The moon controls Earth’s tides through gravitational pull, creating two bulges of water on opposite sides of the planet that produce the rising and falling water levels you see at the coast. The global average tidal range is about three feet, but local geography can amplify that dramatically. Here’s how the whole system works.
Why the Moon Creates Two Tidal Bulges
The moon’s gravity pulls on everything on Earth, but it pulls hardest on whatever is closest. The ocean water on the side of Earth facing the moon gets tugged toward it, creating a bulge of higher water. That part is intuitive. The less obvious part is what happens on the opposite side of the planet.
On the far side of Earth, the moon’s gravitational pull is weaker because of the extra distance. Here, inertia wins out. The water essentially “keeps going” in a straight line rather than being pulled inward, forming a second bulge pointing away from the moon. The result is two simultaneous high tides on opposite sides of Earth at all times, with lower water levels in the areas between them. Because water is fluid, these bulges stay aligned with the moon as Earth rotates beneath them.
Why Tides Shift 50 Minutes Later Each Day
If tides were driven by the sun alone, they’d follow a neat 24-hour schedule. But the moon orbits Earth in the same direction Earth spins, so our planet has to rotate a little extra each day to “catch up” to where the moon has moved. This makes the lunar day 24 hours and 50 minutes long. That’s why high tide arrives roughly 50 minutes later each day compared to the day before.
Most coastlines experience two high tides and two low tides within each lunar day, spaced about 12 hours and 25 minutes apart. But the pattern isn’t identical everywhere.
Why Tides Look Different on Each Coast
The shape of the coastline, the depth of the ocean floor, and the width of bays and inlets all bend the tidal signal into different patterns. Oceanographers classify these into three types:
- Semidiurnal tides: two highs and two lows per lunar day, all roughly equal in height. This is the typical pattern along the U.S. East Coast.
- Mixed semidiurnal tides: two highs and two lows per day, but they differ noticeably in height. The U.S. West Coast follows this pattern, so one high tide each day is clearly bigger than the other.
- Diurnal tides: only one high and one low per lunar day. Much of the Gulf of Mexico experiences this simpler cycle.
Spring Tides and Neap Tides
The sun also exerts a gravitational pull on Earth’s oceans, though it’s weaker than the moon’s despite the sun’s far greater mass (because tidal force drops sharply with distance). Twice a month, during full moons and new moons, the sun and moon line up with Earth. Their gravitational pulls combine, producing especially high high tides and especially low low tides called spring tides. The name has nothing to do with the season; it comes from the idea of the water “springing” up.
About a week after each spring tide, the sun and moon form a right angle relative to Earth (at the first and third quarter moon phases). Their gravitational pulls partially cancel each other out, producing smaller tidal swings called neap tides. This spring-neap cycle repeats roughly every two weeks.
How the Moon’s Distance Changes Tides
The moon doesn’t orbit Earth in a perfect circle. Once a month it reaches perigee, its closest point to Earth, and the stronger gravitational pull at that distance produces above-average tidal ranges. When perigee coincides with a spring tide (a scenario sometimes called a “king tide” or perigean spring tide), coastal water levels can be significantly higher than normal. These events are predictable and happen a few times a year.
The Moon Even Deforms Solid Ground
Tides aren’t limited to water. The moon’s gravity also stretches Earth’s solid crust, raising and lowering the ground beneath your feet by about 30 centimeters (roughly a foot) twice a day. You can’t feel it because the change is gradual and everything around you moves together. But precision instruments like GPS stations and particle accelerators have to account for this land tide to maintain accuracy.
Extreme Tides: The Bay of Fundy
Geography can funnel tidal energy into dramatic results. The Bay of Fundy in Nova Scotia holds the record for the world’s highest tides. Near Wolfville in the Minas Basin, the water level at high tide can be 53 feet (16 meters) higher than at low tide. That’s roughly the height of a five-story building. The bay’s funnel shape and the natural resonance of the water sloshing back and forth amplify the tidal range far beyond what open-ocean physics alone would produce.
Tides Are Slowly Pushing the Moon Away
The tidal interaction between Earth and the moon isn’t a one-way street. As Earth rotates beneath the tidal bulges, friction between the ocean water and the seafloor creates drag that gradually slows Earth’s spin. That lost rotational energy gets transferred to the moon, nudging it into a slightly higher orbit. NASA measurements show the moon is drifting away from Earth at about 1.5 inches (4 centimeters) per year. Over geological time, this same process has already lengthened Earth’s day from about 6 hours (billions of years ago) to the 24 hours we have now.
High Tide Flooding Is Increasing
As sea levels rise, even normal high tides can push water over the threshold into low-lying streets and storm drains. NOAA tracks these “high tide flood days,” and the numbers are climbing. For the 2025 to 2026 period, the U.S. is projected to experience a median of 4 to 9 high tide flood days nationally. Some regions face more: the Mid-Atlantic coast is projected for 8 to 13 flood days, the Western Gulf for 6 to 15, and the Western and Eastern Pacific Islands for 8 to 20. These floods don’t require storms. They happen on otherwise clear days when the tidal cycle, sea level, and sometimes wind align just right.
This means understanding tides isn’t just an academic exercise. For coastal communities, the predictable rhythm of the moon’s pull now intersects with rising baselines in ways that affect property, infrastructure, and daily life.