The gravitational pull of the Moon is the primary phenomenon that causes Earth’s oceans to bulge. More precisely, the interaction between the Moon’s gravity and Earth’s own rotational inertia creates two simultaneous bulges on opposite sides of the planet, producing the rise and fall of tides roughly twice a day. The Sun contributes a secondary bulge through the same mechanism, and the combined effect of both is what drives the tidal patterns you observe at any coastline.
How the Moon Creates Two Bulges at Once
Gravity weakens with distance. The Moon pulls on the side of Earth facing it with noticeably more force than it pulls on the far side, about 8,000 miles away. This difference in gravitational strength across Earth’s diameter is called the tidal force, and it’s the engine behind ocean bulges.
On the side of Earth closest to the Moon, lunar gravity is stronger than the centrifugal force produced by the Earth-Moon system orbiting their shared center of mass. That net inward pull draws ocean water outward toward the Moon, creating what NOAA calls the “direct tide.” On the opposite side of Earth, the situation reverses: the Moon’s gravitational pull is weaker because of the extra distance, but the centrifugal force remains constant everywhere on Earth. Inertia wins out, and the water on the far side moves away from Earth’s center, forming a second bulge called the “opposite tide.”
The result is an elongated shape, with ocean water piling up on two sides of the planet simultaneously. As Earth rotates beneath these two bulges, most coastlines experience two high tides and two low tides every 24 hours and 50 minutes (the extra 50 minutes accounts for the Moon advancing in its orbit each day).
The Sun’s Supporting Role
The Sun also exerts tidal forces on Earth’s oceans, but despite being vastly more massive than the Moon, it’s about 390 times farther away. Because tidal force depends on the difference in gravitational pull across Earth’s diameter, distance matters enormously. The Sun contributes roughly 30% of the total tidal force, while the Moon is responsible for about 60%, with smaller factors accounting for the rest.
When the Sun, Moon, and Earth line up during new and full moons, the solar and lunar bulges reinforce each other. High tides get a bit higher, low tides drop a bit lower, and the combined effect is called a spring tide. This happens twice every lunar month, year-round, and has nothing to do with the spring season. The name comes from the idea of tides “springing forth.”
About seven days after each spring tide, the Sun and Moon sit at right angles relative to Earth. The Sun’s bulge partially cancels the Moon’s, producing smaller tidal swings called neap tides. During neap tides, high tides are lower than average and low tides are higher than average. This alternating pattern of spring and neap tides repeats every two weeks like clockwork.
Why Tides Look So Different Depending on Location
In the open ocean, far from any continent, the actual height of a tidal bulge is modest. Mid-ocean islands typically see tides of one meter or less. The dramatic tides you hear about, like those in Canada’s Bay of Fundy reaching over 16 meters, are products of coastal geography. Narrowing bays, shallow continental shelves, and funnel-shaped inlets amplify the incoming tidal wave, compressing the same volume of water into a shrinking space.
There are also spots in the ocean where the tidal range drops to nearly zero. These are called amphidromic points, locations where the tidal wave essentially pivots. The water rotates around these nodes rather than rising and falling. Near the coast of Qinhuangdao in China, for instance, an amphidromic point keeps the maximum tidal range to just 1.5 meters, far smaller than most other Chinese coastal areas. The global ocean has dozens of these points, and their positions determine how tidal energy distributes across ocean basins.
Other Forces That Make Oceans Bulge
Tidal forces aren’t the only phenomenon that pushes ocean water upward. Earth’s rotation itself creates a permanent, slight bulge at the equator through centrifugal force. The planet’s equatorial radius is 6,378.1 km, about 0.34% greater than its polar radius of 6,356.8 km. The oceans, along with all of Earth’s material, have settled into this oblate shape over billions of years. Unlike tidal bulges that shift with the Moon, this equatorial bulge is fixed and constant.
Weather systems produce temporary, localized bulges as well. At the center of a hurricane, low atmospheric pressure allows the ocean surface to rise slightly, forming a mound of water. This pressure-driven bulge accounts for about 5% of storm surge height, with wind doing the rest of the work by physically shoving water toward shore.
On a much longer timescale, warming ocean temperatures cause water to expand in volume, a process called thermal expansion. More than 90% of the excess heat trapped by greenhouse gases is absorbed by the oceans, and this warming has been responsible for roughly one-third of the global sea-level rise measured by satellites since 2004. This isn’t a bulge in the tidal sense, but it is a measurable, sustained increase in ocean volume driven by physics rather than gravity from another body.
Why the Moon Wins Over the Sun
It’s natural to assume the Sun, which is about 27 million times more massive than the Moon, would dominate tidal forces. The key is that tides aren’t driven by raw gravitational pull. They’re driven by the gradient of that pull, meaning how much it changes from one side of Earth to the other. Gravitational force drops off with the square of distance, but the tidal force drops off with the cube of distance. The Moon is close enough (about 384,400 km away) that the difference in its pull between Earth’s near and far sides is substantial. The Sun, at roughly 150 million km, pulls on both sides of Earth almost equally, so its tidal gradient is much smaller despite its immense mass.
This is why a relatively small, nearby body like the Moon is the dominant force shaping ocean bulges, and why the tides you see at the beach follow the lunar cycle more closely than the solar one.