How Tides Are Governed by Lunar and Solar Gravity

The Bay Where the Ocean Rises Five Stories

Twice each day, the Bay of Fundy between Nova Scotia and New Brunswick experiences tidal ranges that can reach 16 meters—roughly the height of a five-story building. At low tide, the bay floor lies exposed for kilometers. Six hours later, the same ground sits under 16 meters of water. Approximately 160 billion tons of water flow in and out with each tidal cycle. This extreme phenomenon is driven by the same force that keeps the Moon in orbit around Earth: gravity. But tides aren't simply the Moon pulling water upward. The mechanism is subtler, involving differential gravitational force across Earth's diameter—a concept Newton first explained in 1687.

How Tidal Forces Actually Work

The Moon's gravitational pull is stronger on the side of Earth facing the Moon than on the side facing away, because gravitational force decreases with distance. This difference—not the absolute pull—creates tides. The near side of Earth is pulled toward the Moon more strongly than Earth's center, and Earth's center is pulled more strongly than the far side. The result is a stretching effect that produces two tidal bulges: one facing the Moon and one on the opposite side.

Common misconceptions about tides:

• Tides are not caused by the Moon simply "lifting" water. The tidal force is only about one ten-millionth of Earth's surface gravity—far too weak to noticeably lift anything.
• The far-side bulge exists because Earth's center is pulled away from the water on the far side, not because some force pushes that water outward.
• The Sun also generates tides—its tidal effect is about 46% as strong as the Moon's, despite its enormously greater gravitational pull, because tidal force depends on the gradient of gravity, which decreases with the cube of distance.

Spring Tides and Neap Tides

The interaction between lunar and solar tidal forces produces a predictable biweekly cycle of higher and lower tidal ranges.

Spring tides have nothing to do with the season. The term comes from the Germanic springen, meaning to leap or surge. During spring tides, high tides are higher and low tides are lower than average. During neap tides, the Sun's tidal force partially cancels the Moon's, producing a compressed range.

Tidal Patterns Around the World

Not all coastlines experience tides the same way. The shape of ocean basins, continental shelves, and coastal geography creates three distinct tidal patterns.

The Bay of Fundy's extreme tides result from resonance. The bay's length and depth create a natural oscillation period of approximately 12.5 hours—almost exactly matching the semidiurnal tidal period. Water sloshes back and forth in the bay like water in a bathtub, amplifying the tidal range far beyond what the Moon's gravity alone produces.

Tidal Locking: Why the Moon Always Shows One Face

The Moon always presents the same hemisphere toward Earth. This isn't coincidence—it's a direct consequence of tidal forces operating over billions of years.

Early in its history, the Moon rotated faster than it does now. Earth's tidal force raised bulges on the Moon's surface. Because the Moon was rotating, these bulges were carried slightly ahead of the Earth-Moon line, creating a torque that gradually slowed the Moon's rotation. Eventually, the rotation period matched the orbital period—a state called tidal locking or synchronous rotation. The process took hundreds of millions of years.

Tidal locking is common in the solar system:

• All four Galilean moons of Jupiter are tidally locked
• Pluto and Charon are mutually tidally locked—each always shows the same face to the other
• Mercury is in a 3:2 spin-orbit resonance (a near-locked state) with the Sun

Earth's Rotation Is Slowing Down

The same tidal interaction that locked the Moon is gradually slowing Earth's rotation. Tidal friction—primarily from ocean tides dragging against the seafloor—converts Earth's rotational energy into heat. The day is getting longer by approximately 2.3 milliseconds per century.

This effect is measurable in the geological record. Growth rings in 400-million-year-old corals indicate that the Devonian year contained approximately 400 days, each roughly 21.9 hours long. Tidal rhythmites—layered sedimentary deposits recording ancient tidal cycles—confirm the progressive lengthening of the day.

As Earth slows, angular momentum transfers to the Moon, pushing it farther away. Laser ranging measurements (bouncing lasers off reflectors left by Apollo astronauts) show the Moon receding at 3.8 centimeters per year. In the distant future—billions of years from now—Earth's day will lengthen to match the Moon's orbital period, and the two bodies will be mutually tidally locked.

Harnessing Tidal Energy

Tidal power has a fundamental advantage over wind and solar energy: perfect predictability. Tidal schedules can be calculated centuries in advance. The challenge is extracting energy economically from a diffuse, low-speed flow.

• The La Rance tidal barrage in France, operational since 1966, generates 240 MW and was the world's first large-scale tidal power station
• South Korea's Sihwa Lake station (254 MW, opened 2011) is currently the largest tidal barrage
• Tidal stream generators, which work like underwater wind turbines, are being deployed in Scotland's Pentland Firth, where tidal currents reach 5 meters per second
• Global tidal energy potential is estimated at 120–400 GW, though only a fraction is economically accessible with current technology

The tides have risen and fallen on Earth for over 4 billion years, since the Moon formed from debris ejected by a Mars-sized impact. They carved coastlines, synchronized the spawning cycles of marine organisms, and provided the rhythmic intertidal environment where many biologists believe the transition from ocean to land life may have begun. The gravitational dance between Earth and Moon shaped the planet's rotation, its geology, and possibly the trajectory of life itself—all from a force gentle enough that you'd never notice it pulling on your body, yet relentless enough to move 160 billion tons of water twice a day.