Why Toilet Bowl Water Twirls Clockwise
You know the legend: In the northern hemisphere, water goes down the toilet clockwise. And it twirls counterclockwise in the southern hemisphere. It's a cool factoid, in itself. And it has a cool name -- the Coriolis Force.
There's no painless way to explain how Coriolis works, though, so gird your intellectual loins for a small war.
The first battle will be to visualize the spinning planet to which the toilet is fastened:
Slice the Earth at the equator like a grapefruit, and flatten the northern hemisphere into a plate. Now spin it counterclockwise. The North Pole, you'll notice, turns quite slowly. Move out to 45 degrees latitude (Minneapolis) on the plate and here the ground is buzzing along at about 730 miles per hour. Move way out to the edge of the plate and the equator is turning at 1,040 miles per hour.
Visual aid:
Picture a line of people walking arm-in-arm, with one end of the line always at the North Pole. To hold their formation, the people on the outside have to run and the people near the Pole have to creep.)
Got it? At the equator the ground moves fastest; at the poles, it moves slowest.
The second battle involves visualizing a sloshy substance hovering over the spinning Earth. Round out the Earth again, and picture a stationary air mass hovering over the equator at, say, the Amazon basin. This air is stationary only in relation to the Earth. Viewed from space, that air is actually moving at about 1,040 miles per hour, keeping pace with the ground beneath it.
Now excise a neat cube of that air, and shove it north to the 45th parallel. The ground here is moving under the cube of air at 740 miles per hour, but the cube of air continues tooling along at 1,040 miles per hour. Whereas it was stationary relative to the Amazon, now it's moving east at 290 miles per hour, relative to its new home on the 45th parallel.
(This is Newton's First Law: Objects in motion, including cubes of air, are obligated to stay in motion until they get permission from a brick wall, or a more subtle force, to slow down.)
To recap: Push a floating object north, and it will appear, relative to the earth, to pick up speed and move east.
The third battle is a cakewalk. When you pushed air north from the equator, it appeared to gain speed and move east. Now take a cube of air from over the slow-spinning North Pole, and nudge it south toward the 45th, where the earth sweeps beneath it faster: As a cube of air that was stationary near the pole moves south, it appears to slow down, and veer westward.
(Visual aid: Betty, at the center of a merry-go-round, throws a ball to Billy at the merry-go-round's edge. By the time the ball reaches the edge, however, the merry-go-round has moved out from under it. Billy sees the ball fall "west" of him.)
OK, hold those thoughts: Either way you shove a block of air, from north to south, or south to north, it appears to be deflected to its right -- or clockwise. Now consider a toilet in Minneapolis. The toilet is connected to the earth, but the water is merely sloshing around over it, like a mass of air. The whole contraption, however, is whipping around the earth's axis at 730 miles per hour.
The catch is that water floating at the north end of the bowl has a scidge less ground to cover per second; and the water hovering over the south end has a skidge more ground to cover. So the water at the north gets a little bit ahead, the water at the south gets a little bit behind, and when you flush, the clockwise twirl comes to fruition.
(To get the southern-hemisphere view, revisit your spinning plate, walk to its underside, and you'll note that instead of turning from your right to your left, the planet is now turning from your left to your right. Take my word for it: Everything else is reversed, too.)
Ah, but this has all been a cruel and painful joke.
It turns out that even a toilet a mile wide might still be too tiny to exhibit Coriolan tendencies -- the water simply isn't hovering over enough latitudes to feel The Force. It takes a mass of air many miles in diameter to demonstrate the infamous toilet twirl, and even then, Coriolis is often foiled by friction with the ground and barometric high jinks.
So while all of this spinning and shoving can help explain prevailing winds and other large-scale phenomena, the twirl in your toilet is determined by jets of water filling the bowl, the shape of the drain, or, for those who cannot let this myth go -- and I've encountered many of them -- the Coriolis Fairy.
Vocabulary
fictitious force, n. Coriolis isn't even a real force, since it doesn't make anything speed up or slow down -- it only explains why things appear to speed or slow as the world spins out from under them. This sort of impostor is known as a fictitious force.