Why Do Tides Follow the Moon?

Tides rise and fall on a clock set by the moon. The link is gravity — but the details are more interesting than "the moon pulls the water up". Here is how it actually works.

Gravity makes two bulges, not one

The moon’s gravity pulls on Earth’s oceans, raising a bulge of water on the side of Earth nearest the moon. That part is intuitive. The surprise is the second bulge on the opposite side of Earth. It forms because the moon pulls the solid Earth slightly toward itself more strongly than it pulls the far-side water — so the far-side water is, in effect, "left behind", bulging outward. The result is two bulges on opposite sides of the planet.

Why most coasts get two high tides a day

Those two bulges stay roughly lined up with the moon while Earth rotates underneath them once a day. As your stretch of coast spins through a bulge you get a high tide; between the bulges you get a low tide. Pass through both bulges in one rotation and you experience about two high tides and two low tides every 24 hours and 50 minutes.

That extra 50 minutes is why high tide arrives a little later each day: the moon has moved along its orbit, so Earth must turn slightly more to bring your coast back under the bulge.

Spring tides and neap tides

The sun raises tides too. It is vastly more massive than the moon but also much farther away, so its tidal effect is about 46% as strong. The two combine depending on the moon’s phase:

• Spring tides — at new moon and full moon, the sun and moon line up, so their pulls add together. High tides are higher and low tides lower. (Nothing to do with the season "spring".)
• Neap tides — at the first and last quarter, the sun and moon pull at right angles and partly cancel, so the tidal range is smaller.

Why local tides still vary so much

Gravity sets the rhythm, but the size of a tide depends heavily on local geography — the shape of the coastline, the depth of the sea, and bays that funnel water. That is why some places (like the Bay of Fundy) see enormous tides while others barely change. The moon tells you when; the coastline tells you how much.

Want to know which phase the moon is in right now — and when the next new or full moon (and the strongest spring tides) will be? Open the Moon Phase tool, and read moon phases explained for the full cycle.