This is the reason why toilets in both hemispheres do not swirl in opposite directions.

There’s a saying that goes like, “common and persistent misconceptions tend to be rooted in truth.” In fact, hurricanes swirl in opposite directions in both hemispheres due to the Earth’s rotation; and this idea has led to the belief (probably alleged) that water closet toilets also swirl in opposite directions as well. Like what? Yeah, ask any frequent equator-crossing traveler and they might confess what really happens. Spoiler Alert: toilets don’t (they never, and will ever) swirl in opposite directions in both hemispheres. The catch? There’s a real physical phenomenon fueling this misconception. It’s called the Coriolis effect.


Related media: The Coriolis Effect Explained


The Swirly Goes Round

The Coriolis effect is a natural geographic phenomenon that makes wind, water, and virtually every other free-moving thing curve with the rotation of the Earth relative to it’s axis. This effect is made possible by two basic concepts: the Earth’s spherical geometry and Newton’s first law of motion, also known as inertia. The Earth has a circumference of about 38,624 kilometers (24,000 miles) at the equator, which makes a complete rotation once a day.

What this means is that, if you’re standing still at the equator, you’ll actually move as the Earth rotates roughly 1,600 kilometers per hour (1,000 miles per hour) than someone standing still at either of the poles. Now, let’s compare these two hypothetical people to you, if you happen to be standing still at the 45th parallel north or south — say the border between …, or the border between …, for instance. Weird, huh?

Here, the earth beneath your feet is moving at a pace of roughly … kilometers per hour (750 miles per hour) relative to the equator and to either of the poles. Newton’s First Law states that objects in motion tend to stay in motion. What this also means is that, if you stood still at … (your location), and threw a ball so hard enough to reach the equator, you’d miss the target. Because the equator rotates faster along the axis of the Earth than it does at your location.



The Turning, The Swirling; Is All The Difference

Image: NASA Earth Observatory

Now, let’s also imagine an oncoming hurricane. The difference in velocity of the Earth’s rotation pretty much affects the swirling of the hurricane northernmost edge and it’s southernmost edge. They could be several hundreds of kilometers wide, and that’s a pretty big difference. Hurricanes swirl counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Their enormous size is what account for that huge difference in ground velocity between the northernmost and southernmost edges of the storm system.

This is the Coriolis effect in reality, but for it to have an impact on smaller fluid systems, like water closet toilets and you staring your coffee in the morning, well, this is more like sci-fi than actually science. The catch? The Coriolis effect does have an impact on flushing toilets, but to such a tiny degree that, well, you need the patience to observe or even prove it. Challenge on? Hell yeah!



We Just Don’t Swirl Alike

In 1962, fluid mechanics expert Asher Shapiro from the Massachusetts Institute of Technology (MIT) performed an experiment to observe at firsthand the Coriolis effect in a large, shallow dish of water. He conducted the experiment with enough precision, even to compensate for any minuscule forces in air pressure that could account for a change of a tiny counterclockwise swirl as the water drained.

On the other half of the planet, another team in Sydney, Australia later performed the same experiment and demonstrated the opposite results. Sounds fair enough. What this means in practice is that, whatever direction water closet toilets flush in has more to do with the shape and size of the bowl and the inertia of the fluid than that of the rotation of the Earth itself. Another legend successfully debunked!


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Written by: Nana Kwadwo, Fri, Apr 05, 2019.

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