Tides, the regular rise and fall of sea level, are caused mainly by the Moon’s gravity pulling on Earth’s oceans, with a smaller pull from the Sun. These forces create two bulges of water, one facing the Moon and one on the opposite side. As Earth spins, most coasts get two high and two low tides a day.

What Causes Tides? The Moon, the Sun and Ocean Tides

Why two bulges?

The Moon pulls hardest on the side of Earth closest to it, drawing water into a bulge. On the far side, the Moon’s pull is weakest, so the water there is “left behind” relative to the rest of Earth, forming a second bulge. Earth spins beneath both bulges every day.

Because the Moon also moves along its orbit, the cycle takes about 24 hours 50 minutes, so high tide comes about 50 minutes later each day.

The Sun’s role

The Sun is far more massive than the Moon, but much farther away. Its tidal effect is a bit less than half the Moon’s.

Tide	When	Moon and Sun	Result
Spring tide	New moon and full moon	Lined up	Bigger range: higher highs, lower lows
Neap tide	First and last quarter moon	At right angles	Smaller range

“Spring” here has nothing to do with the season; it comes from an old word meaning to “leap up.”

Where tides are biggest

Place	Tidal range
Bay of Fundy, Canada	Up to about 16 m (the world’s largest)
Bristol Channel, UK	About 14–15 m
Mediterranean Sea	Often less than 0.5 m

The shape of coastlines and the seabed strongly affect tides. Funnel-shaped bays amplify them.

Tides and life

Tide pools are home to crabs, starfish and anemones that survive daily exposure to air.
Some fish, like California grunion, spawn on beaches at the highest tides.
Tidal power stations use moving water to generate electricity.

The Moon is slowly leaving

Tidal friction gradually slows Earth’s spin, making days slightly longer, and pushes the Moon about 3.8 cm farther away each year.

For the Moon’s other effects on what we see, read why the Moon looks bigger near the horizon. For the oceans themselves, see the largest ocean.

What causes tides?