So it works on large scales. But on small scales like in your toilet, sink, or bucket , the rotation of the Earth itself at a decidedly pokey rate of one rotation per day is much weaker than other forces -- like the force of water jets in a toilet, or the force of water hitting slopes in a sink. In tracking down where this drain-direction myth originated and how it got so firmly lodged in the heads of people like me, many sources discuss the otherwise awesome Michael Palin documentary Pole to Pole , in which Palin visits the equator in Kenya and observes a tourist trap in which a man "demonstrates" via fakery the draining of water in different ways on the equator itself, and just north and south of it.
Palin doesn't point out that it's fake. I remember seeing this documentary when it came out, and it may be where I picked up the notion -- it seems like such an appealing demonstration of science, such an "ah-ha! We're all tiny ants on a huge spinning globe! What wonders! Sadly, it's BS. Again, Fraser has a good write-up ; here's a snippet:. Indeed, a colleague of mine, who witnessed the performance first hand and knew it was a cheat, was not able to spot how the fraud was perpetrated.
It is an interesting sidelight that when back on the bus, he informed his fellow tourists that they had just witnessed fakery the Earth did not cause the rotation they had just seen there was widespread disappointment. The tourists preferred the fantasy to the reality. Fraser proceeds to explain how you can fake it yourself. According to various sources , it is possible to demonstrate a Coriolis effect on water on a small scale, but only under extremely controlled circumstances -- involving predictably shaped water vessels, long periods of time of waiting for water to become as still as possible, carefully removing a stopper in the bottom of the vessel without adding spin, and other such crazy stuff.
But in your typical toilet or sink, the Coriolis force is so small as to be undetectable relative to other forces. Even holding a bowl of water and turning around introduces sufficient spin to get things going in one direction or another.
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Reddit Pocket Flipboard Email. Explainers The big questions about Covid booster shots. Explainers 5 things to know about the big climate conference in Glasgow. A toilet or sink is just not large enough. The Coriolis effect influences because wind velocities may be hundreds of times greater than the motions in a sink and because the distances involved are far larger than the tiny draining diameter in a sink or toilet.
The net motion in the water becomes much more pronounced as the water is forced to move in toward the center of evacuation, causing the normally invisible flows in the water to become visible as the water nears the drain. The ultimate direction of that flow is random--it can go one way one time, the other way the next.
But you will find that the faucet is almost always off center or that there is some other asymmetry in the sink. As a result, filling the sink consistently gives it some net rotation in the same direction, which you see as the normal direction of evacuation. Toilets will always drain and fill the same way, for the same reason.
Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Brad Hanson, a staff geologist with the Louisiana Geological Survey, presents the argument of why--in theory--water going down the drain would indeed spin in different directions depending on which hemisphere you're in: "The direction of motion is caused by the Coriolis effect.
Decker, professor emeritus of oceanic and atmospheric science at Oregon State University notes, however, that the Coriolis effect may actually have little to do with the behavior of real-world sinks and tubs: "Really, I doubt that the direction of the draining water represents anything more than an accidental twist given by the starting flow.
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