Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing “Hoax” - Philip Plait (2002)

Part I. Bad Astronomy Begins at Home

Chapter 2. Flushed with Embarrassment: The Coriolis Effect and Your Bathroom

t's a pretty scene. Nanyuki is a small town situated just north of the equator where it cuts across Kenya in Africa. The town was founded early in the twentieth century, and still has something of a frontier feel to it.

It's a frequent stop for tour buses on their way to nearby Mount Kenya. It has the seemingly mandatory gift and curio shops, but it also features a local man named Peter McLeary. As tourists gather around, McLeary shows them a demonstration they are not likely to forget. More's the pity.

McLeary takes the tourists to a line drawn on the floor of an old burned-out hotel, and tells them that it's the actual location of the equator. A glib speaker, he explains that water swirls down a drain clockwise north of this line and counterclockwise south of it, an effect caused by the rotation of the Earth.

He then goes on to prove it. He takes a small, roughly square pan about 30 centimeters across and fills it with water. He places some matchsticks in it so that his audience can see the rotation more easily. Walking to one side of the line and turning to face his audience, he pulls out a stopper, letting the water drain out. Sure enough, when he does this demonstration north of the equatorial line the water drains clockwise, and when he repeats the experiment south of the line, the water drains the other way. Proof positive that the Earth is spinning!

The demonstration is convincing, and McLeary has done it for many years, raking in tips from the credulous tourists. It has been seen by countless travelers, and was even featured on the PBS series Pole to Pole, in which former Monty Python silly man Michael Palin tours the world, taking in interesting sights. In this particular episode Palin watches McLeary do his thing and adds, "This is known as the Coriolis effect … it does work."

Actually, no, it doesn't. Palin, and who knows how many tourists before and after him, are being taken in by a fraud. And it doesn't end there. This hoary idea is used to explain why toilets flush in different directions in the northern and southern hemispheres, as well as the way northern and southern sinks and bathtubs drain. Many college students claim that their high school science teachers taught them this fact. The problem is, it's no fact. It's bad astronomy.

The Coriolis effect is real enough. By the 1800s, it had been known for years that cannonballs fired along a north-south line tended to deviate from a straight path, always landing west of their mark if fired toward the south, and east if fired to the north. In 1835 the French mathematician Gustave-Gaspard Coriolis published a paper with the unassuming title of, "On the Equations of Relative Motion of Systems of Bodies." In it, he describes what has become known as the Coriolis effect.

Imagine you are standing on the Earth. Okay, that's easy enough. Now imagine that the Earth is spinning, once a day. Still with me? Okay, now imagine you are standing on the equator. The rotation of Earth takes you eastward, and after a day you have swept around a big circle in space, with a radius equal to the Earth's radius. On the equator, that means you have traveled about 40,000 kilometers (25,000 miles) in one day.

Now imagine you are on the north pole. After one day, you have rotated around the spot on which you are standing, but you haven't actually gone anywhere. The north pole is defined as the spot where the Earth's rotation axis intersects the ground, so pretty much by definition you don't make a circle there. You just spin, making no eastward motion at all.

As you move north from the equator, you can see that your eastward velocity decreases. At the equator you are moving nearly 1,670 kilometers per hour (1,030 miles per hour) to the east (40,000 kilometers in 24 hours = 1670 kph). At Sarasota, Florida, at a latitude of about 27 degrees north, you are moving east at 1,500 kph (930 mph), and by the time you reach Wiscasset, Maine, at 44 degrees north latitude, you are moving east at only 1,200 kph (720 mph). If you brave the chill of Barrow, Alaska, you'll be at latitude 71 degrees north and moving at a leisurely 550 kph (340 mph). Finally, at the north pole, you aren't moving east at all; you just make your tiny circle without any eastward movement.

Let's say you stop in Sarasota, which is a reasonable thing to do, given the climate there compared to Barrow. Now imagine someone on the equator due south of your position takes a baseball and throws it directly north, right toward you. As it moves northward, its velocity eastward increases relative to the ground. Relative to you, that baseball is moving 1,670 kph - 1,500 kph = 170 kph (1,030 mph - 930 mph = 100 mph) or so to the east by the time it reaches you. Even though the fastball is aimed right at you, it will miss you by a pretty wide margin! By the time it gets to your latitude, it will be a long way to the east of you.

That's why cannonballs are deflected as they travel north or south. When they are first shot from the cannon, they have some initial velocity to the east. But if they are fired north, they reach their target moving faster to the east than the ground beneath them. The cannoneer needs to aim a bit west to compensate. The reverse is true if it is fired south; the cannonball reaches its target moving slower than the ground, and needs to be aimed to the east to actually hit the target.

In our baseball example above, the distances and times involved were large, letting the Coriolis effect gather some steam. In reality, it's a tiny effect. Let's say you are driving a car north at 100 kph (60 mph) in Wiscasset, Maine. The Coriolis effect deflects you by the teeny amount of 3 millimeters (0.1 inches) per second. After a solid hour of driving, that amounts to a deflection of only 10 meters (33 feet). You couldn't possibly notice this.

Still, it is there. It's subtle, but over long distances and large amounts of time it adds up. That can be a mighty sum, given the correct circumstances.

And those circumstances do arise. An area of low pressure in the atmosphere is like a vacuum cleaner, drawing in the surrounding air. Let's take the simplified view that we are in the northern hemisphere, and assume that the air is coming in only from due north and due south. The air coming in from the south is moving faster to the east than the air near the center of the low-pressure system, so it bends to the east. Air moving from the north is moving slower than the air in the center of the system, and deflects west. These two deflections add up to a counterclockwise rotation to the low-pressure system. This is called a cyclonic system.

The opposite is true in the southern hemisphere. A low-pressure system will spin clockwise because air drawn in from the north will be moving faster to the east, and air coming in from the south will be moving slower. The spin is opposite from the northern hemisphere, and is called an anticyclonic system.

If the system is stable for a long time, days or weeks, it can grow massively in strength. Warm ocean water feeds the system, making it stronger. As the air gets closer to the center it moves faster, like an ice skater who spins faster when she draws in her arms. If the winds can gain in strength and blow at a hundred or more kilometers per hour, it becomes a hurricane (or a typhoon if it's in the Pacific ocean).

All that, from that tiny deflection you can't even feel in a car!

Does this sound familiar? Sure! It's the same idea that Peter McLeary uses to explain why water swirls the way it does when he gives his demonstration in Kenya.

But there's a problem: as we already saw, the Coriolis effect only produces a measurable effect over huge distances and long periods of time. Even the most decadent of bathtubs is thousands of times too small and drains way too quickly to ever be affected by it. It can be shown mathematically that random motions in your water are thousands of times stronger than the Coriolis effect, which means that any random eddy or swirl in the water will completely swamp it. If the water always drains one way from your bathtub, then it has far more to do with the detailed shape of your drain than from the rotating Earth.

Obsessive would-be physicists have actually performed experiments using household sinks. They have found that the sink needs to sit still for over three weeks so that random currents die off enough to see an appreciable Coriolis effect. Not only that, they have to let the sink drain one drip at a time to give the effect time to take hold. You're not likely to see this after hand-washing your delicates in the sink.

The same is true for your toilet. This one always makes me laugh: toilets are designed to spin the water. It helps remove, well, stubborn things that don't want to be removed so easily. The water is injected into the bowl through tubes that are angled, so it always flushes the same way! If I were to rip my toilet out of the wall and fly it down to Australia, it would flush in the same direction it does now.

The Coriolis effect is only significant over large distances. A hurricane is born when a low-pressure patch of air draws air in from higher and lower latitudes. Because of the Coriolis effect, in the northern hemisphere the air from the south moves east, and the air from the north moves west, causing a clockwise rotation.

The idea that the Coriolis effect works on such small scales is a pernicious myth. I have seen it in countless television shows and magazine articles; it was once even reported in the Sports Illustrated swimsuit issue. Oddly, they describe walking across the equator from the Central American country of Costa Rica, which is hundreds of kilometers from the equator. Some writer on staff did the figures incorrectly, but then, those aren't the kind of figures the magazine is usually trying to sell. On the other hand, maybe all that walking is how the models stay so slim.

So, if the Coriolis effect doesn't work on something as small as a sink or a pan, how did Peter McLeary pull it off? After all, as Michael Palin commented, it worked for him.

Actually, McLeary cheated. If you watch him do it on Pole to Pole, you can catch the swindle. He stands on his equator line and fills the basin. Then he walks a few meters or so north, and rapidly turns to his right to face his audience. He opens a hole in the bottom of the pan and the water obligingly rotates clockwise as it drains out. Next, he refills it, walks a few meters south of the equator, then rapidly turns to his left to face the audience. Draining, the water spins counterclockwise.

Do you see how this works? By spinning rapidly in opposite directions, he can make the water rotate any way he wants! The squarish shape of the pan helps, too; the corners help push on the water as the pan rotates, making it flow better.

Meteorology professor Alistair Fraser has used this demonstration in his own class. He draws a line down the middle of the classroom and declares it to be the equator (he teaches in Pennsylvania). He then does just what McLeary does and gets the same results.

Still don't believe me? Then think about it: the Coriolis effect should make draining water spin counterclockwise in the northern hemisphere and clockwise in the southern. In the northern hemisphere, water moving north deflects east, moving it counterclockwise. Water coming south from the north deflects west, but that's still counterclockwise. The opposite is true again for the southern hemisphere; the water will spin clockwise.

But this is precisely the opposite of what McLeary demonstrates. He's a fraud!

Your honor, I rest my case.

Well, not really. I have one more tale to tell. While searching for information about Nanyuki, I found one tourist's travelogue that describes three sinks sitting roughly ten meters apart, just outside of town. One is south of the equator, the second is directly on it, and the third is north of it. Perhaps someone else is horning in on McLeary's act. Anyway, the tourist who wrote the travelogue claimed that the northern sink drained clockwise, the southern sink drained counterclockwise, and the one in the middle drained straight down. Evidently the drain holes have been cut in such a way as to force the water to drain the way the designer wanted. Note once again that they drain the wrong way!

It's pretty funny, actually. They go through all that trouble to make a few bucks, and they don't even get the scam right. Somehow, though, I don't think those con artists are starving. Con artists rarely do. They can always put the right spin on their subjects.