Ocean Tides

Use the sim to explore how the Moon causes high and low tides as Earth rotates, then answer the questions to check your understanding.

Hour 0

This model shows one tidal day, from 0 hr 0 min through 24 hr 50 min. The final point is the matching high tide on the next day. The animation stops at the end.

Simplified model: The ocean bulges are drawn far larger than real life so you can see them. In the open ocean the real bulge is less than a meter tall. Real high and low tide times also vary because ocean depth, coastlines, and ocean-basin shape affect how tides move.

Earth, Moon, and the tidal bulges · bulge size greatly exaggerated

Sunlight N Earth high tide bulge high tide bulge Moon = your beach

The water at your beach

high tide mark low tide mark

High tide

Tide graph for this day

high low 0 6 12 18 24 h
Learn more about the science

What causes the tides?

The moon's gravity pulls on Earth and everything on it, including the oceans. Water is free to flow, so it piles up in a bulge on the side of Earth facing the moon. Your beach has high tide whenever Earth's spin carries it into a bulge.

Why are there TWO bulges?

The moon pulls hardest on the near-side ocean, a bit less on Earth itself, and least of all on the far-side ocean. The far-side water gets "left behind" as Earth is tugged away from under it, so it piles up into a second bulge. Check the simulation: the water bulges on both sides at once.

Why two high tides a day?

Earth spins once a day, carrying your beach through both bulges. Bulge, dip, bulge, dip: two high tides and two low tides, roughly 6 hours apart from each other.

This model shows two equal high tides to make the basic pattern easy to see. At real coastlines, the two daily high tides may be equal or unequal. That difference is caused by factors such as the shape of the coastline and ocean basin and the Moon's angle above or below the equator, not simply by whether the beach is on the Moon-facing side or the opposite side.

Why does the matching high tide happen about 50 minutes later the next day?

While Earth spins, the moon also moves along its orbit. After 24 hours, your beach is back where it started, but the moon and the tidal bulges have moved ahead. Earth needs about 50 extra minutes of spinning to catch up. That means the matching high tide is about 50 minutes later the next day, not 50 minutes after the low tide.

Check Your Understanding

Stuck on a question? Scroll up and use the simulation, the “What is happening now?” explanation, or Learn more about the science.

Question 1 of 5