degrees north latitude, the other part of the descending air is deflected north, and turns east again just as before and for the same reason, except this time at the surface. By 60 degrees latitude it's going mainly east again because of Coriolis, and it accumulates and rises, some of it this time making it to the poles before cooling, accumulating, and falling back to Earth, at which point it heads south, tending westward. One more interesting thing, and the lesson is over. At the surface the polar winds flow southwest and the midlatitude surface winds turn northeast so at about 60 degrees they collide; it's the only collision zone of the major winds. All sorts of storms and unpredictable weather spin off, keeping things in the middle latitudes interesting.
So here I am, in the museum, thinking about Coriolis causing things in the northern hemisphere to turn right, being in the northern hemisphere, and noticing the pendulum turning right, but wondering why it continues turning when it continues past the E/W parallel. There would be 0 Coriolis effect at that point.
Mystery. The description on the wall was of no use, and so I turned to Wikipedia. It was not of much use to me either. So when I finally figured it out, I thought I might be able to explain it to others -- give back to humanity. I will lift a veil of confusion by explaining Foucalt to those who have already been inoculated by Coriolis, I thought. So I crafted this statement, signed in as ErickH and published it! It was 4:40 p.m. March 25, 2011.
Foucault's pendulum is not to be confused with the Coriolis Effect which alters the direction of north- and south-bound fluids traveling long distances. A visualization of each may be useful. For Foucault, imagine a very large plate (1,000 kilometer radius) tangent and glued to the Earth about halfway north of the equator. Or picture a saucer glued to a basketball as it rotates on a vertical axis. Increase the eastward rotation to about 5 seconds. From the point of tangency, fire a slow pistol in any direction horizontal to the plate. As the bullet follows a straight path through space, the plate beneath it slips counterclockwise making the trajectory, from the perspective of the ground, always curve to the right. Using the palm of your hand and an imaginary basketball, you will see how this is true, regardless of the direction fired. The most dramatic turns occur at the poles and it diminishes to zero at the equator. In the southern hemisphere all movements are to the left, and when tangent at the equator, the bullet does not curve at all. These slight but constant motions of swing which cause the actual pendulum (which you can see in museums) revolve throughout the day.
.. .this was followed by a short description of Coriolis, for comparison.
Reading it now, I find it a little long. But it was my first post and I didn't know how to edit Wikipedia, so for a while this is what people read when they went to to the entry there for "Focault Pendulum." I can only imagine what a great relief it must have been to read this passage. But, as it turns out, Wikipedia does accept corrections as easily as it accepts posts, and I noticed that at 4:42 the same day -- two minutes later -- someone named "Minimac" had written "citation needed" at the bottom of both paragraphs. I saw his/her comment in the editor's remark: "Both paragraphs require at least a citation."
Hmm. But who would I cite, myself? I thought of trying to find someone who had said something similar, but these thoughts were actually mine. So I decided to let nature take its course and that didn't take very long. On April 15 at 7:49 p.m., 21 days after launch, "ShanRen" wrote in the cliff notes "Removing section with bogus arguments." And he did.
ShanRen was right, my post didn't belong there; Wiki must be concise and what I'd written wasn't very polished anyway. Anyway, I'm satisfied with all the good I was able to accomplish in 21 days. But here's the real lesson. Take a look behind the VIEW HISTORY button NE corner of Wikipedia, and you'll see something wonderful: all the versions, all the revisions, all the corrections and suggestions and removals, the reasons, the times, the people, and even before and after views for every single change.
Foucalt must be one of the more controversial entries because the page has been edited 745 times since PierreAbbat started it at 7:02p.m. August 17 2002. It was edited seven more times last week.
So what's left after many hundreds of tweaks? Just a short but precise description, the necessary formulae, some links to further references and a reasonable bibliography. Darwin would have enjoyed Wikipedia. Not only has the entry under his own name been revised 8,264 times in the past 10 years, but Wikipedia is also one of the finest examples of natural selection you could ask for.
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