The Focault Pendulum is a famous experiment which is alleged to give simple, direct evidence of the earth's rotation. Introduced in the 1851, Léon Foucault claimed that the swinging rotational motions were proof the earth's rotation. Today Foucault Pendulums are popular displays in science museums and universities.
A common criticism of the Focault Pendulum is to point out that when the pendulum experiment has been recreated and put into motion, the pendulum has often been seen to rotate in excess, in shortness, or in an opposite direction from the direction it should have traveled according to theory. At times it does not rotate at all. Those scientists who have repeated the experiment have freely admitted that “it was difficult to avoid giving the pendulum some slight lateral bias at starting.”
In the unmotorized Focault Pendulum experiment the pendulum is generally unpredictable, and because of air resistance, the unmotorized pendulum will only move for a short while before needing to be reset. In motorized Focault Pendulums, as seen in museum exhibits, it is the repetitive machinery which imparts the repeating lateral bias that creates the regular results seen for the museum's visitors.
Thus, the experiment is entirely invalid as a demonstration of diurnal rotation. That a pendulum on a string can rotate as it swings back and fourth has more to do with the initial conditions which set it into motion than the supposed rotation of the earth.
Lady Blount provides the following:
In Earth Not a Globe Samuel Birley Rowbotham informs us that the variation of the pendulum is often non-uniform and unpredictable:
“ First, when a pendulum, constructed according to the plan of M. Foucault, is allowed to vibrate, its plane of vibration is often variable – not always. The variation when it does occur, is not uniform – is not always the same in the same place; nor always the same either in its rate or velocity, or in its direction. It cannot therefore be taken as evidence; for that which is inconstant cannot be used in favor of or against any given proposition. It therefore is not evidence and proves nothing! Secondly, if the plane of vibration is observed to change, where is the connection between such change and the supposed motion of the Earth? What principle of reasoning guides the experimenter to the conclusion that it is the Earth which moves underneath the pendulum, and not the pendulum which moves over the Earth? What logical right or necessity forces one conclusion in preference to the other? Thirdly, why was not the peculiar arrangement of the point of suspension of the pendulum specially considered, in regard to its possible influence upon the plane of oscillation? Was it not known, or was it overlooked, or was it, in the climax of theoretical revelry, ignored that a ‘ball-and-socket’ joint is one which facilitates circular motion more readily than any other? ”
The Wrong Direction
The Focault Pendulum is often seen to move in the wrong direction entirely. See the following quotes:
One alternative explanation that has been suggested by those who do accept the Focault Pendulum is Mach's Principle. Mach's Principle explains that if the earth was still and the all the stars went around the Earth then the gravitational pull of the stars would pull the pendulum. As Mach said "The universe is not twice given, with an earth at rest and an earth in motion; but only once, with its relative motions alone determinable. It is accordingly, not permitted us to say how things would be if the earth did not rotate."