Of Particular Significance

The Best Proof that the Earth Spins

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 02/07/2022

In my last post I gave you a way to check for yourself, using observations that are easy but were unavailable to ancient scientists, that the Earth is rotating from west to east. The clue comes from the artificial satellites and space junk overhead. You can look for them next time you have an hour or so under a dark night sky, and if you watch carefully, you’ll see none of them are heading west. Why is that? Because of the Earth’s rotation. It is much more expensive to launch rockets westward than eastward, so both government agencies and private companies avoid it.

In this post I want to describe the best proof I know of that the Earth rotates daily, using something else our ancestors didn’t have. Unlike the demonstration furnished by a Foucault pendulum, this proof is clear and intuitive, involving no trigonometry, no complicated diagrams, and no mind-bending arguments.

The Magic Star-Pointing Wand

Let’s start by imagining we owned something perfect (almost) for demonstrating that the Earth is spinning daily. Suppose we are given a magic wand, with an amazing occult power: if you point it at a distant star, any star (excepting the Sun), from any location on the Earth, it will forever stay pointed at that star. Just think of all the wonderful things you could do with this device!

As a simple example, imagine you go to the equator near Quito, Ecuador, on the day when Mintaka, the right-most star in the belt of the constellation Orion, is directly overhead at midnight. Right at midnight, you point the magic wand straight up at Mintaka. By 6 am, Mintaka is setting in the west, and the wand points west too. At noon, with the Sun overhead, Mintaka is directly opposite the Sun, on the opposite side of the Earth; and so your wand points straight down. At 6pm Mintaka rises, and the wand points toward it in the east. And when Mintaka is again overhead, the wand points straight up. From your perspective, it has rotated once during the 24 hour period.

[Actually, it takes 23 hours and 56 minutes, not 24 hours. That’s a subtle point, which we can return to in a later post.]

And from then on, through this second night and into the third and fourth, the wand follows Mintaka on its path around the sky. It doesn’t matter if it’s cloudy or stormy or clear; it doesn’t matter if it’s day or night. Always, like magic, the wand points at Mintaka.

Of course, if the Earth is indeed rotating, the right way to understand the wand’s motion is that it is an illusion, and instead the star is fixed on the sky, and the wand is fixed in space. It is you who are being rotated, carried along as the Earth rotates. This is illustrated in the figure below.

Left: The Earth viewed from above the North Pole. A person on the equator points a magic wand at a distant star at midnight. As 24 hours pass, the Earth carries the person around in a circle, while the wand continues to point at the star. Right: From this person’s point of view, during these 24 hours the wand has rotated around in a vertical plane. This plane is perpendicular to the Earth’s rotation axis, which is shown with an arrow pointing toward northern regions.

There’s nothing special about the location, the star, or the timing. You could have first pointed the wand at 3:16 am instead of midnight. You could have pointed the wand at Mintaka from, say, Oslo or Buenos Aires; the wand will again rotate in a plane once a day, though the orientation of the plane, which is perpendicular to the Earth’s spin axis, will change depending on your latitude.

If persons elsewhere on the Earth (shown here with north at the top) point at the same star as in the previous figure, they too will see the wand rotate in a plane. But the orientation of the plane will be different from their perspectives. (Top right) For someone at the north pole, the plane will be horizontal. (Bottom right) Someone living midway into the southern hemisphere will see the plane as upside-down and tilted.

Or you could have aimed at other bright stars, such as Sirius, or Vega, or Arcturus, or at dim stars with no special names. You could have pointed the wand at the star Betelgeuse at the moment it rises above the trees outside your home on December 14th, or at the Large Magellenic Cloud of stars on February 2nd at 11:15 p.m. in your favorite southern vacation spot. It matters not at all: the wand, once aimed, never lets go of its target.

And that means, as each night and day go by, the wand will seem to perform a little pirouette, coming back to its starting point about 24 hours later. Rather than rotating in a plane, the end of the wand pointed at a random star will go around in a smaller circle, while the motion of the wand as a whole outlines a cone. The center of that cone will point along the Earth’s rotation axis, right at the North Star (called Polaris), if the target star is north of the equator, or at the South Star (called Polaris Australis) if the target star is south of the equator. If pointed straight at the North or South Stars, it will not budge, for those stars’ apparent locations in the sky remain fixed even as the Earth rotates.

No matter where they are on Earth, persons who point the wand at a randomly chosen star will see the wand’s end move in a circle, while the wand as a whole rotates in a cone whose center points along the Earth’s axis. Top right: a person at the north pole will see the wand rotate around the vertical, because the Earth’s axis looks vertical to them. Bottom right: a person on the equator will see it rotate around the north-south horizontal, since that is where the Earth’s axis lies for them. (Note this person, whose arms point north-south, has turned 90 degrees from the person in the first figure, whose arms point east-west.)

Since this works at every location on Earth and with every star in the sky, would this prove the Earth is rotating? No. Our magic wand only proves that either the Earth is rotating, moving you around in a circle with the wand standing still, or that the stars are rotating, carrying the wand with them. This wand gives us an opportunity to measure how we are moving relative to the stars, or vice versa, but by itself doesn’t determine who is moving.

But maybe, with a little more knowledge of magic, we could learn which is which. If only we had such a magic wand.

The Wand Exists?

In reality, we don’t have a wand that works by such magic. We have something better: a wand that works by physics. Rather than something occult created by a wizard, it’s a concrete object created by one or more engineers, and it’s quite easy to understand and use, at least in principle. And when we understand how it works, it becomes clear that the stars are standing still, and it is the Earth that is rotating.

This wand, called a gyroscope, is a wonderful device. It was invented in 1812 by German physicist Johann Bohnenberger, though Foucault, the inventor of the famous pendulum, gave it its name in 1852. Foucault became interested in gyroscopes for the same reason we are discussing them here: he wasn’t satisfied with his pendulum as a demonstration of the Earth’s rotation, because it is somewhat counter-intuitive. In particular, it’s not easy to describe to a non-expert (or even a physics student) why it rotates every 24 hours at the north or south pole, every 32 hours in Paris, and every five and a half days in Caracas. Foucault wanted something simpler and more obvious. The behavior of a gyroscope, as a mathematician friend apparently suggested to him, would fit the bill.

The simplest gyroscopes are simply spinning tops: a piece of metal or other material, whose shape is not essential except that it must be carefully engineered to be as round and balanced as possible around a central axis. To become a gyroscope, it simply needs to be set spinning, and the axis around which it spins plays the role of our magic wand. It’s therefore essential that the axis be free to rotate in any direction, which requires a nested set of gimbals. It has to spin for 24 hours if we’re to use it as I’ve described, and it has to be absolutely free from any stray forces that might cause it to lose its sense of direction.

You can buy a cheap toy gyroscope at a store, but that won’t do at all; it’s not nearly good enough. Better ones are available at physics departments, where they serve for important demonstrations and experiments; later on I’ll link to a video that shows one. But even these aren’t good enough for our task. Foucault, using the best techniques he had available back then, only managed to design a small one that could spin for 10 minutes… enough to demonstrate that the Earth rotates, but not enough to make a spectacle on the same level as the majestic swing of a pendulum hung high in a cavernous room.

But a gyroscope of extreme tenacity and stability has all the properties of our magic wand. No matter where you live and no matter what time of day you choose and no matter where you choose to point it, its axis seems to move in a circle every 24 hours (except when pointed right along the Earth’s spin axis, in which case it remains fixed). That means 360 degrees rotation per day, which is 15 degrees per hour, or one degree every four minutes. (Almost exactly.) Unlike Foucault’s pendulum, its apparent rotation occurs at the same rate as the Earth’s real rotation, no matter where it is on Earth.

Like the imaginary wand, this device would demonstrate either that the Earth spins every 24 hours, or the stars zip around the Earth every 24 hours. But unlike the mythical wand that operates by magic, we know exactly how a gyroscope works. There’s no magic: any connection with the stars is irrelevant. You can point it at a gap right between two stars, and it will still point at that gap indefinitely.

Why the apparent magic if there’s no special relationship between this terrestrial gadget and the stars? It is a coincidence, though a dramatic one.

  1. Because of a principle of physics, (conservation of angular momentum, which I’ll describe in a moment), a well-constructed stable isolated gyroscope always rotates around a fixed direction.
  2. Because the stars are so incredibly far away, their rapid motion relative to one another and to our Sun is too slow for us to notice over periods of years or even lifetimes.

In short, the gyroscope’s fixed by principle, the stars by mere accident; and the combination of the two explains why a gyroscope tracks whatever star (or absence of star) its axis is aimed at.

Importantly, you can check that a gyroscope’s axis is stable even without knowing why! Before you point the gyroscope at a star and leave it alone, you should play with it a few minutes — a time short enough that the Earth’s rotation (if any) would be unnoticeable. Try lifting it, or turning its housing upside-down or sideways, or tilting the table it might be sitting on; you will see the axis of its spin does not budge. Beautiful demonstrations of this can be seen in this video, where high quality gyroscopes at the Caltech physics department are demonstrated. See especially 00:45-01:00, and again, perhaps more visibly, at 02:15-03:45. Of course if you strike the gyroscope, or shake it hard enough, or you unbalance it in any way, you’ll cause it to lose its aim. Indeed, you’ll see evidence that even the gyroscopes in the video, while good for demonstrations, aren’t stable enough for the task of showing the Earth rotates. But as long as you are gentle with it, a perfectly-engineered gyroscope will successfully maintain its orientation.

In space, it’s easier to get a gyroscope to point reliably. In this European Space Agency video from the international space station, you can see that even a toy gyroscope will do a pretty good job in the apparently weightless environment. But of course we want a gyroscope that can be stable on Earth, and a toy won’t do.

Now that you’ve played with this perfect gyroscope for a while and convinced yourself that it maintains its spin axis, you can point it at a star. There are three things that can happen.

  • The gyroscope is not as well made as you need, and so its axis wanders due to its imperfections. If you point it at another star, it will again wander, but in a different way, since the imperfections have nothing to do with the motion of the stars.
  • The gyroscope is high-enough quality, and the Earth does not rotate. Then the spin axis will remain pointed at exactly the same spot in your room — at your ceiling, or your window, or the blotch on the floor, or the dirty sock under the bed — all day long. It will not follow the stars, but neither will not wander; it will seem to you to remain fixed.
  • The gyroscope is high-enough quality to remain stable, and the Earth does rotate. Then the gyroscope will behave like our imaginary magic wand, and appear to track the stars and, from your point of view, to rotate daily.

So just try out the gyroscope for a few days and draw your own conclusion. If you do this experiment and find behavior that mimics the imaginary magic wand we started with, you may safely conclude that the Earth rotates once a day, around an axis that points at the North Star in the north and at the South Star in the south. Its once-a-day rotation explains in an intuitive way that the Earth spins, and that the stars only seem to rotate around the Earth daily.

This is the clear demonstration of Earth’s spin that Foucault was seeking. Why does it work?

Conservation of Angular Momentum

A balanced, isolated gyroscope won’t lose its orientation because of the principle of nature called the “conservation of angular momentum”. Here “angular momentum” is a measure of spinning that accounts not only how fast the object spins but also the axis around which it spins, and “conservation” means “never changing”. You can see this principle at work in this video from MIT.

Conservation of angular momentum has many consequences — it explains why spinning figure skaters can spin faster when they pull their arms in toward their bodies — but for our purposes, the only consequence we care about is this: an isolated spinning object will maintain its spin axis and spin rate indefinitely.

In fact, the Earth itself is such an object — it’s one of the best gyroscopes we know. That’s why the axis of the Earth has pointed for centuries at the two stars Polaris (the North Star) and Polaris Australis (the corresponding South Star), which are the stars that appear to be essentially stationary throughout the night. The other planets also are natural gyroscopes, as are the Sun, other stars, black holes, and indeed most large objects in the universe.

But reality is never perfect. No gyroscope is ever perfectly isolated, and interactions with other objects tend to alter its spin rate and gradually destabilize its spin axis. Even the Earth’s axis wobbles, though slowly compared to a human lifetime. For the gyroscope we want, sitting on the Earth, there will be friction at the gyroscope’s supports which will slow its spin. Even if these are countered by a motor that keeps it spinning, there are usually other small effects which, over a day’s time, can cause this wand to lose its magic. To design a mechanical gyroscope which is stable enough to perform a complete daily rotation is not an easy task at all. Foucault, in 1852, did quite well to make one which was stable for ten minutes!

State of the Art?

The fact that gyroscopes have still not replaced Foucault’s pendulum in science museums around the world reflects the technological challenge involved, and it has remained a topic of research. Consider Andre Geim, who after first winning an Ig Nobel prize in 2000 for levitating frogs using powerful magnets, later shared the 2010 Nobel prize for the discovery of the wonder material called graphene. In 2001 he wrote a paper (unfortunately behind a paywall) entitled “Detection of Earth Rotation with a Diamagnetically Levitating Gyroscope”. This paper was co-authored with his hamster, Tisha, who apparently participated in the levitation experiments. Despite the co-author, the paper is written in a serious vein. In it Geim claims that he built a magnetically levitated gyroscope that operated for two hours (limited only by the cost of running his powerful 15 Tesla magnet) and rotated at the expected 15 degrees per hour. He then discusses the technical advantages and limitations of this approach and how it might be improved. That said, I have not yet found a single citation of this paper that reproduces or directly uses his result, nor can I find any talk in which he or anyone else describes this phenomenon, so I’m still not sure how seriously to take this paper. Let me know if you know more than I do.

In any case we are beyond the current limits of my knowledge and far beyond my expertise. There are a number of modern gyroscope-imitators, some of which operate on different principles from the mechanical gyroscopes I’ve been discussing, and I don’t understand all of them. It would be great if there’s one whose operation was intuitively clear, which could do the job of maintaining its orientation for more than a day, and whose cost might someday drop enough to make it affordable, at least to museums; but I don’t see one yet. If anyone reading this is more knowledgeable about this subject that I am, please chime in in the comments.

The Ideal Spinning World

Here’s my dream for the future. Someday, I hope that every major science museum — maybe even many universities — will own three sophisticated, stable, affordable gyroscopes. Why three? Because there are many lovely things you can do with three. You could, for example, pick three stars at random — that is, point the three gyroscopes in any directions you choose — and then watch how the gyroscopes rotate during the day. The three cones that the gyroscopes’ axes sweep out over the day will all be centered on the Earth’s axis. You’ll also notice that the angles between the gyroscopes never change, as you’d expect if they are truly at fixed orientations and it’s you who are rotating.

Three gyroscopes pointed at random stars will all seem to circle the Earth’s rotation axis daily, while maintaining the angles between them.

A second option would be to point one gyroscope at the North Star (Polaris), a second at a bright star elsewhere, such as Aldebaran, and the third at the Sun. When you check the next day, the first gyroscope won’t have moved, the second will still be pointing at Aldebaran, but the third will no longer be pointing quite at the Sun; it will be about 1 degree off. This change per day will continue. Half a year later, the gyroscope would point opposite the Sun, out into deep space. And if you could watch it all year, the Sun would come back into alignment with the third gyroscope only after the full year had passed. This, of course, is a sign that the Sun orbits the Earth once a year — or is it the other way round?

A gyroscope pointed at the North Star won’t seem to move; one pointed at a random star, such as Aldebaran, will rotate in a cone and remain pointed at that star; but one pointed at the Sun, though it will also rotate in a cone, will fail to follow the Sun by about 1 degree per day, or a full 360 degrees per year.

A last option I’ll mention is that you could arrange the three gyroscopes so that they point at right angles, setting up what’s known as a Cartesian coordinate system. The simplest way to do this would be to point one at Polaris and place the other two in the plane perpendicular to the first one and to each other. Then, over the day, the first gyroscope wouldn’t budge, but the other two would seem to rotate smoothly around it.

If three gyroscopes are set at right angles, they define a set of Cartesian coordinates; if one of the three is pointed at the North Star, it will remain fixed while the other two will rotate in the plane perpendicular to it. I have drawn in dashed lines the Earth’s rotation axis and the other two Cartesian axes.

The Best Proofs Come from the Most Unlikely Sources

Until and unless these kinds of mechanical gyroscopes become possible, we may have to satisfy ourselves with laser gyroscopes, which are far less intuitive than mechanical ones but have the same magical ability to know where they point. (I won’t try to describe how they work here, since it’s quite subtle, and I’d rather see someone build a mechanical one anyway.) I haven’t found any free videos that clearly show these in action, but they are definitely capable of demonstrating that the Earth rotates. This was done on camera in Netflix’ movie Behind the Curve, in which one of these laser gyroscopes is seen to rotate 15 degrees per hour and thus 360 degrees per day.

There is resonance, in this story, with the 17th century scientists who laughed at spin-earthers because cannonballs on a rotating Earth wouldn’t fly straight; we laugh back, because in fact they do not fly straight (i.e. the Coriolis effect, known long before Coriolis but clarified by him in 1835.) In the case at hand, the experiment that proves the Earth rotates was performed by someone who didn’t believe the Earth rotates. The fellow in question, a flat earther, was confident that if he bought a good enough gyroscope, reportedly at a cost of $20,000, it wouldn’t rotate at all, and thus he fully expected he would prove, on camera, that anyone who claims the Earth spins is ridiculous. Of course he was wrong — the gyroscope rotated at 15 degrees per hour. We thank him for his efforts.

But naturally, being a flat earther, rather than recognizing that he’d perfectly reproduced the prediction of the competing theory and admitting defeat, he set about to make reality fit his beliefs. He described his discovery and his reaction in this video, where he claimed the sky must be causing the gyroscope’s rotation. To test this notion, he tried putting the gyroscope in metal containers in an attempt to shield it from these unknown sky effects, but, hey, the gyroscope just kept rotating. Sensible people will be giggling about this for a long time, assuming the whole thing wasn’t staged. (It probably wasn’t; you can’t make this stuff up, can you?)

To see the full Netflix video requires that you pay. At the moment, I can’t find any free videos that allow you to observe the Earth’s rotation convincingly. But perhaps a company that sells ring laser gyroscopes would like to set one up as a combination public service/marketing opportunity? Just point a camera at it and let it sit, recording its orientation hour by hour, day by day, as the Earth turns?

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25 Responses

  1. I heard a Russian skater, Trusova, say they are working on landing quintuple rotation jumps. Somewhat analogous to the satellite launching logic, does she need to spin in the right direction to need the least energy to make the spin? Should she look at water going down the drain rotation to know for each hemisphere how to orient her spin? How can I calculate how much it matters?

    1. Actually, the effect is tiny, and not related to the satellite launching logic, which is much simpler. To understand this, go to the simplest places: the equator, and the poles.

      At the equator, if you face east, you’re moving forward at 1100 miles per hour. That means if you launch a rocket eastward, you’re taking advantage of that forward motion to go on a orbit that goes even faster forward. But spinning is another matter. The fact that you’re moving forward doesn’t help you or hurt you when you spin to the left (toward the north) or to the right (toward the south) because at the equator there’s nothing different about north versus south. At the equator, the Earth’s rotation makes east different from west, but not north different from south. By the same token, a Foucault pendulum does not rotate at the equator, and the Coriolis effect is zero — that’s why there are no huge rotating storms at the equator. So our skater gets zero advantage at the equator from the Earth’s rotation, in contrast to a rocket.

      At the north pole, a rocket is standing still before it launches, just slowly rotating once a day. It gets no advantage from the north pole, and in fact, it can’t even be launched eastward, because from the north pole, all directions are *south*. Our skater, on the other hand, can get an advantage from the Earth’s spin. Before she attempts to spin, she is already spinning to her left, as the Earth rotates underneath her. But that rotation is quite slow, making 360 degrees in one day. That’s 15 degrees per hour, or 1/4 of a degree per minute, or 1/24 of a degree per second. That’s 0.004 degrees per second. Since she will be trying to spin about 5 times in at most 2 seconds, she will be spinning about 5/2 turns per second, or about 5/2*360=900 degrees per second. Now, the extra 1/240 degree per second is an advantage, but relative to 900 degrees per second it’s an advantage of *5 parts per million.*

      One way to visualize that is if she gives herself a twist that will allow her at the equator to turn 1800 degrees in two seconds — the five spins, 360*5=1800 — then at the pole, if she spins one way, she will spin 1800.008 degrees, or 5.00002 turns, while if she spins the other way she will only manage 4.99998 turns. I don’t think the judges will reward or penalize her for this difference!

      In between the poles and the equator, the effect of the Earth’s rotation on spin is less than at the poles but more than at the equator, and its effect on rocket launches is more than at the poles but less than at the equator. So I’ve given you the two extremes; rocket launches eastward are somewhat easier at the equator, a 5% benefit for low-earth orbits, while spinning is easiest at the pole, but for a skater it’s an effect of less than a thousandth of a percent. Essential, our skater is too small and too fast for there to be any difference.

      The Earth’s rotational effects on spinning things really only start to matter when you start talking about many miles and/or many minutes. That’s why it’s so hard to tell the Earth is rotating when you’re a person operating on the time scales of seconds and on the distance scales of houses. The idea that the Earth’s spin affects how water goes down a drain is a fairy tale; unless you do an extremely careful experiment where you first let the water settle for minutes and pull the plug very gently, currents in the water or asymmetries in the tub will easily overwhelm the tiny effects of the Earth’s rotation on a bathtub or toilet.

  2. Matt,
    For me as a layman – How about „Gravity Probe B“ experiment? High precision gyroscopes where used to test Einstein’s theory of GR. They have also measured the so called “frame-dragging effect”. Can this not be used as an argument the other way round to prove that the earth is spinning? I mean the frame-dragging effect has been observed and for my understanding it is the consequence of a spinning planet earth. I know my argument sounds a little bit weird as I’m reversing the ‘cause and effect’ principle and I guess there are loopholes for other possible ways of interpretation.

    1. This is true, but kind of back to front. Indeed, if gravity probe B had been located outside a non-spinning planet, the frame-dragging effect would have been absent. And yes, gyroscopes were crucial for the experiment (and were unfortunately a partial failure.) However, this is a very tough and expensive way to go! Gravity Probe B cost much, much more than a laser quasi-gyroscope, and has to look at much more subtle effects. Using general relativity to prove the Earth spins is a bit like using an ocean liner to study ripples in water; there are easier ways to go.

  3. > Of course he was wrong — the gyroscope rotated at 15 degrees per hour. We thank him for his efforts.

    Thanks, Bob!

      1. Maybe try eating some earth, then taking a bite of an Eggo waffle, and comparing the taste.

        I see that Eggo makes many varieties of waffles, so you need to repeat the experiment until you find one that tastes like earth. Don’t forget to take a mouthful of earth for every waffle so you can properly compare the taste.

    1. Yes, this is very nice, I’d never read it before. It encapsulates the scientific view: science is not a path that claims pure right over pure wrong, but instead is a path toward ever-improving approximations to the truth. The demonstration that the approximations keep improving is the ever-more-powerful technology that the better-and-better approximations allow.

    1. That’s true of special relativity. General relativity is more clever. But your point is still essentially correct. It’s not an inertial reference frame, and the fact that gyroscopes rotate helps prove that.

  4. Doesn’t relativity tell us that every frame of reference is valid and equally correct. If I take the view that where I am standing on Earth and is the cosmos is the center and that the Earth doesn’t spin, but the Sun circles me. Isn’t that a valid frame of reference?

    1. This is a subtlety worthy of further discussion. Just because a system of coordinates is valid doesn’t mean you should use it… because it mangles other things. For instance, I can use flat coordinates on the Earth, but then they have a singularity somewhere… so the coordinates are fine for some things, but not advisable because they make other things terribly hard to understand. I could, for instance, take a point that lies between Jupiter and the Sun, about 1/3 of the way out, and write that point as the center of the cosmos. That would be just as arbitrary as putting the Earth at the center… or, for that matter, the Sun at the center, since the Sun itself is in orbit around the Milky Way. So we do have to be careful what we mean when we say the Earth is spinning — and specifying that gyroscopes on the Earth’s surface rotate is one very good way to do it!

    1. Hmm isn’t that backwards? *How* do we know that rotating crystal spheres don’t exist? Or even if you and I know, how are we to convince someone else? How do we answer the question, “Well, have you been out in the stars to check for yourself?” It’s a reasonable question, since it’s not obvious that there aren’t heavenly laws of physics that permit such spheres and might even power them indefinitely… That’s why it’s important to understand how to give a systematic answer to such a question. Otherwise our knowledge is (or at least seems to be) based on hearsay.

      1. > *How* do we know that rotating crystal spheres don’t exist?
        One thing that comes to mind is that we know that stars have different parallaxes so they must be in different distances from earth. To me it would be unrealistic to think that billions of spheres with different centers and radii will all coordinate to turn in unison to trace the star trails that we observe.

        1. So, the problem with parallaxes is that they are really, really tiny, and even though they’d convince you and me, the fact that (until very recently) very few stars actually show them opens the door to long, complex discussions — and before you know it our skeptic’s eyes glaze over, and nothing is accomplished. The point I’m making is that it’s better, psychologically and conceptually, to have something that every person can see for themselves — the Foucault pendulum is much more effective a proof than some discussion of tiny little shifts in star positions, and the gyroscope much more effective even than that.

      2. > Hmm isn’t that backwards? *How* do we know that rotating crystal spheres don’t exist?

        Observable planets exist, and their motion was well-known long before the physics involved in this article had been invented, and it rules out “rotating crystal sphere”. Kepler gave the “definitive” answer about their motion but even ancient Greeks (Hipparchus, 2 century B.C.) had a model accurate enough to rule out such hypothetical “rotating crystal sphere”.

        Overall, using Newtonian physics as a proof for this feels a bit like using Special relativity for this purpose…

        1. So, there’s two points to consider here. First of all, although Artistarchus proposed the Earth spins, Hipparchus argued successfully (to his contemporaries) that it does not, precisely because he (Hipparchus) had accurate enough data and modeling to rule out a circular orbit for the Earth. If someone around him had been brave enough to accept an elliptical orbit, the Greeks might have discovered what we all now know; maybe someone did and their results were lost to history. Second, we’re talking about the wrong issue here. I’m not trying to explain that the Earth rotates to *you*, or to *me*; we both know dozens of arguments that the Earth rotates. It’s the child of 14, or the non-science-literate adult who is rational but uninformed, who needs an argument which gets to the point and is clear as a bell. (And it’s important that you know it too, because you’re going to have to explain it to someone who doesn’t want to hear some detailed argument about ellipses with 2% eccentricity which are extracted from high-precision astronomical data. That kind of argument simply doesn’t resonate with most people, and those it resonates with don’t need convincing!

          1. Aren’t you mixing up the rotations here? To the ancient Greeks Heliocentrism vs Geocentrism was a different debate than whether or not the earth spins.

            And when its comes to teaching kids or non-science literate, do you really think the argument in this blog post is more intuitive than pictures like this : https://en.wikipedia.org/wiki/Star_trail#/media/File:All_In_A_Spin_Star_trail.jpg ?

            As your “flat-earther playing with a gyro” example illustrate: in science we cannot “prove” anything: all we can do is provide a *model*, with its set of *predictions* and then show that they resist challenges. Facing someone with enough bad faith to change their model on the flight, adding small “corrections” to their model to cope with the observations (“it must be an effect from the sky”), you’ll eventually end up needing data of higher and higher accuracy to contradict them. (And that’s basically the story of astronomy in the few centuries after Galileo …)

            1. I agree I’m being somewhat careless on the ancient history, but this is mainly because my goal here is to focus on the present, not the ancient past.

              Nothing wrong with your star picture — I’d show it too, if giving a class — but again, that picture does not prove the Earth rotates. That proves that *either* the Earth rotates *or* the sky rotates. To distinguish them, you have to prove that the Earth’s a non-inertial frame, and then, to convince non-scientists, you have to explain to people that this is what you did. (Moreover, you still need to do some work to convince students that the geometry makes sense… that the point at the center is above the Earth’s axis. Most people have no idea where the Earth’s axis points; at best, they’ll point horizontal-north, or horizontal-south.)

              As for the word “proof” and “models”: one of the challenges of writing about science is that each article has to pick an audience, with the inevitable result that one is always too precise for some readers and too imprecise for others. You can’t successfully explain science if you’re 100% precise in your language 100% of the time. I think the best compromise requires being a little less precise until you’ve captured the interest of readers, and then expose them to more subtle issues and to deeper understanding. This article is clearly intended for a broad audience, so I would ask your patience regarding these issues.

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