Have you ever been to the beach?
If you’re from California like me, then I’m betting you have. If you’re from a place that’s not near an ocean and you’ve never been near the water all your life, then I’ll tell you a little bit about the tides.
They happen every day, twice a day. If you find yourself a nice comfortable spot overlooking the beach, you can see the waves come into the shore and then gently roll out again. If you stay for hours on end, you’ll see the water level eventually rise a bit.
And if you stay even longer, you’ll see the water level lower back down. When it’s high, it’s called high tide, and when it’s low, it’s called low tide.
The tides are partially responsible for the myth that the moon’s gravity affects you in some kind of metaphysical way. But this isn’t true at all.
So why do the tides happen?
It has to do with the moon’s gravity.
We know from Newton’s third law of motion and from universal mutual gravitation that just as the Earth’s gravity attracts the moon, the moon’s gravity attracts the Earth. We know that the two orbit around their center of mass, which is a point in space within Earth’s interior.
But we also know that gravity doesn’t affect the whole Earth equally. Gravity weakens the farther out it gets, and Earth is big enough that not all of it feels the same force of gravity from the moon.
Earth is a special planet in the solar system in that it’s covered in liquid water. And that liquid water is very, very movable. It sloshes around. The moon’s gravity actually pulls the oceans out a bit from the Earth’s sphere.
Here, the blue band around the Earth represents the sea level at high tide and low tide around the globe. Of course, the ocean doesn’t come out that far, but this diagram emphasizes the tides in order to make clear what’s happening.
Naturally, Earth’s interior is pulled towards the moon as well as it orbits around the center of mass. The oceans on the other side of the planet are just as sloshy as those on the side facing the moon, and they get left behind a little as Earth’s interior is pulled.
Essentially, high tide on the side of the Earth facing the moon is caused by the moon’s gravity. But high tide on the other side is caused by inertia—matter’s tendency to stay exactly where it is (or move exactly how it is).
The Earth gets pulled, and that half of the oceans gets left behind.
But only a little bit, obviously. Tides don’t change by much—they don’t flood the coastline, just creep up it a bit. They don’t get pulled out nearly as far as the diagram shows.
The tides are the most obvious by the ocean, of course because water moves more freely than the rest of the Earth. But like I said, the moon’s gravity reaches all parts of Earth, and different parts of Earth’s rocky material feel that gravity differently.
Would you believe me if I told you that the Earth’s rocky bulk actually gets deformed a little over time?
It’s true. The Earth’s surface actually expands and contracts by a few centimeters as the Earth rotates. You never notice it, but the effect is there. That’s the moon’s gravity tugging on the Earth.
This effect is much more pronounced on an object like the moon. You realize, if Earth has tides because of its moon, so does every other planet in the solar system with a moon (or moons). Here’s an image of the effect of the tides on another moon in the solar system.
Meet Saturn’s moon, Iapetus! As we’ll explore in much later posts, Iapetus is a land of incredible contrasts. But it’s also small enough compared to Saturn to have tides of its own…
Yes, that mountainy ridge there formed for the same reason Earth has tides.
Why doesn’t our moon have a ridge like that? Probably because its planet is so much smaller than Iapetus’s planet, so the planet’s tidal forces on the moon aren’t strong enough to deform the land like that.
Saturn, after all, is nearly the size of Jupiter—and Jupiter isn’t too far from being the size of a tiny dwarf star.
Don’t worry, we’ll talk about that more later. I have plenty of posts planned for exploring the solar system.
So, we know how the tides form. The moon tugs on the Earth and makes the ocean bulge out a bit on either side. But how come the tides come in and out? Why do they change?
Well, that’s all because of the Earth’s rotation.
Okay, this diagram has a lot of information. Let’s break it down.
The tidal bulge, as it’s called, always stays oriented with the moon. That means that one high tide region is always facing the moon, and the other is always facing away from the moon.
Well, mostly. We’ll get into why the diagram has the tides rotated a bit in just a second.
Anyway, the tides stay angled the same relative to the moon. But that means they follow the moon in its month-long orbit. The Earth rotates all the way around once a day, so it essentially rotates inside the two tidal bulges.
What does this mean? As the tide “comes in” on a beach, the tide isn’t so much coming in as the Earth’s rotation carries you into the tide.
The thing is, though, the oceans aren’t completely movable. The continents act as friction against the tidal bulges, and so does the sea floor. So as the moon pulls on the tidal bulge in the opposite direction of the Earth’s rotation, Earth’s rotation slows down.
Seriously. Just 620 million years ago, Earth’s days were less than 22 hours long. The rotation has slowed down to 24 hours.
This explains why the tides aren’t perfectly lined up with the moon. The continents hold them back a bit.
But here’s another thing. According to universal mutual gravitation, if the moon pulls on the tidal bulges, they must also pull on the moon. And since they’re angled a bit ahead of the moon in its orbit, they pull it forward in its orbit…
…and believe it or not, they actually speed the moon up a bit.
Speeding up the moon actually brings it that much closer to escape velocity, the velocity needed to escape Earth’s gravitational pull.
That won’t happen for thousands of years, but eventually, we will lose the moon.
But…wait a second. If the tides can pull on the moon, and the Earth can pull on the moon, and the moon can pull on the tides and the Earth, and the sun obviously pulls on the Earth since we’re in orbit, shouldn’t the sun control the tides, too?
The answer is yes…sort of.
The sun doesn’t have nearly as dramatic an effect on the tides as the moon. It may have stronger gravity than the moon, but it’s also much farther away, making its pull on the oceans quite weak.
What it can do is change how severe the moon can make the tides. As you can see above, during new moon and full moon when the sun and moon pull at the oceans in the same direction, tides are much more dramatic and rise higher up the coastlines.
During first and third quarter moon, however, the sun actually sort of cancels out the moon’s pull on the oceans. This makes tides much less dramatic.
When the sun increases the tides, they’re called spring tides, named for the way the ocean “springs” up from the ground a bit more. When the sun cancels out the tides, they’re called neap tides, taken from an Old English word that means “lacking power to advance.”
Understanding tides on Earth is the key to understanding tides throughout the solar system—and, indeed, the universe as a whole. That’s the wonder of a universe where the laws of physics are the same throughout.
We can always apply what we know of Earth to the bigger picture. And that’s what keeps us discovering.