Water going down a bathtub drain might seem like a simple phenomenon, but it's triggered a long and very complicated debate. Seriously. As you can see from videos like this one, some tub-watchers out there insist that water always swirls one direction in the Northern Hemisphere and the opposite direction in the Southern Hemisphere.
That argument sounds plausible, especially when the well-known Coriolis effect is invoked (more on that later). But does the argument really hold water--or is the different-direction-in-opposite-hemisphere crowd all wet?
"For household sinks, tubs and toilets this is a myth," Dr. Paul Doherty, senior scientist at the Exploratorium in San Francisco, told The Huffington Post. "People can check it for themselves. Drain all the sinks and tubs in their house and see which way they swirl. Usually some sinks would drain clockwise and some counterclockwise, which means either it is a myth, or they live on the equator."
Dr. Doherty was joking about the equator part. But he explained that water can swirl either way anywhere on earth--and that which way it swirls is effectively random. So why do so many people still believe otherwise? Because the notion that draining water should swirl differently in different hemispheres could be true in theory. Sort of.
To understand why this is the case, you need to know a bit about the Coriolis effect--it explains the phenomenon in which the rotation of the earth can cause things moving in a straight line to appear to follow a curving path. The Coriolis effect can be helpful for understanding large-scale movements (such as storms, airplanes, and wind), but it's inconsequential when it comes to something as small as a bathtub.
How inconsequential? The Coriolis effect is roughly 30 million times less significant than the force of gravity when it comes to describing water going down a typical drain, Doherty said, referring to a 1962 calculation made by MIT scientist Dr. Ascher Shapiro.
"He got a basin of water, filled it six inches deep, covered it with plastic so the air wouldn’t blow on it and put a match on the water, just like in the [BBC] video, and then pulled the plug," Doherty said. "After about 15 seconds the match began to rotate counterclockwise."
As the experiment was conducted in the northern hemisphere, Doherty said, Shapiro's colleagues in Australia repeated the test--and the match rotated the other way.
"So if you do this with scientific precision far enough from the equator, it actually has been done," Doherty said, adding that many scam artists can manipulate this test. "It went the wrong way in the experiment in the [BBC] video."