Pendulum the pendulum could be used to measure the force of gravity. During his expedition to Cayenne, French Guiana in 1671, Jean Richer found that a pendulum clock was minutes per day slower at Cayenne than at Paris. From this he deduced that the force of gravity was lower at Cayenne. In 1687, Isaac Newton in "Principia Mathematica" showed that this was because the Earth was not a true sphere but slightly oblate (flattened at the poles) from the effect of centrifugal force due to its rotation, causing gravity to increase with latitude. Portable pendulums began to be taken on voyages
Foucault pendulum Earth, "F" is the component of the Coriolis force in the "x"-direction and "F" is the component of the Coriolis force in the "y"-direction. The restoring force, in the small-angle approximation, is given by Using Newton's laws of motion this leads to the system of equations Switching to complex coordinates , the equations read To first order in Ω/"ω" this equation has the solution If time is measured in days, then and the pendulum rotates by an angle of −2π sin("φ") during one day. Many physical systems precess in a similar manner to a Foucault pendulum. As early as 1836,