Back when I was a teenager, I went on a vacation to Africa with a friend’s family. We were being driven around in a van, and at some point, our driver pulled over at a small village on the side of the highway. We were shown a stone tablet on the ground marking the line through which the equator passed through the village. A well dressed man with an official looking ID pinned to his shirt then conducted a demonstration of the way water swirls down drains differently in different hemispheres. He produced a plastic basin with a hole at the botton that he stopped with his finger, and then filled with water. He then led our group 20 feet away from the supposed ‘equator’, unplugged the hole and the water swirled away in a clockwise direction. He repeated this 20 feet away on the other side of the line, and this time it went in a counterclockwise direction. He then took the container to the equator line, put it on the ground, filled it up and picked it up. The water didn’t swirl much at all, it seemed to go straight down.

My sixteen year old self was suitably impressed. Also, thats my defence for being taken in by the trick. I was sixteen, and my skep-dar hadn’t kicked in yet. Because, really, all the stuff we’ve heard about water swirling down the drain in different directions in different hemispheres, because of the coriolis effect isn’t true. Bad Science.

Good science:
In physics, the Coriolis effect is an apparent deflection of moving objects when they are viewed from a rotating reference frame. The common perception, propagated by most people, including some high school teachers is that water flowing down a drain will go counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. In reality, the Coriolis effect is a few orders of magnitude smaller than various random influences on drain direction, such as the geometry of the container and the direction in which water was initially added to it. So, factors such as the container being square or round, the way water was added, or how the container was agitated play much larger roles in the direction water flows.

I’m not alone though. No less than Michael Palin was fooled, when he featured the guy in Kenya in his BBC series, from Pole to Pole. It’s also a belief propagated by most people, including some high school teachers. We should know better.

The Bad Metereology blog explains how this is done:

There are two clues to the successful fakery of the drainage changing direction at the whim of the mountebank. One is revealed in each of the two images (above) which I captured from the TV program:

Non-circular pan
The non-circular pan allows the faker to easily introduce rotation into the pan after he begins. Let us imagine that the pan is filled very carefully so that there is no rotation initially. Indeed, one wants to be able to show the audience that any rotation introduced while filling has died out. Now what? If the pan were circular, it would be harder to start the water spinning by turning the pan itself, but by having the pan nearly square, the water must turn if the pan does.

Turn to face the audience
But how does one turn the pan without the audience becoming suspicious. Obviously, it must be done in such a way that the audience does not attribute the action to part of the fakery, but to a courtesy which enables them to see better.

The procedure is as follows:

- Find the materials
You want two plastic containers: one which is to be the pan to be drained, and the other is a bucket for the storage of water. The pan should be non-circular. I found a quasi-square one (made by Sterilite) for $1.28 in the local Wal-Mart. It is the three-cup size (.7 liter). It works well. Drill a hole (say, 1/4 inch or 3/8 inch) through the center bottom. You don’t need a special stopper for the hole: your finger placed underneath works just fine. The other container can be any small plastic bucket with the only restriction that your pan should be able to sit on top of it so that the bucket can catch the water as it drains.

- Fill the pan
Choose the spot that you wish to claim is the equator (the center of your classroom will do). Fill the pan from the bucket there and let the motion from filling die out. (You can insert and then gently remove a vertical object, such as index card, into the water to dampen the motion.) You can then show that there is little or no circulation by floating a match stick (or sprinkling pepper) on the water.

- Covertly add the chosen rotation
If you are going to the north side of the equator, stand in front of the pan facing south. Pick it up, turn around by turning to your left, walk to the north end of the room, turn around by turning to your left, and face the audience. As you have a non-circular pan, you have now introduced counter-clockwise rotation (which is cyclonic in the northern hemisphere) into the water.

- The coup-de-grace
Add a tracer (such as matches or pepper). Remove the stopper (your finger) and let the water drain. Lie through your teeth by claiming that it is the rotation of the Earth here in the north which is causing the water to circulate the way it does.

- The other hemisphere
Go back to the equator but this time stand on the south side looking north. After filling the pan, turn around by turning to your right, walk south, again turn around by turning to your right to face your audience (cyclonic rotation in the southern hemisphere is clockwise), and remember to complete the demonstration by lying again.

- The equator
This is the hardest part of the fakery, because it is actually very difficult indeed to eliminate all rotation from a pan of water. It really should sit for a very long time (and your finger might get tired). If the pan is fairly deep, and the hole small, it takes a moment or two before the rotation is apparent. This seems to be the dodge followed by the Kenyan faker.

(You can read the entire explanation of how this is done by clicking: here )

This is a relatively benign myth. But it’s symptomatic of a wider lack of skepticism. We tend to accept what we’re told when it comes to understanding science, without trying to understand the underlying fundamentals, or whether it even makes sense. It’s usually understandable to do this when it’s coming from an authority figure, such as a teacher or parent. Most of the time, this works out fine, but it’s how common fallacies like thinking homeopathy or chiropractic are legitimate fields of medicine are propagated. We usually get it from people in authority, and if we grew up with it we’re less likely to question it.

_________
To learn more about the science behind the Coriolis effect, I recommend:
http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html
http://en.wikipedia.org/wiki/Coriolis_effect

To learn more about the “science” underlying homeopathy, chiropractic and astrology, I recommend:
- Skeptoid on Homeopathy
- Skeptoid on Chiropractic
- The Bad Astronomer on Astrology