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Imagine for a moment that you are trapped aboard a ship in the ocean without a map, a radio, GPS, or any landmarks. Is there a way to determine at least which hemisphere you are in? This scene played out in the movie Escape Plan (2013) in which the main character, Breslin, played by American actor Sylvester Stallone, tries to determine the location of the prison ship within which he is trapped by watching the path of the water in a toilet as it slides into the drain. He mentions to his fellow inmates that “water goes counterclockwise above the Equator” and determines that the prison ship is off the coast of Morocco. Is this possible? Can you actually use a flushing toilet to figure out whether you are in the Northern or in the Southern Hemisphere?

Sadly, you cannot, because toilets tend to angle the jets that pass water into the bowl to drive the direction of draining water. The scene in the movie, however, does introduce the concept of Coriolis force or the Coriolis effect, which may affect the path of fluids, such as air or water, but this bears some explanation. The effect of the Coriolis force is an apparent deflection of the path of an object that moves within a rotating coordinate system.

On Earth an object that moves along a north-south path will appear to veer to the right in the Northern Hemisphere but to the left in the Southern Hemisphere. There are two reasons for this phenomenon: first, Earth rotates eastward; second, the relative speed and direction of a point on Earth is a function of latitude (it is essentially zero at the poles, and it attains a maximum value at the Equator). Thus, if you were to fire a ball from a cannon northward from a point on the Equator, the ball would land to the east of its due north path. (A similar displacement occurs if the cannonball is fired in any direction.) In the Southern Hemisphere, however, since Earth also rotates eastward, the deflection would be to the left.

The Coriolis force also works on air and water in a similar fashion but only if the volumes of these fluids are extremely large. So, in a toilet, even if the influence of the jets filling up the bowl was insignificant, the volume of water in the toilet is too small to be affected by Earth’s rotation (and the Coriolis force) in any appreciable way.