Despite what you may have heard, it doesn't make water go down the drain one direction or the other. But it does have an effect: The Coriolis Effect can turn ships off course and change the weather.

I first learned about the Coriolis Effect when watching that great educational show; The X-Files. Mulder notices at a school that the water in the drinking fountain drains clockwise. Due to the Coriolis Effect, it should drain counter-clockwise, and so clearly something strange is going on. More concrete proof of this was given when the substitute teacher turned into a snake and ate the PTA. It turns out that The X-Files lied about the likelihood of at least one of those things.

(The Simpsons was also in on it.)

The Coriolis Effect does exist, though. It describes how the rotation of the earth has an effect on objects. Living, as we do, on the constantly-moving surface of the earth, we see this moving body as a stationary object. Throughout our ordinary lives, we're able to maintain this illusion. Although the rotation of the earth affects us, the effect is small. If, however, we were plucked out of our lives and asked to stand near the north pole and fire an object south to hit a target, we would feel the effect of the earth's rotation.

We'd aim the object carefully, let it fly – and see it veer off to the right of our target. The further the throw, the further the object would veer. Of course, the object doesn't veer to the right. The earth underneath it rotates to the left, or eastward. The object appears to swerve off to the right.

If you were to stand on the equator, and launch an object northward, the object would appear to swerve to the left, correct? It makes sense. The earth is turning towards the right (from your perspective). The object you launch would land to the left of your target. Oh, but no!

If you are standing on the equator, you are whizzing through space faster than anyone else on the globe. You have to travel the circumference of the globe once each day, when you are on the equator. Someone up north, or south, has to make a comparably smaller rotation each day, and a person on the pole stands still.

When standing on the equator, looking north, you have a massive rightward velocity. When you launch your object northward, it's flying to the right along with you. The target is not moving right nearly as fast, and so the object you throw has a rightward motion in respect to the target. You will see your perfectly ‘straight' throw veer off to the right. In fact, the Coriolis Effect makes any untethered object in the northern hemisphere appear to deflect to the right. In the southern hemisphere, it flips, making all objects appear to deflect to the left.

Although this effect can theoretically be demonstrated with water in a basin with a drain, it can't be done in most sinks. Sinks that are not circular, that have irregular sides, or that have large drains have enough of an effect on the water in them that Coriolis forces are entirely eclipsed. It's possible to demonstrate a slight Coriolis Effect with a meter-wide circular sink with shallow, smooth sides and a tiny drain.

In my next post on the Coriolis Effect, we'll see how it makes storms swirl to counterclockwise in the northern hemisphere and clockwise in the southern hemisphere.

Via University of Oregon and the University of Montana.