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 motions were proof the earth's rotation. Today Foucault Pendulums are popular displays in science museums and universities.
A common criticism is to point out that when the pendulum is put into motion it has often been seen at times to rotate in excess, in shortness, or in an opposite direction from the direction it should have traveled. At times it does not rotate at all. Those who have repeated the experiment have asserted that “it was difficult to avoid giving the pendulum some slight lateral bias at starting.”
In the unmotorized Focault Pendulum experiment the pendulum is entirely 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 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:
“This pendulum, modern scientists tell us, affords a visible proof that we are living on a whirling globe, which, according to a ‘work on science’ now before me, is spinning upon its so-called axis at the rate of over 1,000 miles an hour at the equator; and, in addition to other motions, is rushing on an everlasting tour round the sun (the diameter of which is said to be 813,000 miles, and its weight 354,936 times greater than the earth from which it is said to be about 93,000,000 miles distant,) at the rate of over 1,000 miles per minute. Now to prove that the earth really has these motions a pendulum is suspended at the show; the showman sets motion, and bids the gaping world of thoughtless men and women to ‘behold a proof’ that we are living on a whirling globe which is rushing away through space!”
—The Romance of Science, Lady Blount
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?”
—Earth Not a Globe, Samuel Birley Rowbotham
The Wrong Direction
The Focault Pendulum is often seen to move in the wrong direction entirely. See the following quotes:
“Astronomers have made experiments with pendulums which have been suspended from the interior of high buildings, and have exulted over the idea of being able to prove the rotation of the Earth on its ‘axis,’ by the varying direction taken by the pendulum over a prepared table underneath – asserting that the table moved round under the pendulum, instead of the pendulum shifting and oscillating in different directions over the table! But, since it has been found that, as often as not, the pendulum went round the wrong way for the ‘rotation’ theory, chagrin has taken the place of exultation, and we have a proof of the failure of astronomers in their efforts to substantiate their theory.”
—100 Proofs the Earth is Not a Globe, William Carpenter
“We believe, with all due deference to the pendulum, and its proprietor, that it proves nothing but the craftiness of the inventor; and we can only describe the show and showman as deceptions. A thing so childish as this ‘pendulum proof’ that it can only be described as one of the most simple and ridiculous attempts to gull the public that has ever been conceived. It has been said that the pendulum experiment proves the rotation of the earth, but this is quite impossible, for one pendulum turns one way; and sometimes, another pendulum turns in the opposite direction. Now we ask does the earth rotate in opposite directions at different places at one and the same time? We should like to know. Perhaps the experimenters will kindly enlighten us on this point … If the earth had the terrible motions attributed to it, there would be some sensible effects of such motions. But we neither feel the motion, see it, nor hear it. And how people can stand watching the pendulum vibrate, and think that they are seeing a proof of the motions of the earth, almost passes comprehension. They are, however, brought up to believe it, and it is thought to be ‘scientific’ to believe what the astronomers teach.”
—The Romance of Science (8-10), Lady Blount
As an alternative explanation, 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."