Newton’s laws of motion mathematically describe how objects move when forces are exerted on them.
Earth is spinning like a top, even though to us who are standing on Earth, it seems that we are not moving. Newton did not account for Earth’s spin in his equations. The Coriolis force appears as an extra term when Newton’s laws are transformed to account for Earth’s spin.
Italian scientist Giovanni Battista Riccioli described the effect in 1651, explaining that Earth’s rotation causes a cannonball fired to the north to deflect to the east. Gaspard-Gustave Coriolis published a paper in 1835 describing the force mathematically.
The Coriolis force acts in a direction perpendicular to Earth’s rotational axis. Objects in the Northern Hemisphere are deflected to the right, while objects in the Southern Hemisphere are deflected to the left.
The magnitude of the Coriolis force depends on the speed of the object and its latitude. The Coriolis force is zero at the equator and increases toward the poles.
The Coriolis force also is proportional to Earth’s rotation rate. Earth completes one rotation per day, so for everyday motions, like throwing a ball or an apple falling from a tree, the Coriolis force is very small compared to other forces and is negligible. Its effects become noticeable only for motions occurring over large distances and long periods of time.
Because Earth spins, we need to account for the Coriolis force to correctly analyze the large-scale movement of air in the atmosphere and water currents in the ocean. It is too small to explain the rotation of draining water in sinks and toilets.