See Inside September 2009

Coriolis Effect

The earth's spin influences hurricanes but not toilets

In the final year of World War I, when the German military pointed its largest artillery at Paris from a distance of 75 miles, the troops adjusted the trajectory for many factors that could be ignored with less powerful guns. In particular, a subtle influence from the rotation of the earth—the Coriolis effect or force—would have shifted all their shots by about half a mile.

Decades earlier a Parisian scientist by the name of Gaspard-Gustave de Coriolis had written down the equations describing that effect as a part of his 1835 paper analyzing machines with rotating parts, such as waterwheels. The Coriolis effect can arise in any situation involving rotation. If you stand anywhere on a counterclockwise-turning carousel, for instance, and throw a ball in any direction, you will see the ball’s trajectory curve to its right. Someone standing next to the carousel will see the ball move in a straight line, but in your rotating frame of reference the ball's direction of motion swings around clockwise. A new force appears to act on the ball. On the spinning earth, we see a similar (but much weaker) force acting on moving objects.

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