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Foucault Pendulum

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Léon Foucault puts the pendulum into motion for an audience

The Foucault 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 of the earth's rotation. The pendulum swings back and forth, and over time, slowly seems to rotate over its arena or "pit". It is explained that the earth is rotating beneath the pendulum. Today Foucault Pendulums are popular displays in science museums and universities.

A common criticism of the Foucault 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 Foucault Pendulum experiment the pendulum is, as we will read below, generally inconsistent in its movements. Because of air resistance and the impossibility of perpetual motion, the unmotorized pendulum will only move for a limited period before needing to be reset. In motorized Foucault 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 line 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 The Romance of Science:

  “ 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! ”

Historical Reception

The Popular and Scientific Reception of the Foucault Pendulum in the United States
by Michael F. Conlin, Ph.D. (bio)
Full Text Link

History professor Michael Conlin gives us a historical account of the Foucault Pendulum and its reception. We read that, although the Foucault Pendulum was publicly supported by Royal Astronomer George Airy and others, it was privately rejected:


  “ In private correspondence, Airy had repeatedly dismissed the Foucault pendulum experiment as a "fraud." He regarded the latitude-dependent formula for the period of the apparent precession of the pendulum as a "mathematical curiosity having no application whatever to the soi-disant experiment." Attending no demonstrations, Airy based his conclusion on oral accounts of the experiment. Concurring with Airy, Powell accepted the theory but held that as a "practical question" the experiment was "doubtless open to every kind of doubt." After learning of successful demonstrations by British scientists, Airy conducted his own experiments. Although it was possible to conduct the experiment properly, he concluded that the "difficulty of starting a free pendulum, so as to make it vibrate at first in a plane, is extremely great."

Although Airy and Powell kept their opinions private, popular journals learned of their rejection of the experiment. Linking Airy's and Powell's doubts to recent unsuccessful popular demonstrations of the experiment, these journals questioned the validity of the Foucault pendulum. The London Literary Gazette recommended caution to those who would attempt the experiment because "persons unqualified by previous habits of research and accurate investigation" had failed. The Literary Gazette knew of several exhibitions "in which, to the horror of the spectators, the earth has been shown to turn the wrong way." The Illustrated London News expressed similar reservations, observing that the "experiment is now giving rise to much controversy, and it is hard to conceive that there is not some fallacy lurking at the bottom of it." ”


  “ In Newark C. Dowden, a correspondent of the AAAS [American Association for the Advancement of Science], conducted an unsuccessful repetition in July. In an article for Appletons' Mechanics' and Engineers' Journal, Dowden explained that scientists had "jumped to a hasty and premature conclusion" in asserting that a vibrating pendulum could provide a visible proof of the earth's rotation because of two "unjustifiable" assumptions: that friction at the point of suspension was nothing and that the pendulum could vibrate independently of the earth's motion. Dr. William Kitchell, a member of the AAAS, and two other gentlemen repeated the experiment with a pendulum thirty feet in length at the New Jersey Art-Union Gallery in Newark. After numerous trials made over several days, they reported to the Newark Advertiser that the result was "invariably ELLIPTICAL MOTION!" While it might have been theoretically possible to take sufficient precautions to repeat the experiment successfully, they "reluctantly" concluded that air resistance and the impossibility of bringing the pendulum completely to rest before starting were "insuperable obstacles to its practical performance. "After consultation with Dowden, Kitchell reasoned that Foucault's demonstration at the Pantheon and subsequent American exhibitions must have been "vitiated" by "unobserved errors." They boldly proposed another method to demonstrate the earth's motion with a pendulum by measuring the deviance between the calculated motion and the observed motion of the apsides of the ellipse. They hoped that Strong would determine the angular motion of the ellipse of a given pendulum so that they might perform the experiment. Assuring them that the Foucault pendulum demonstrated the earth's rotation, Strong offered to repeat the experiment for doubters. Dowden remained unconvinced, believing that he had a "positive duty" to reject the Foucault pendulum because its "advocates cannot agree among themselves," its proofs were "impossible," and its workings "absolutely absurd."2 ”

The work goes on at length on the public controversy surrounding the experiment.

Systematic Error

Prof. A.C. Longden of Knox College found that the pendulum was subject to mechanical errors and bias. From "On the Irregularities of Motion of the Foucault Pendulum" in the April 1919 edition of The Physical Review we read:

  “ Neverless the pendulum behaves as if it had two periods. Furthermore it always started rotating clockwise, never counterclockwise. This fact remained a puzzle until another wire was substituted for the wire which I had been using. The two wires were cut from the same piece, and were certainly as nearly alike as two wires could well be, and yet the pendulum now invariably started rotating counterclockwise--never clockwise.

...After starting the pendulum a number of times with the new wire and always getting the same results, counterclockwise motion at the start, I rotated the wire 180 degrees on its own axis, without disturbing either the pendulum ball or the support. The wire may be rotated by turning the screw G, Fig. 7, without disturbing the hanger, and a similar arrangement at the bottom enables us to rotate the wire without rotating the ball. Now, the pendulum started rotating clockwise. ”

When the wire was replaced or rotated, the pendulum moved in the opposite direction. This may be a reason for why some who have performed this experiment have claimed to see consistency in direction.

Non-Uniform Variation

In Earth Not a Globe Samuel 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 Foucault Pendulum is often seen to move in the wrong direction entirely. See the following from A Hundred Proofs The Earth is Not a Globe:

  “ 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. ”

From The Romance of Science (8-10) by Lady Blount and Zetetes we read:

  “ 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. ”

Museum Exhibit Devices

We read the following from an installation guide: (Archive)

  “ Pay close attention to the photo beams alignment. This adjustment can effect the Ball’s precession around the pit. It may require a couple of days to determine if precession is operating properly. Precession is a function of the Earths rotation. ”

We are instructed to spend several days adjusting the alignment of the photo beams, which affects the pendulum's precession, an element which is supposedly a function of the earth's rotation, until we have determined that the "precession is operating properly".

Dr. Schoepffer

Dr. Schoepffer (bio), an eye-witness of the experiment, says:

  “ In an introductory speech Dr. Menzzer at Quedlinburg showed that until then there had been no proof for the Copernican hypothesis, the so-called proofs being, after close investigation, just as many confutations, until the Foucault pendulum showed the rotation of the earth uncontrovertibly. The pendulum was tied, the string was burnt, the swingings began, but the pendulum deviated to the left, instead of to the right. It was hastily brought to rest. New burning of the string. This time the deviation was the one desired, and we were invited again to be present in the church the next morning at eight o’clock, to be convinced that the deviation agrees with the theory. On the following morning, however, we saw that the pendulum during the night had changed its mind, and had from the deviation to the right again returned to the left. To me this new proof did not seem to be quite in order. My belief in the Copernican doctrine was shaken by the speech of Dr. Menzzer, and I concluded to go to Berlin for an explanation.

After seeing the pendulum-experiment here also and, strangely, again with a deviation to the left, I went to Alexander v. Humboldt, who was indeed ever the first refuge of those seeking information. He received me very friendly and spoke the memorable words: "I have known, too, for a long time, that as yet we have no proof for the Copernican system, but I shall never dare to be the first to attack it. Don’t rush into the wasps’ nest. You will but bring upon yourself the scorn of the thoughtless multitude."

Furthermore, I have found, by careful experiments, that a skillful experimenter can let the pendulum deviate either to the left or to the right. And we must not overlook the fact that the deviations may be caused by air-currents, electricity, earth-magnetism, special apparatus, and perhaps many other causes. Blunt and Cox observed the most curious and contrary swingings. Phillips of New York found very great hourly deviations in the swinging-line. Walker observed a peculiarly swift deviation when the pendulum swings in the magnetic meridian. D’Oliveira at Rio de Janeiro stated that the pendulum deviates to the right in the direction of the meridian, but to the left in the direction of the parallel. This deviation, diametrically opposed to the theory, was seen very often. And sometimes the pendulum does not deviate at all. Much more could be said against this “beautiful experiment.” Though beautiful it may seem to the theorist, it certainly is far from being irrefragable evidence for the earth’s motion. ”

Dr. Schoepffer's quote from Alexander von Humboldt (bio) appears again in The Earth Stands Fast.

South Pole Pendulum

Foucault Pendulum at South Pole

In 2001 a Foucault pendulum device was installed at the "South Pole" in the stairwell of a newly constructed research station. It has been claimed that this verified Foucault’s theory. However, excerpts from the report show that adjustments had to be made to obtain the desired results. (Archive)

  “ A hole was drilled through a length of 2×4 through which the pendulum’s wire was fixed in such a way that it would have freedom of movement in all directions. The 2×4 was then fixed to the top of the stairwell and the wire suspended down into the stairwell.

...There was no mechanism for keeping the pendulum swinging and the amplitude decayed within a couple of hours so it had to be restarted periodically over the 24 hour period.

...Our first attempt with the pendulum showed the Earth spinning backward from what was expected. We didn’t notice this at first because we’re all from the Northern Hemisphere and are accustomed to the earth spinning in an anticlockwise direction. We then realized that from our frame of reference the earth should be spinning clockwise so we had to modify the pendulum.

...Our second attempt showed the earth rotating in the proper direction but at an angular velocity twice what is expected (ire., 12 hour days instead of 24). We suspected some kind of government conspiracy but decided to make a further modification and try it again.

...It was difficult to make the pendulum swing in a plane instead of an ellipse. After several attempts with various techniques of holding the bob and dropping it we always got some kind of ellipse instead of a plane. This adds to our error because it is more difficult to locate and mark the pendulum arc’s apex. A way to do it is to suspended the bob by tying it off with a piece of string and letting it settle, then burn through the string. The bob would then drop without any outside force and swing in a plane. Since it is against the Antarctica Treaty to have any open flames at the South Pole we could not do this. After much practice Mike Town got very adept at dropping the bob so that it arced in a plane. ”

Mach's Principle

One alternative explanation that has been suggested by those who do accept the Foucault 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."

External links:

See Also

Flat Earth Topics on Rotation and Revolution

  • Michelson-Morley Experiment - Light velocity experiment which suggests a lack of Earth's motion around the Sun
  • Sagnac Experiment - Experiments which show that light's velocity is indeed affected by detector motion
  • Airy's Failure - An experiment which suggests that the stars are in motion, rather than the Earth
  • Time Dilation by Latitude - The predicted time dilation caused by Earth's rotation does not occur
  • Aviation - Mechanical air flight assumes a flat, non-rotating Earth

Round Earth Topics on Rotation