Rotating Earth: the proofs or significance of Leon Foucault’s pendulum – Part 1


In an earlier post, we had discussed proofs of the round shape of the Earth. This included some ancient and some modern proofs. There was, in general, a consensus that the shape of the Earth was spherical and not flat and the proofs were given since the time of ancient Greeks. Only in the middle ages, there seems to have been some doubt regarding the shape of the Earth. But amongst the learned people, there was never a doubt about the shape of the Earth. Counter-intuitive it may seem when you look at the near horizon, it is not that counter-intuitive. We can find direct proofs about it by looking around and observing keenly.

But the rotation of Earth proved to be a more difficult beast to tame and is highly counter-intuitive. Your daily experience does not tell you the Earth is rotating, rather intuition tells you that it is fixed and stationary. Though the idea of a moving Earth is not new, the general acceptance of the idea took a very long time. And even almost 350 years after Copernicus’ heliocentric model was accepted, a direct proof of Earth’s rotation was lacking. And this absence of definitive proof was not due to a lack of trying. Some of the greatest minds in science, mathematics and astronomy worked on this problem since Copernicus but were unable to solve it. This included likes of Galileo, Newton, Descartes, and host of incredibly talented mathematicians since the scientific revolution. Until Leon Foucaultin the mid-1800s provided not one but two direct proofs of the rotation of the Earth. In this series of posts, we will see how this happened.

When we say the movement of the Earth, we also have to distinguish between two motions that it has: first its motion about its orbit around the Sun, and second its rotational motion about its own axis. So what possible observational proofs or direct evidence will allow us to detect the two motions? In this post, we will explore how our ideas regarding these two motions of the Earth evolved over time and what type of proofs were given for and against it.

Even more, there was a simple geometrical fact directly opposed to the Earth’s annual motion around the Sun and there was nothing that could directly prove its diurnal rotation. (Mikhailov, 1975)

Let us consider the two components of Earth’s motion. The first is the movement around the Sun along the orbit. The simplest proof for this component of Earth’s motion is from the parallax that we can observe for distant stars. Parallax is the relative change in position of objects when they are viewed from different locations. The simplest example of this can be seen with our own eyes.

Straighten your hand, and hold your thumb out. Observe the thumb with both the eyes open. You will see your thumb at a specific location with respect to the background objects. Now close your left eye, and look at how the position of the thumb has changed with respect to the background objects. Now open the right eye, and close the left one. What we will see is a shift in the background of the thumb. This shift is related by simple geometry to the distance between our eyes, called the baseline in astronomical parlance. Thus even a distance of the order of a few centimetres causes parallax, then if it is assumed that Earth is moving around the Sun, it should definitely cause an observable parallax in the fixed stars. And this was precisely one of the major roadblock

Earth moving around an orbit raised mechanical objections that seemed even more serious in later ages; and it raised a great astronomical difficulty immediately. If the Earth moves in a vast orbit, the pattern of fixed stars should show parallax changes during the year. (Rogers, 1960)

The history of cosmic theories … may without exaggeration be called a history of collective obsessions and controlled schizophrenias.
– Arthur Koestler, The Sleepwalkers

Though it is widely believed that Copernicus was the first to suggest a moving Earth, it is not the case. One of the earliest proponents of the rotating Earth was a Greek philosopher named Aristarchus. One of the books by Heath on Aristarchus is indeed titled Copernicus of Antiquity (Aristarchus of Samos). A longer version of the book is Aristarchus of Samos: The Ancient Copernicus. In his model of the cosmos, Aristarchus imagined the Sun at the centre and the Earth and other planets revolving around it. At the time it was proposed, it was not received well. There were philosophical and scientific reasons for rejecting the model.

Aristarchus’s model of the heliocentric Universe

First, let us look at the philosophical reasons. In ancient Greek cosmology, there was a clear and insurmountable distinction between the celestial and the terrestrial. The celestial order and bodies were believed to be perfect, as opposed to the imperfect terrestrial. After watching and recording the uninterrupted waltz of the sky over many millennia, it was believed that the heavens were unchangeable and perfect. The observations revealed that there are two types of “stars”. First the so-called “fixed stars” do not change their positions relative to each other. That is to say, their angular separation remains the same. They move together as a group across the sky. Imagination coupled with a group of stars led to the conceiving of constellations. Different civilizations imagined different heroes, animals, objects in the sky. They formed stories about the constellations. These became entwined with cultures and their myths.


The second type of stars did change their positions with respect to other “fixed stars”. That is to say, they changed their angular distances with “fixed stars”. These stars, the planets, came to be called as “wandering stars” as opposed to the “fixed stars”.

Ancient Greeks called these lights πλάνητες ἀστέρες (planētes asteres, “wandering stars”) or simply πλανῆται (planētai, “wanderers”),from which today’s word “planet” was derived.


So how does one make sense of these observations? For the fixed stars, the solution is simple and elegant. One observes the set of stars rising from the east and setting to the west. And this set of stars changes across the year (which can be evidenced by changing seasons around us). And this change was found to be cyclical. Year after year, with observations spanning centuries, we found that the stars seem to be embedded on inside of a sphere, and this sphere rotates at a constant speed. This “model” explains the observed phenomena of fixed stars very well.

The unchanging nature of this cyclical process observed, as opposed to the chaotic nature on Earth, perhaps led to the idea that celestial phenomena are perfect. Also, the religious notion of associating the heavens with gods, perhaps added to them being perfect. So, in the case of perfect unchanging heavens, the speeds of celestial bodies, as evidenced by observing the celestial sphere consisting of “fixed stars” was also to be constant. And since celestial objects were considered as perfect, the two geometrical objects that were regarded as perfect the sphere and the circle were included in the scheme of heavens. To explain the observation of motion of stars through the sky, their rising from the east and setting to the west, it was hypothesized that the stars are embedded on the inside of a sphere, and this sphere rotates at a constant speed. We being fixed on the Earth, observe this rotating sphere as the rising and setting of stars. This model of the world works perfectly and formed the template for explaining the “wandering stars” also.

These two ideas, namely celestial objects placed on a circle/sphere rotating with constant speed, formed the philosophical basis of Greek cosmology which would dominate the Western world for nearly two thousand years. And why would one consider the Earth to be stationary? This is perhaps because the idea is highly counter-intuitive. All our experience tells us that the Earth is stationary. The metaphors that we use like rock-solid refer to an idea of immovable and rigid Earth. Even speculating about movement of Earth, there is no need for something that is so obviously not there. But as the history of science shows us, most of the scientific ideas, with a few exceptions, are highly counter-intuitive. And that the Earth seems to move and rotate is one of the most counter-intuitive thing that we experience in nature.

The celestial observations were correlated with happenings on the Earth. One could, for example, predict seasons as per the rising of certain stars, as was done by ancient Egyptians. Tables containing continuous observations of stars and planets covering several centuries were created and maintained by the Babylonian astronomers. It was this wealth of astronomical data, continuously covering several centuries, that became available to the ancient Greek astronomers as a result of Alexander’s conquest of Persia. Having such a wealth of data led to the formation of better theories, but with the two constraints of circles/spheres and constant speeds mentioned above.

With this background, next, we will consider the progress in these ideas.

A stabilised image of the Milky Way as seen from a moving Earth.


Quatermass and The Pit

One of my favourite genres of literature is science fiction. Two of the classic science fiction authors at the start of the previous century were H. G. Wells, Edgar Rice Burroughs and Jules Verne. The golden era of science fiction began in late 1930s and 1940s and flourished in the post-war era, which saw the big three (Issac Asimov, Arthur C. Clarke, and Robert Heinlein) along with many others bring out their finest.  Some of the other remarkable authors from that era include (my personal preference, and by no means a representative list) Ray Bradbury, Philip K. Dick, Stanisław Lem, Frank Herbert, Ursula K. Le Guin, and Kurt Vonnegut. Though novels were there, the mass format was the pulp science fiction magazines which published short or serialised stories from various authors. Many of the famous novels were first published as short stories in these pulp magazines. A selection of them with fantastic full-colour covers and some with black and white illustrations on the inside. Some of the prominent titles that come up are Amazing Stories, Astounding Science Fiction, Worlds of IF, Galaxy Science fiction among others.

The post-war era was an era where people believed that we will have permanent bases on the Moon by end of the 20th century and space travel would be commonplace. But we now know, it will be perhaps a few decades if not centuries for space travel to become common. The optimism in the 50s and 60s perhaps was fuelled by the cold war space race, which saw both the West and Soviets invest huge sums to research and development in developing space technologies. This optimism gradually waned as the Soviet empire fell.

With television becoming the newest technology to reach out to the audience, it is not surprising that many of the programmes were tuned to science fiction. I happened to stumble upon one such programmes while scanning the treasures at The Internet Archive. This was a British production titled Quatermass and The Pit created by Nigel Kneale. This is third in a series of Quatermass episodes.

Warning: Spoilers ahead

The six-part television series from 1958 (each episode is 30-35 minutes) is set in post-war London at Hobbs lane where during an excavation for a building some fossil skulls are discovered.  Dr. Matthew Roney, a paleontologist from a nearby museum begins to investigate the discovery. The fossil skulls and subsequent bones are found to be a new previously unknown dwarf hominid species, perhaps the missing link and are dated to roughly 5 MYA. Roney’s head assistant Barbara Judd, creates a reconstruction of the species which is present to the press.

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Soon after, when they continue digging a strange smooth object is found in the pit. The object resembles an unexploded World War II-era bomb and police and subsequently, the military is contacted for its safe disposal.


The bomb disposal squad works slowly and does not care about the archaeological aspects of the pit. This makes Roney impatient, who then contacts his experimental physicist friend Prof. Bernard Quatermass to hasten the disposal of the bomb disposal squad.

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Quartermass is involved in rocket research, which he intends to use for peaceful purposes. And this creates a rift between him and the military personnel he is working with. This has some moral and ethical implications for the purpose of scientific research and whether the scientists are responsible for their research being used for military purposes. The military intends to develop bases on the Moon and Mars in order to achieve supremacy in space which is against the principles of Quatermass.

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Quatermass and Colonel Breen visit the site in order to look at the discovery. When the supposed bomb is excavated deeper more fossils are found and the true shape of the artefact is revealed. And it turns out that the artefact cannot be cut by gas cutter, even after raising the temperature to order of 3000 degrees.

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Further digging, provides a disk and an opening to the artefact. Soon, the shape of the complete artefact is revealed. Rest of the hollow space is emptied out, yet the hull of the artefact remains close shut. There is a pentacle on the smooth inner surface of the hull.

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From the outside, the artefact looks like a rocket, which leads Breen to speculate that it is indeed a German rocket which fell here during the war. Also, traces of artificial radioactivity are found in the soil, which indicates that the artefact might be propelled by a nuclear engine. But Quatermass taking into account the age of the fossils speculates that the artefact itself might be of ancient origin. One of the bomb-disposal unit member has a strange hallucinating experience inside the artefact. He sees a dwarf-like figure pass through the walls.

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To open the hull, they try to drill it with a borazon boron nitride drill which makes no impact. But the action of the drills sets out weird vibrations which make everyone frightened and uneasy.

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Everyone is in a state of panic after this. Quatermass, Roney, and Judd run a parallel investigation after hearing out an old local couple about the neigbouring house being haunted. They dig older records and find episodes of haunting dating back till 1300s through to the present. For Quatermass and Roney this is too much of a coincidence and they begin to speculate about the ancient origins of the artefact.

Just after the drilling, a hole automatically appears in the pentacled hull. Roney looks inside and sees what seems like an eye. They force open the hull and find three insects inside the hull who are decaying.

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Roney immediately tries to stop the decay and preserves the specimens and takes them to the museum. They are unlike any insects known and are tripods. Quatermass and Roney speculate the extra-terrestrial origin of these insects, most probably from Mars.

When the drill operator is taking out his equipment, he triggers more poltergeist activity from the artefact and sets a panic across the street. He finally lands in a church in a state of delirium. He describes to Roney and Quatermass hallucinating visions of the insects found in the artefact killing each other.

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Like good scientists, they further investigate the visions using a Roney’s optic-encephalogram, a device that records impressions from the optical centers of the brain. It turns out Judd is the most sensitive of the lot to these visions and they record them. The visions show large-scale culling of the mutations of insects. Seeing these recordings as a “proof” of their theory of extra-terrestrial origins of the artefact. This evidence along with his theory is presented to the military brass. The theory is ridiculed as a fantasy, and a common-sense approach that artefact and the insects being propaganda from Nazi Germany is preferred. They want to dispel the myth that the artefact is that old or it is indeed extra-terrestrial.

The theory as developed by Quatermass is as follows taking into account the evidence he has:

The Martian race of insects is selected to weed out any mutants. So there is a tendency to have large scale purges, which are seen in the hallucinations of people. The Martians came here 5 MYA, and tried to genetically re-programme our ancestors in their own image. During this reprogramming, the human ancestors were given telepathytelekinesis and other psychic powers. And they were set back to Earth. The artefact found was one such space-ship which crashed while bringing modified hominids back to the Earth. Now in the vicinity of the space-ship, some of these long-forgotten powers are awakened. The spaceship itself induces the visions and poltergeist phenomena seen when the ground near the ship was disturbed. Quatermass fears that a large scale activation of such powers might lead to mass killings of humans as seen in the hallucinations.

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A media event is organised in order to address this once and for all. Quatermass pleads that this event must be stopped but in vain. Just as the live event is about to start, the power cables in the vicinity of the artefact, activates it fully. This sets chaos about everywhere and people are trying to kill each other. Somehow Quatermass comes out and is saved by Roney. Entire London is seen to be under mass panic and people killing each other and destroying things.

I will stop here and won’t ruin the climax for you.

You can watch the entire series at The Internet Archive

Episode One “The Halfmen” 

Episode Two “The Ghosts”

Episode Three “Imps and Demons”

Episode Four “The Enchanted”

Episode Five “The Wild Hunt”

Episode Six “‘Hob'”

Some reflections (as seen by a reader from 21st century):

The easy flow of information and relatively free access to the press seem to be unrealistic. For example, one of the reporters gets in easily and takes photos at will of the pit, the artefact and insects. In fact, even after the mysterious nature of the artefact is made known, no attempt at hiding it from the public is done. This is perhaps due to the fact that the military brass firmly believes it to be WW II era find, yet even in this case the free access to press is questionable.

The other aspect is the depth of the characters, which are frankly speaking one dimensional. But perhaps this is keeping in mind the general state of science fiction from that era. Most of the stories, films were like this which did not involve multiple levels of the plot. For example, another fantastic TV series from the era The Twilight Zone (1959) has similar storylines. The acting also looks over the top at times (not at all subtle at any point really), but perhaps this is again a reflection of that era and influence of theatre on films.

Quatermass, like a good scientist, considers evidence from the pit itself (the artefact with seemingly advanced technology, the alien bodies, the 5 MYA fossils), and from people (the visions, and the hallucinations, the elder couple who tell about haunting in the area) and historical records. The evidence of the artefact being old, is right there from the beginning, yet it takes Quatermass and others a long time to consider extraterrestrial origin. Perhaps, we, as readers in the current age, are more agreeable to such a possibility, hence we may find it a bit naive. But then we are trying to judge a production from another era with standards of another.

Some of the themes could be considered on a deeper level. For example, how does evidence from evolutionary aspects corresponds to this explaining? We can perhaps develop another story which takes this forward…