What causes tides?

Tides are the predictable rise and fall of sea level along coastlines, driven almost entirely by gravity. The Moon pulls on the Earth’s oceans, and to a lesser extent on the solid Earth and atmosphere itself. Because gravity weakens with distance, the side of Earth facing the Moon is pulled a bit harder than the center of the planet, and the far side is pulled a bit less. NOAA’s Ocean Service explains that this difference stretches the ocean into two opposing bulges: one on the side closest to the Moon and one on the side farthest from it. As Earth rotates through those bulges every 24 hours and 50 minutes, every coastline passes through high tide and low tide roughly twice per day.

The Sun matters too, even though its pull on Earth is about 179 times weaker than the Moon’s at any single point. The reason tides care about the Sun is the same differential stretching, and the Sun delivers about 46 percent as much tidal force as the Moon. When the Sun, Earth, and Moon line up at new moon or full moon, the Sun’s pull reinforces the Moon’s and tides swing wider than average. NOAA calls these “spring tides” — the word has nothing to do with the season. When the Sun and Moon are at right angles relative to Earth, at the quarter moons, the pulls partially cancel and tides run smaller. Those are “neap tides.” The biggest tides of the year usually come at the equinoxes in March and September, when the Sun is over the equator and its tidal vector lines up especially well with the Moon’s.

The shape of the coastline and the depth of the seafloor dramatically reshape these global forces. Wide, shallow continental shelves amplify tides, which is why the Bay of Fundy in eastern Canada sees tidal ranges of more than 50 feet, among the largest on Earth. Narrow inlets and harbors can develop extreme tidal bores, including the Qiantang River in China and the Severn in the U.K. The open Pacific coast of the U.S. sees a typical daily tide range of about 3 to 5 feet, while the Gulf of Mexico often sees less than a foot. Far from shore, in the deep ocean, the actual water movement from a tide is only a foot or two, but it is enough to drive currents that move energy around the planet.

Tides matter in real, practical ways. NOAA tide tables drive shipping schedules, harbor entrance planning, fishing, and coastal construction. Tidal energy is a growing source of renewable electricity; several prototype projects have captured the predictable flow of tides with underwater turbines. The Sihwa Lake tidal power station in South Korea is currently the largest operating facility, with a capacity of about 254 MW. In other places, tidal locking restricts marine traffic to a short window around high water. Tides also mix coastal waters, bringing nutrients up from the depths that fuel kelp forests, oyster beds, and rich fisheries.

The Moon’s pull on the solid Earth itself is small but measurable, and it causes the entire planet to flex by about a foot each day. Earth tide instruments pick up the same forces that drive ocean tides, and they are sensitive enough to register atmospheric pressure changes and distant earthquakes. That is part of why precise GPS stations near coasts need tide corrections to maintain their accuracy.

Climate change is now affecting tidal patterns too. As sea level rises and coastlines reshape, the resonance that amplifies tides in places like the Bay of Fundy shifts, and high-tide flooding in cities like Miami, Norfolk, and Annapolis is hitting far more often than it did a generation ago. NASA and NOAA are now tracking those nuisance floods because they point to where infrastructure is starting to lose its margin of safety. For more on the ocean’s other long-running motion, see the ocean salinity guide and the Moon phases explainer.