Ocean Tides Explained: Lunar Gravity, Spring Tides & Power

The Bay of Fundy Rises 16 Meters. The Mediterranean Barely Moves.

At the head of the Bay of Fundy between New Brunswick and Nova Scotia, the tidal range exceeds 16 meters — roughly the height of a five-story building — and the tidal bore surges upriver at speeds that outpace a person running. At the center of the Mediterranean Sea, the same gravitational forces produce a tidal range of less than 30 centimeters. Both bodies of water experience the same lunar and solar tidal forcing. The vast difference in tidal range reflects not differences in gravitational pull but differences in basin geometry, resonance, and ocean depth — factors that turn the global tidal signal into an extraordinarily local phenomenon.

The Physics of Tidal Forcing

Newton's law of universal gravitation states that gravitational force falls off with the square of distance. Tidal forces arise from the gradient of gravitational attraction across a body's diameter — not the total force itself. The Moon pulls the side of Earth nearest to it more strongly than the center of Earth, and the center of Earth more strongly than the far side. This differential force stretches Earth's oceans along the Earth-Moon axis, creating two tidal bulges: one pointing toward the Moon and one pointing away from it.

The tidal force from the Moon is proportional to 1/r³ (the third power of distance) rather than 1/r² (gravitational force). This means tidal influence falls off much faster with distance than gravity itself. The Sun is approximately 27 million times more massive than the Moon but 390 times farther away. Its tidal effect is approximately 46% as strong as the Moon's — significant but secondary.

• Moon's tidal acceleration at Earth's surface: ~1.1 × 10⁻⁶ m/s²
• Sun's tidal acceleration at Earth's surface: ~0.5 × 10⁻⁶ m/s²
• When Moon and Sun align (new or full moon), their tidal forces add — producing spring tides approximately 20%–40% larger than average.
• When Moon and Sun are at right angles (first or third quarter moon), their tidal forces partially cancel — producing neap tides approximately 20%–30% smaller than average.

Spring vs. Neap Tides

The tidal cycle has two overlapping periodicities: the daily (diurnal) or twice-daily (semidiurnal) flood and ebb driven by Earth's rotation, and the fortnightly spring-neap cycle driven by the Moon's orbital position.

Global Tidal Range Comparison

Tidal Locking: A Consequence of Tidal Forces

The same tidal forces that raise ocean tides on Earth have, over billions of years, synchronized the Moon's rotation to its orbital period — a condition called tidal locking or synchronous rotation. The tidal bulge raised on the Moon by Earth's gravity (early in the Moon's history) gradually transferred angular momentum from the Moon's spin to its orbital motion, slowing the Moon's rotation until its day equaled its year (27.3 days). The Moon now always presents the same face to Earth.

Earth's rotation is slowing by the same mechanism in reverse: the Moon's tidal forces on Earth are gradually transferring angular momentum from Earth's spin to the Moon's orbit. Earth's day was approximately 22 hours long 620 million years ago (established through tidal rhythmite sediment counting). The Moon is receding from Earth at approximately 3.8 cm per year as a consequence. In roughly 50 billion years — far beyond the Sun's main-sequence lifetime — Earth and Moon would be mutually tidally locked.

Tidal Power Generation

Tidal energy is renewable, predictable (unlike solar or wind), and concentrated in specific geographic locations where tidal ranges and currents are extreme. Two primary generation technologies exist.

• Tidal barrage: A dam-like structure across a tidal estuary that traps water at high tide and releases it through turbines as the tide falls. La Rance Tidal Power Plant in Brittany, France — the world's first large tidal barrage, operational since 1966 — generates 240 MW from a 10 km barrage across the Rance estuary with a 13.5 m tidal range. Annual output averages approximately 500 GWh. The Sihwa Lake Tidal Power Station in South Korea (254 MW, operational 2011) is the world's largest by capacity.
• Tidal stream generators: Underwater turbines positioned in fast-moving tidal currents, conceptually similar to wind turbines but operating in water approximately 800 times denser than air. Orbital Marine Power's O2 floating tidal turbine (rated 2 MW), installed off Orkney, Scotland in 2021, is among the world's most powerful tidal stream devices. The Pentland Firth between mainland Scotland and Orkney is estimated to hold a technically exploitable resource of 1.9 GW — capable of meeting approximately 40% of Scotland's current electricity demand.

Tidal barrages alter estuarine ecosystems by permanently modifying salinity gradients, sedimentation, and fish migration routes — environmental trade-offs that have constrained development of proposed projects such as the Severn Barrage (UK). Tidal stream generators have a smaller ecological footprint and are the preferred development pathway in most new projects.