What causes tides?

Drawing of Earth showing the polar jet streams and the subtropical jet streams.

Sea stars and sea urchins in a tide pool tank at the Cabrillo Aquarium.
Image source: Wikimedia Commons, Jllm06

Tides and Time

High tides and low tides come and go, as the level of the sea goes up and down. This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world.

Why is that?

Tides are really all about gravity, and when we’re talking about the daily tides, it’s the moon’s gravity that’s causing them.

As Earth rotates, the moon’s gravity pulls on different parts of our planet. Even though the moon only has about 1/100th the mass of Earth, since it’s so close to us, it has enough gravity to move things around. The moon’s gravity even pulls on the land, but not enough for anyone to really tell.

When the moon’s gravity pulls on the water in the oceans, however, someone’s bound to notice. Water, being a liquid and all, has a much easier time moving around. It bulges toward the moon, and that bulge follows the moon as Earth turns beneath it.

Drawing of Earth showing the polar jet streams and the subtropical jet streams.

Water bulges toward the moon because of gravitational pull.

Note: The moon is not this close to Earth. Thirty
Earths could fit between the moon and Earth.

That explains the first high tide each day, but what about the second high tide?

The ocean also bulges out on the side of Earth opposite the moon.

Wait, what?

If the moon's gravity is pulling the oceans toward it, how can the ocean also bulge on the side of Earth away from the moon? It does seem a little weird.

Gravity is the major force causing tides, but inertia is playing a part too. Inertia is matter’s resistance to change. It wants to keep doing whatever it’s doing, whether that’s moving in a straight line or staying still, until another force acts on it.

While the water closest to the moon is getting pulled, the water farthest from the moon is staying right where it is. Both sides are experiencing gravity and inertia, but one always overpowers the other.

On the side by the moon, gravity wins. On the side away from the moon, inertia wins.

These two bulges explain why in one day, there are two high tides and two low tides.

Drawing of Earth showing the polar jet streams and the subtropical jet streams.

The two water bulges stay in line with the moon as Earth spins.

Drawing of Earth showing the polar jet streams and the subtropical jet streams.

Here is how one month of high and low tides looks as the moon makes one full trip around Earth.

Again, the moon is not this close to Earth.
It's just easier to show it this way.

Are these high and low tides the same height everywhere on the planet?


If Earth were perfectly round and completely covered in water, then high and low tides would be equally proportioned everywhere. But Earth is not a perfect sphere, and there are big continents getting in the way of water flowing and bulging in the direction of the moon. That’s why in some places, the difference between high and low tide isn’t very big, and in other places, the difference is drastic.

Extreme high and low tide.

High and low tide in the Bay of Fundy in Canada.
Image Source: Wikimedia Commons, Tttrung. Photo by Samuel Wantman.

Does anything else affect tides?

The sun has a part to play in tides as well. For instance, when the sun’s gravitational pull lines up with the moon’s gravitational pull, the tides are more extreme.

Drawing of Earth showing the polar jet streams and the subtropical jet streams.

When the gravitational pull of the sun and moon are combined, you get more extreme high and low tides. This explains high and low tides that happen about every two weeks.

Note: This figure is not to scale. Not at all, really.
The sun is much bigger and farther away,
but you already knew that.

Wind and weather patterns also can affect tides. Strong offshore winds can move water away from coastlines, exaggerating low tides. Onshore winds can push water onto the shore, making low tides much less noticeable.

High-pressure weather systems can push down sea levels, leading to nice sunny days with particularly low tides. Low-pressure systems that lead to cloudy, rainy days often cause tides than are much higher than predicted, so watch out!