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.

As well as deflecting the paths of long-range artillery shells and ballistic missiles, the Coriolis effect is what causes cyclones (which includes hurricanes and typhoons) to spin clockwise south of the equator and counterclockwise north of it. Indeed, the Coriolis effect is the reason that winds in general tend to flow around regions of high and low pressure, running parallel to the lines of constant pressure on a weather map (“isobars”), instead of flowing directly from high to low pressure at right angles to the isobars. In the Northern Hemisphere, air flowing radially inward across the isobars toward the low pressure would be deflected to the right. The motion reaches a steady state with the wind encircling the low-pressure area—the pressure gradient pushing inward and the Coriolis force outward.

A popular factoid claims that water running down a drain turns in one direction in the Southern Hemisphere and the opposite way in the Northern Hemisphere. That idea is a myth: although the Coriolis force is strong enough to direct the winds of hurricanes when acting over hundreds of miles for days, it is far too weak to stir a small bowl of water in the scant seconds the water takes to run down the drain.