The Coriolis effect is the apparent deflection of objects from a straight path if the objects are viewed from a rotating frame of reference. The effect is named after Gaspard-Gustave Coriolis, a French scientist who described it in 1835, though the mathematics appeared in the tidal equations of Pierre-Simon Laplace in 1778. One of the most notable examples is the deflection of winds moving along the surface of the Earth to the right of the direction of travel in the Northern hemisphere and to the left of the direction of travel in the Southern hemisphere. This effect is caused by the rotation of the Earth and is responsible for the direction of the rotation of cyclones: winds around the center of a cyclone rotate counterclockwise on the northern hemisphere and clockwise on the southern hemisphere.
The Coriolis effect is caused by the Coriolis force, which appears in the equation of motion in a rotating frame of reference. Sometimes this force is called a fictitious force (or pseudo force), because it does not appear when the motion is expressed in an inertial frame of reference. In an inertial frame of reference, the motion is explained by the real impressed forces, together with inertia. In a rotating frame, the Coriolis and centrifugal forces are needed in the equation to correctly describe the motion.
Contrary to popular belief, the Coriolis effect is not the determining factor in the rotation of water in toilets or bathtubs (see the Draining bathtubs and toilets section below).