How to Tell that the Earth Spins

Continuing with the supplementary material for the book, from its Chapter 2. This is in reference to Galileo’s principle of relativity, a central pillar of modern science. This principle states that perfectly steady motion in a straight line is indistinguishable from no motion at all, and thus cannot be felt. This is why we don’t feel our rapid motion around the Earth and Sun; over minutes, that motion is almost steady and straight. I wrote

• . . . Our planet rotates and roams the heavens, but our motion is nearly steady. That makes it nearly undetectable, thanks to Galileo’s principle.

To this I added a brief endnote, since the spin of the Earth can be detected, with some difficulty.

• As pointed out by the nineteenth-century French physicist Léon Foucault, the Earth’s rotation, the least steady of our motions, is reflected in the motion of a tall pendulum. Many science museums around the world have such a “Foucault pendulum” on exhibit.

But for those who would want to know more, here’s some information about how to measure the Earth’s spin.

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The spin of the Earth was first detected through what is known as the Coriolis effect, which causes objects far from the equator and moving in long paths to seem to curve gently. The reasons for the apparent curved paths, as well as the consequent impacts on navigation and weather, are discussed in this post from 2022, one of a series in which I showed how you can confirm basic facts of astronomy for yourself.

(Regarding the Coriolis effect: there’s a famous tourist trap in which trained professionals cause water to spin down drains in opposite directions, depending on which side of the equator they are standing on. But this is a magician’s trick, intended to obtain a nice tip from impressed travelers. The Coriolis effect is tiny on the scale of a sink; it’s only easy to observe on scales of miles (kilometers). It is even tinier right near the equator — that’s why there are no hurricanes at very low latitudes — and so it has no effect on the trickster’s draining water. Here’s someone’s webpage devoted to this issue.)

In the same post, I described the basics of a Foucault pendulum, the simplest device that can visibly demonstrate the Earth’s rotation. It’s really nothing more than an ordinary pendulum, but very tall, heavy, and carefully suspended. Unfortunately, although such a pendulum is easy to observe and is common in science museums, it is conceptually confusing. It is easily understood only at the Earth’s poles, where it swings in a fixed plane; then the Earth rotates “underneath” it, making it to appear to spectators that it rotates exactly once a day. But at lower latitudes, the pendulum appears to rotate more slowly, and at the equator it seems not to rotate at all (because, again, there’s no Coriolis effect near the equator.) Depending on how close the pendulum is to the equator, it may take weeks, months or years to rotate completely. To understand these details is not straightforward, even for physics students.

A better device for measuring the Earth’s rotation, which Foucault was well aware of and which I discussed in the next post in that same series, is a gyroscope — for example, a spinning top, or indeed any symmetrical, rapidly spinning object. Conceptually, a gyroscope is extremely simple, because its pointing direction stays fixed in space no matter what happens around it. Once pointed at a star, the gyroscope will continue to aim at that star even as the Earth turns, and so it will appear to rotate once a day no matter where it is located on Earth.

So why don’t science museums display gyroscopes instead of Foucault pendula? Unfortunately, even today, it is still impossible to build a mechanical gyroscope that is stable enough over a full day to demonstrate the Earth’s rotation. Ring laser gyroscopes, which use interference effects between light waves, are much more stable, and they can do the job well (as a flat-earther discovered to his embarrassment — see the last final section of that same post.) But their workings are invisible and nonintuitive, making them less useful at a science museum or in a physics classroom.

Now here’s something worth thinking about. Imagine a intelligent species living forever underground, perhaps without vision, never having seen the sky. Many such species may exist in the universe, far more than on planetary surfaces that are subject to potentially sterilizing solar flares and other catastrophes. Despite complete ignorance of their astronomical surroundings, these creatures too can prove their planet rotates, using nothing more than a swinging pendulum or a spinning top.