The idea that the water in the toilet spins the opposite direction in the Southern Hemisphere is simply not true. The myth comes from the effect of Earth’s rotation, a force also known as the Coriolis effect. While the toilet bowl water may be too small scale to truly experience this effect, the Coriolis force does have a very real presence in the motion of both oceanic and atmospheric processes.

The basic definition of the Coriolis effect is the deflection of a moving object from a straight path when in a moving frame of reference. In our case, the moving frame of reference would be the spinning Earth. The moving objects can also be thought of as the forward momentum of ocean currents or atmospheric wind. Due to the rotation of the Earth, these types of large

physical processes will deflect moving water or air to the right in the Northern Hemisphere and left in the Southern

Hemisphere. It is the same physics that will lead storms in the North Hemisphere to rotate counter-clockwise, whereas storms in the Southern hemisphere rotate clockwise. As well, large ocean gyres will experience the same rotation as they transport water across the globe. 

So what about that toilet bowl theory? Shouldn’t it do the same? Well, not really. The rotation of the earth is a much larger scale influence and is only a very small fraction of magnitude on a small body of moving water. This influence is nothing noticeable compared to the more dominating physical factors in the toilet, such as the toilet bowl’s structures or at what angle the water initially enters the bowl.  

 Katie Jackson

Surfline.com

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