Ocean Tides

Explore how the Moon’s gravity and Earth’s rotation affect ocean tides.

Hour 0

Times are approximate. One tidal day lasts about 24 hr 50 min. The last high tide starts the next tidal day; it is not an extra high tide within the first day.

Simplified model: Sizes, distances, and bulges are not to scale. This shows the Moon’s effect, not a tide forecast for a real beach.

Earth, Moon, and the tidal bulges

View from above the North Pole Sunlight N Earth Moon = example beach

The water at your beach

high tide mark low tide mark

High tide

Tide graph: one tidal day

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

What causes tides?

Tides are regular rises and falls in ocean water level. The Moon’s gravity is the main cause. It pulls on Earth itself and the oceans on both sides. The Sun’s gravity also affects tides, but this model focuses on the Moon.

Why are there two bulges?

The Moon’s gravity pulls on Earth itself and its oceans. The Moon’s pull is strongest on the ocean facing it, weaker at Earth’s center, and weakest on the ocean on the opposite side. This difference creates two bulges of water, one facing the Moon and one on the opposite side. Both bulges cause high tides.

Why do many beaches have two high tides and two low tides?

Earth rotates, or spins. In this model, that carries the beach through two bulges and two lower-water areas each tidal day. A bulge means high tide; a lower-water area means low tide. High to low takes about 6 hours 13 minutes. High to the next high takes about 12 hours 25 minutes.

This model has two equal high tides and two equal low tides. Real tides can have different heights and timing, and some places have only one high tide and one low tide per tidal day. Coastlines, ocean depth, ocean-basin shape, and the Moon’s angle relative to the equator affect the pattern. Real high tides do not have to line up with the Moon.

Why does a tidal day last about 24 hours 50 minutes?

The Moon moves along its orbit while Earth spins. After 24 hours, Earth needs a little more rotation to line the beach up with the Moon again. That takes about 50 extra minutes. The last button shows the start of the next tidal day, not a third high tide within the first one.

Explore the science: NASA: Tides · NOAA: The Lunar Day · NOAA: Tidal Patterns

Check for 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