What if you played catch on a merry-go-round?

Imagine you and a friend sit on a merry-go-round. You roll a ball across to your friend.

If the merry-go-round is spinning, does the ball still reach them?

Rolling a ball to a friend across a still merry-go-round is easy: aim straight and it arrives. But what if the merry-go-round is turning while the ball rolls?

This lab is a thought experiment. You never need to throw anything on a moving ride; we will imagine it carefully, then try a safe version on a paper plate. The surprising answer helps explain why winds curve and hurricanes spin.

Rolling a ball across a rotating platform poses a classic question about frames of reference: does the ball's path depend on who is watching?

The answer leads to the Coriolis effect, which governs the large-scale turning of winds and ocean currents on our rotating planet. Here we reason it out without anyone throwing balls on a moving ride.

When the merry-go-round is still, the ball rolls straight to your friend.

Nothing is turning, so it is easy to aim.

On a merry-go-round that is standing still, rolling a ball to the friend across from you is simple and straightforward, as National Geographic puts it. The ball travels in a straight line and your friend stays put, so it arrives.

The ground under our feet is spinning too, though we can't feel it. Earth turns once every 24 hours, and at the Equator the ground races along at nearly 1,600 kilometers (1,000 miles) per hour.

With the platform at rest, the ball and the target share one stationary frame and a straight-line roll succeeds.

Earth, however, is a rotating platform. It turns once a day, so a point on the Equator moves at nearly 1,600 km/h while points nearer the poles move more slowly (National Geographic). This difference in speed is the key to what happens when things travel long distances across Earth's surface.

Now pretend the merry-go-round is spinning.

You roll the ball straight. But to you, it looks like it curves away from your friend!

Now imagine the merry-go-round turning while the ball rolls. To the riders, the ball seems to curve away and misses the friend. But a friend standing on the ground nearby sees something different: the ball rolls in a straight line.

The ball isn't really curving. While it rolls, the riders are carried around, out of its path. That is the heart of the Coriolis effect.

Seen from the ground (an inertial frame), the released ball moves in a straight line, since no sideways force acts on it. Seen from the rotating platform, the riders move out of its path, so the ball appears to swerve.

Physicists describe that apparent swerve with a 'Coriolis force' in the rotating frame. National Geographic stresses that no physical force is involved: it is the ground moving at a different speed from an object in the air.

The ball goes straight. But you and your friend are moving in a circle.

By the time the ball gets there, your friend has moved!

On a spinning merry-go-round, everyone riding is carried around in a circle. The rolling ball isn't: once it leaves your hand, it simply goes straight. So it ends up where your friend used to be.

On Earth, the same thing happens to air and water moving long distances. In the Northern Hemisphere, moving air seems to bend to the right; in the Southern Hemisphere, to the left. This is named after Gaspard-Gustave de Coriolis, a French mathematician who studied spinning things like waterwheels.

Earth rotates west to east, so the surface moves fastest at the Equator and slowest near the poles. Air moving north from the Equator carries its faster eastward speed and outruns the ground beneath it, appearing to swing right; air moving toward the Equator lags and also appears to swing right in the Northern Hemisphere (National Geographic).

NOAA describes the result: circulating air is deflected right in the Northern Hemisphere and left in the Southern. The effect is named after Gaspard-Gustave de Coriolis (1792–1843). It grows with speed and distance and vanishes at the Equator.

Big storms called hurricanes spin around.

The spinning Earth helps them turn. North of the Equator, they turn one way. South of it, they turn the other way.

The Coriolis effect shapes Earth's winds. Air flowing back toward the Equator is turned into the steady trade winds. When air rushes toward the center of a hurricane, it is deflected, so the storm spins: counterclockwise north of the Equator and clockwise south of it.

And one popular story is not true: you can't see the Coriolis effect in a flushing toilet or a draining sink. Those swirl because of their shape or how the water was moving.

Deflection organizes the trade winds and makes cyclones rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern. National Geographic notes that cyclones need the Coriolis effect to circulate, so hurricanes almost never form in equatorial regions, where it is absent, and never cross the Equator. Pilots must allow for its influence on prevailing winds.

The effect matters only for large distances or high speeds. National Geographic calls drain-direction stories an urban legend: basin shape and stray motions dominate.