Earth’s tides vary because of shifting gravitational relationships between the Earth, Moon, and Sun, combined with the shape of coastlines, orbital cycles that play out over weeks and decades, and even changes in atmospheric pressure. No single factor controls the tides. Instead, several forces layer on top of each other, producing patterns that repeat on daily, monthly, yearly, and multi-decade timescales.
The Moon and Sun Pull Together or Apart
The Moon is the dominant driver of tides, exerting about 2.2 times more tidal force than the Sun despite being far smaller. That’s because tidal force depends heavily on distance, and the Moon is roughly 390 times closer to Earth than the Sun. The Sun still matters, though, and the constant interplay between these two gravitational sources is the single biggest reason tides change from one week to the next.
During new and full moons, the Earth, Moon, and Sun fall roughly into alignment. Their gravitational pulls reinforce each other, producing higher high tides and lower low tides than average. These are called spring tides, a term that has nothing to do with the season and comes from a word meaning “to well up.” Spring tides happen about every two weeks.
At the first- and third-quarter moon phases, the Sun and Moon pull on the oceans at right angles to each other, partially canceling each other out. High tides are lower and low tides are higher than average. These diminished tides are called neap tides, from a Greek word meaning “scanty.” So in any given month, you get two rounds of large spring tides and two rounds of smaller neap tides, creating a steady pulse of variation.
How Orbital Distance Changes the Range
The Moon doesn’t orbit Earth in a perfect circle. Once a month it reaches its closest point (perigee), and about two weeks later it reaches its farthest point (apogee). At perigee, the Moon’s gravitational pull is stronger, producing above-average tidal ranges. At apogee, ranges shrink. When perigee coincides with a new or full moon, you get what’s sometimes called a “perigean spring tide,” with noticeably larger swings between high and low water.
The same principle applies to Earth’s orbit around the Sun. Earth is closest to the Sun around January 2 each year (perihelion) and farthest around July 2 (aphelion). Tidal ranges are slightly enhanced in early January and slightly reduced in early July. This effect is smaller than the Moon’s monthly cycle, but it’s measurable. When perigee, perihelion, and a new or full moon all line up at roughly the same time, the result is considerably larger tidal ranges. The reverse combination, apogee plus aphelion plus a quarter moon, produces notably reduced ranges.
The 18.6-Year Lunar Nodal Cycle
One of the less well-known sources of tidal variation operates on a nearly two-decade timescale. The plane of the Moon’s orbit wobbles slowly, completing a full rotation every 18.6 years. This is called the lunar nodal cycle, and it changes how high above or below the equator the Moon travels during each orbit. That shift alters the distribution and intensity of tidal forces across the planet.
According to research by the U.S. Geological Survey, this 18.6-year cycle dominates the annual averages of high water, low water, and tidal range at harbors along the U.S. East Coast, including Boston. It also weakly influences annual mean sea level. The cycle is significant enough that scientists analyzing long-term sea level trends need to account for it. Using data that doesn’t span complete multiples of the 18.6-year cycle can distort conclusions about whether sea levels are truly rising or falling at a given location.
Why Tides Differ So Much by Location
If Earth were a smooth sphere covered entirely by water, every coast would experience two nearly equal high and low tides per day. In reality, large continents block tidal bulges from sweeping freely around the globe. This forces the tides into complex patterns within each ocean basin that can differ dramatically from basin to basin or even within different parts of the same ocean.
The result is three broad categories of tidal patterns. Semidiurnal coasts get two high tides and two low tides of roughly equal height each day, which is common along the U.S. Atlantic coast. Diurnal coasts, like parts of the Gulf of Mexico, see only one high and one low per day. Mixed coasts get two highs and two lows, but with significant differences in height between them. Which pattern a given location experiences depends on the geometry of its ocean basin and how tidal energy bounces around within it.
Earth’s rotation also creates points in the ocean, called amphidromic points, where the tidal range is essentially zero. There are about 140 of these scattered across the world’s oceans. Tidal range increases with distance from these points. The interaction between Earth’s rotation and basin geometry causes tidal waves to rotate around amphidromic points rather than sloshing back and forth, which is why neighboring coastlines can have surprisingly different tidal ranges.
Coastal Shape and Water Depth
Local geography can amplify tides far beyond what gravitational forces alone would produce. The global average tidal range is about 3 feet. But at Wolfville, in Nova Scotia’s Minas Basin at the head of the Bay of Fundy, the difference between high and low tide can reach 53 feet (16 meters). That’s not because the Moon pulls harder on Nova Scotia. It’s because the Bay of Fundy acts like a natural amplifier.
Funnel-shaped bays and estuaries concentrate tidal energy into a narrowing channel, forcing the water higher. Deep channels amplify tides more effectively because convergence (the funneling effect) overpowers the friction of the seafloor. In shallower channels, bottom friction dominates and tends to dampen tidal energy instead. The width of a continental shelf matters too: wide, shallow shelves allow tidal waves to build, while steep drop-offs give them little room to amplify. Some bays also experience resonance, where the natural oscillation period of the basin closely matches the timing of the incoming tide, causing the water to slosh higher with each cycle, much like pushing a child on a swing at just the right moment.
Weather and Atmospheric Pressure
Atmospheric pressure acts as an invisible weight on the ocean surface. When pressure drops, as during a storm system, the sea surface rises slightly. When pressure increases, it pushes the surface down. The theoretical relationship is straightforward: every 1 millibar increase in atmospheric pressure depresses sea level by about 1 centimeter. In practice, satellite measurements show the real-world response is somewhat weaker, roughly 0.6 to 0.7 centimeters per millibar over most of the ocean. At high latitudes the response is closer to the full theoretical value, while near the equator it’s almost nonexistent.
This means that a strong low-pressure system can add several centimeters to predicted tide levels, while a high-pressure system can suppress them. Wind compounds this effect. Sustained onshore winds push water toward the coast, raising levels beyond what the tide alone would produce. These weather-driven additions are called storm surge when they’re extreme, but even ordinary weather patterns cause daily deviations from predicted tide tables. It’s why actual water levels at a given harbor almost never match the astronomical prediction exactly.
How All These Cycles Stack Up
What makes tidal prediction both fascinating and complicated is that all of these cycles operate simultaneously. On any given day, the tide you observe is the combined result of where the Moon sits in its monthly orbit, whether it’s near perigee or apogee, the current phase of the spring-neap cycle, Earth’s distance from the Sun, the stage of the 18.6-year nodal cycle, the shape of the local coastline, and whatever the atmosphere happens to be doing overhead. Modern tide prediction breaks the total tidal signal into dozens of individual components, each with its own period and strength, then adds them back together to produce the forecasts you see in tide tables.
The layering of these cycles also explains why some years produce memorable coastal flooding while others are unremarkable. A perigean spring tide occurring near perihelion, during the high phase of the nodal cycle, with a low-pressure system and onshore winds, can push water levels well above normal. None of those factors alone would cause a problem, but their coincidence creates conditions that matter for coastal communities, infrastructure, and ecosystems.