Time and tide, as the ancient saying goes, wait for no one.
But what is a tide?
The word “tide” is a bit confusing; most people think of it as the ebb and flow of the ocean at the beach, caused by the moon as it orbits Earth. But astronomers think of it as a force—the change in a force, specifically—that causes the water to wash in and out.
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Not only that, but this implacable force is also why the moon only shows one face to us and why it’s slowly receding from Earth, and why our planet’s rotation is slowing. It even shapes stars and whole galaxies!
Gravity is what holds the moon in orbit around Earth. But gravity has a critical characteristic: it gets weaker with distance. In our daily life on or near our planet’s surface, this isn’t noticeable because the change in the strength of the pull of Earth’s gravity only becomes perceptible over large distances.
How large? Let’s consider the size of the moon because our natural satellite is the main source of Earth’s familiar tides. The moon is, on average, 384,000 kilometers from Earth, but based on the lunar diameter, the moon’s near side is 3,400 km closer to us than the far side. The force of Earth’s gravity is stronger on the moon’s near side than the far side and tugs on it harder. This has the effect of stretching the moon, literally causing it to bulge out into an oval (or, more accurately, an ellipsoidal) shape.
This change in the force of gravity is what astronomers call the tidal force.
You might think that only the near side bulges outward, but oddly the far side does as well. One way to think of this is that the stretching force is symmetric; gravity can’t pull on just one side of the moon, so the far side must bulge out relative to the center. Another way to understand this is that because the moon’s near side is tugged harder than its center—and because the center is tugged harder than the far side—the far side is forced to stretch away from the center, too.
Importantly, the reverse situation is also true: the gravity of the moon stretches Earth, causing our planet to bulge. This means there are two tidal bulges on Earth—one on the side facing the moon and the other on Earth’s opposite side.
In the meantime, Earth rotates once per day. When a part of our planet passes through a tidal bulge, the gravity of the moon pulls it harder, away from Earth’s center. Water flows easily in response to such shifts, so we get a rise in ocean height on that bulge: high tide. This happens on both bulges, toward and away from the moon, which is why we get roughly two high tides per day.
There’s also a tide in the ground, too, but it’s less noticeable than the water tide because the solid Earth doesn’t move as freely as the water does. The ground beneath your feet rises and falls by about a meter twice daily because of the Moon’s tides, but this cyclical change is imperceptible to you because it occurs slowly and across the large expanse of Earth’s surface.
This is where things get interesting. Earth is spinning and isn’t terribly flexible. That means the bulge in Earth’s shape isn’t directly under the moon; the planetary spin sweeps it forward a little bit, ahead of the imaginary line connecting the centers of Earth and the moon. That’s critical, because the moon’s gravity tugs on that bulge, constantly trying to pull it back into line. This acts like friction and slows Earth’s rotation ever so slightly—a mere two milliseconds every century. But over the eons, this adds up; in the distant past our planet spun much faster, and the day was much shorter than its present 24-hour duration.
Earth’s gravity is much stronger than the moon’s, though, so our planet’s corresponding tidal effect on our satellite has been much more profound—so strong, in fact, that long ago it dramatically slowed lunar rotation so that the moon’s spin and orbital motion were synced. In other words, the duration of a lunar day became equal to the time the moon takes to orbit Earth once. Once that happened, the moon’s tidal bulge aligned with the direction to Earth, and the slowing stopped. In the lingo of astronomy, the moon became “tidally locked.”
This is why the moon always shows one face to Earth! It’s not a coincidence. In fact, it’s inevitable when you have a satellite orbiting a planet for long periods of time. All the big moons in the solar system are locked to their host world in this way.
This also means Earth should eventually slow so much that it too will tidally lock with the moon and spin once every time the moon orbits once, eternally presenting the same hemisphere to our lunar companion. Earth’s larger mass means our planet is far harder to slow, however, so this process is ongoing and will take billions more years. Our sun will most likely expand into a red giant and consume our planet before Earth’s tidal locking can happen.
I’ll note that the sun has a tidal force on Earth about half as strong as the moon’s, so it also raises tidal bulges on Earth, although they are smaller. When the sun and moon are in alignment in the sky (at new and full moon), these effects add together, and we get spring tides: higher high tides and lower low tides. This can cause flooding in coastal areas, especially when the moon is near perigee, the point in its orbit when it’s closest to Earth.
There’s another effect of tides, too. There is energy locked up in the moon’s rotation and orbital motion and Earth’s rotation. When the moon’s spin slowed over time, that rotational energy had to go somewhere. Some of that energy turned into friction in the moon’s interior, but some of it was dumped into the moon’s orbital motion. When you pump energy into an orbiting object, it moves into a higher orbit, away from its primary host. This means the moon started out much closer to Earth and has moved farther away over the eons. And as it continues to slow Earth, our planet’s rotational energy is pumped into the moon’s orbit as well, so our satellite is still receding from us at the glacial rate of roughly four centimeters per year. That’s very roughly the speed of continental drift or at which your fingernails grow. This outward drift is, among other things, the reason why our moon’s near-perfect eclipses of our sun are so cosmically coincidental; in a few hundred million years from now, the moon will have moved too far from Earth to ever again fully block the sun in our sky, and total solar eclipses will no longer be seen.
Tides affect all large objects in the universe. Two stars in a close binary orbit can stretch each other out, which affects their rotation and orbital motion and even their evolution over time. If two galaxies pass close by each other, the tides pull and distort them like taffy, creating long, curving “tidal tails” of stars and gas, like in the iconic Tadpole galaxy seen by the Hubble Space Telescope. Closer to home, if a moon gets too close to its planet—within a distance called the Roche limit—the resulting tidal forces can be strong enough to rip that moon apart. Some ancient satellite of Saturn may have suffered this very catastrophe, giving rise to the planet’s rings. Even if it’s not destroyed, a moon orbiting a gas giant can be significantly heated by tidal friction, causing its interior to liquefy; icy moons such as Saturn’s Enceladus and Jupiter’s Europa have oceans of liquid water under their eternally frozen surface because of this!
Tides really do wait for no one. While much of human history and economy has been affected by them, they are more far-reaching than our own provincial and colloquial use of the word implies, both in time and in space.