The Foucault pendulum swings in a constant plane or direction, and thus reveals the rotation of the Earth turning underneath. The Foucault pendulum finally answered an often-voiced criticism of Copernicus (posed, for example, by Tycho and Riccioli), that a cannon ball fired northward should result in a deflected trajectory due to the Earth’s rotation. The Foucault pendulum was not conceived until 1851, three centuries after Copernicus.