How the Moon Causes Ocean Tides

The moon is the main reason the ocean rises and falls twice a day. It does it with gravity, the same force that keeps you on the ground. Here is the plain version.

The moon pulls the water

The moon's gravity tugs on everything on Earth, but water is free to move, so it responds the most. On the side of Earth facing the moon, the ocean bulges out toward it. That bulge is a high tide.

Why there are two high tides

Here is the part that surprises people. There is a second bulge on the far side of Earth, away from the moon, at the same time. That happens because the moon pulls the solid Earth slightly toward itself, leaving the water on the far side behind. So two bulges, on opposite sides.

As Earth spins once a day, any given coast passes through both bulges and both low points. That is why most places get two high tides and two low tides each day.

Where the sun comes in

The sun pulls on the oceans too, with about half the effect of the moon. When the sun and moon line up, at new and full moons, their pulls add together and you get the biggest tides, called spring tides. When they are at right angles, around the half-moon phases, the tides are gentler, called neap tides.

So the phase predicts the tides

Knowing the moon phase gives you a rough idea of the tide strength. Full and new mean big swings, half-moons mean smaller ones.

Check the current phase on moontrack, or read the 8 moon phases explained.