Why are there usually two high tides a day?

The tide-generating forces raise bulges of water on both the side of Earth facing the Moon and the opposite side, so as the planet rotates most coasts pass through two high and two low tides each day.

What causes spring and neap tides?

When the Sun and Moon align, their tidal forces add to give large spring tides; when they are at right angles, the forces partly cancel to give smaller neap tides, cycling about every two weeks.

Tides and Tidal Dynamics

The gravitational pull of the Moon and Sun raises a predictable rhythm of rising and falling seas, but it is the shape of ocean basins and the rotation of the Earth that turn that forcing into the rich variety of tides observed at the coast.

Definition

Tides are the periodic rise and fall of sea level caused by the gravitational attraction of the Moon and Sun combined with Earth's rotation; tidal dynamics is the study of how these forces drive the motion of the ocean.

Scope

This topic covers the gravitational origin of the tide-generating forces, the equilibrium and dynamical theories of tides, the harmonic constituents used for tidal prediction, the formation of rotating tidal systems (amphidromes) in ocean basins, the amplification of tides in shallow seas, and the tidal currents that result.

Core questions

What gravitational forces generate the tides?
Why does the equilibrium theory fail to predict real tides, and what does the dynamical theory add?
How are tides predicted from harmonic constituents?
Why do tidal range and timing vary so much from place to place?

Key theories

Tide-generating force and equilibrium tide: The difference between the Moon's and Sun's gravitational pull across the Earth and the orbital acceleration produces tide-generating forces, whose idealized response is the equilibrium tide.
Dynamical theory and amphidromic systems: Because the ocean cannot respond instantly, tides propagate as waves that, deflected by the Coriolis force and constrained by basins, rotate around amphidromic points of near-zero range.

Mechanisms

The Moon and Sun exert gravitational forces that vary across the Earth, producing tidal bulges; as the Earth rotates and the bodies orbit, these forces drive tidal waves that travel through ocean basins. Constrained by coastlines and turned by the Coriolis force, the waves form rotating amphidromic systems, and in shallow seas and bays resonance can amplify the range dramatically.

Clinical relevance

Tidal prediction is essential for navigation, port operations, and coastal engineering; tidal currents disperse pollutants and nutrients, drive mixing, carry energy that can be harvested for power, and combine with storm surge to set coastal flood risk.

History

Newton explained the tide-generating force and the equilibrium tide; Laplace formulated the dynamical tidal equations, George Darwin developed harmonic analysis, and Kelvin (William Thomson) built the first tide-predicting machines, establishing the accurate tidal prediction still used today.

Key figures

Isaac Newton
Pierre-Simon Laplace
George Darwin
William Thomson

Seminal works

pughWoodworth2014
pinet2019

Frequently asked questions

Why are there usually two high tides a day?: The tide-generating forces raise bulges of water on both the side of Earth facing the Moon and the opposite side, so as the planet rotates most coasts pass through two high and two low tides each day.
What causes spring and neap tides?: When the Sun and Moon align, their tidal forces add to give large spring tides; when they are at right angles, the forces partly cancel to give smaller neap tides, cycling about every two weeks.