The idea that water drains in opposite directions in the northern and southern hemispheres has been going around for years. Everyone from PBS and the BBC, to The Simpsons and Obi Wan himself have weighed in on the debate. But amazingly, they’ve all been wrong – and so was I.
In their latest, and perhaps most epic collaboration to date, science communicators Destin of Smarter Every Day and Derek Muller of Veritasium joined forces to settle the score, once and for all. Not only do the two cross-continental videos bring together some of our favorite YouTubers, but thanks to the editing wizardry of Gordon McGladdery, they were made to be played simultaneously. The result is something I’ve never seen before (even better if viewed on two devices), so bust out your gear, press play, and get ready for a double dose of science.
To unravel this mystery, Destin and Derek set up the exact same experiment in both hemispheres: first, in Sydney, Australia (34-degrees-south latitude), and then two-years later in Huntsville, Alabama (34-degrees-north latitude). In each location, a 1.5 meter (5 ft) kiddie pool was filled with water, left to settle, and drained. Surprisingly, the water actually did whirl in different directions.
The experiment revolves around the Coriolis effect, a force caused by the rotation of the Earth on its axis. This effect causes hurricane winds to rotate counter-clockwise in the northern hemisphere, and clockwise in the southern hemisphere, so in theory, it should do the same to draining water. But as we’ve seen time and time again in past experiments, it doesn’t.  So, what’s going on?
The problem with most previous attempts, the team explains, isn’t actually the science, but the experiments themselves. For starters, most toilets have jets under the rim, which determine direction of flow. So conducting a Coriolis experiment in your porcelain throne is automatically a no-go. “And in any container of water,” adds Muller, “there’s always going to be some kind of angular momentum – meaning the water isn’t going to be perfectly still [to begin with].”
The Earth rotates one time every 24 hours, but the water in your sink or bathtub makes a rotation in just a few seconds. That water is spinning ten thousand times faster than Earth, which minimizes the Coriolis effect’s visibility. This means even small ripples, like the ones caused by reaching your hand in to pull the drain plug, are enough to mask it completely.
By using a valve to open the drain in their pools, Muller and Destin created a situation where no forces were acting on the water, save for the one exerted by our big blue dot. “You can see what a tiny little effect it is, and what extreme lengths we had to go to, to see it,” says Muller. “But it’s real, it’s freaking there!” adds Destin with a laugh.
Slow clap, guys. Slow effing clap.
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IMAGES: Veritasium, Smarter Every Day
