A moon tide is simply a tide caused by the moon’s gravitational pull on Earth’s oceans. While the sun also influences tides, the moon is the dominant force, responsible for about twice as much tidal pull as the sun. Every coastal tide you’ve ever seen is largely a moon tide, though the term is most often used to distinguish the moon’s role from the sun’s or to describe tides that occur in sync with specific lunar phases.
How the Moon Creates Tides
The moon’s gravity tugs on everything on Earth, but it pulls hardest on whatever is closest. Water on the side of Earth facing the moon gets pulled toward it, creating a bulge of higher water. What’s less intuitive is that a second bulge forms on the opposite side of Earth at the same time. This happens because of the way Earth and the moon orbit each other.
Earth and the moon don’t simply sit still. They revolve around a shared center of mass, called a barycenter, which sits inside Earth but off-center toward the moon. This revolution generates an outward centrifugal force at every point on Earth, always directed away from the moon. On the far side of Earth, the moon’s gravitational pull is weakest (because it’s the farthest point away), but the centrifugal force is exactly the same as everywhere else. That imbalance pushes water outward on the far side, creating the second bulge.
These two bulges are the high tides. The areas between them, roughly 90 degrees away, are where water has been pulled away from, creating low tides. As Earth rotates, different coastlines pass through these bulges and troughs, producing the rhythmic rise and fall of water levels throughout the day.
Why Tides Shift by 50 Minutes Each Day
If tides were driven only by Earth’s rotation, they’d arrive at the same time every day. But the moon is also moving, orbiting Earth in the same direction that Earth spins. By the time a spot on Earth completes one full rotation, the moon has crept forward in its orbit, so Earth needs an extra 50 minutes to “catch up” and realign with the moon. This makes a lunar day 24 hours and 50 minutes long instead of 24 hours. It’s why high tide arrives roughly 50 minutes later each day.
Because Earth rotates through both tidal bulges in one lunar day, most coastlines experience two high tides and two low tides in that 24-hour-and-50-minute window.
The Sun’s Role Compared to the Moon’s
The sun is vastly more massive than the moon, but it’s also much farther away. Tidal force depends heavily on distance, not just mass. The result: the sun’s tidal influence is only about 44% of the moon’s. That’s still significant. When the sun and moon work together, tides get bigger. When they work against each other, tides shrink.
Spring Tides and Neap Tides
Twice a month, during the new moon and full moon, the Earth, moon, and sun line up (or nearly so). Their gravitational forces combine, producing spring tides. The name has nothing to do with the season; it comes from the idea of the tide “springing forth.” During spring tides, high tides are higher than average and low tides are lower, creating a larger overall tidal range. These are sometimes called king tides.
About seven days after each spring tide, the moon moves to a position at a right angle to the sun relative to Earth. This happens during the first-quarter and third-quarter moon phases, when the moon looks half full. In this arrangement, the sun’s pull partially cancels out the moon’s, producing neap tides. High tides are lower than average and low tides are higher, so the water level stays more moderate throughout the day.
How the Moon’s Distance Changes Tides
The moon’s orbit isn’t a perfect circle. It’s slightly elliptical, which means the moon is sometimes closer to Earth (perigee) and sometimes farther away (apogee). When a spring tide coincides with perigee, you get a perigean spring tide, the strongest regular tidal event. High tides during a perigean spring tide can be more than a foot higher than high tides during a spring tide at apogee. In places with already dramatic tides, like Anchorage, Alaska, which has a tidal range over 30 feet, the difference can reach 3 feet or more.
The most extreme tidal range on Earth occurs in Nova Scotia’s Bay of Fundy, where the water level can swing as much as 53 feet (16 meters) between high and low tide. The global average tidal range, by comparison, is about 3 feet.
Tidal Patterns Vary by Location
Not every coastline experiences tides the same way. The shape of the ocean floor, the width of bays, and the contours of the coastline all influence how tidal energy moves through water. This creates three main tidal patterns.
- Semidiurnal tides produce two high tides and two low tides of roughly equal height each lunar day. This is the most common pattern along the U.S. East Coast.
- Mixed semidiurnal tides also produce two highs and two lows, but they differ noticeably in height. One high tide is significantly taller than the other. This pattern is typical along the U.S. West Coast.
- Diurnal tides produce just one high tide and one low tide per lunar day. Parts of the Gulf of Mexico follow this pattern.
So while the moon drives tides everywhere on Earth, the local experience of those tides depends heavily on geography. Two cities at the same latitude can have completely different tidal rhythms because of how their coastlines channel the water the moon is pulling around.