An idealised tide calculated as though the planet were covered by deep water that adjusts instantly to the Moon and Sun. It predicts the correct periods and the astronomical timing of each constituent, which is why it supplies the reference phase for harmonic analysis. It badly predicts real heights and times, because continents, basin resonance, friction and the Earth's rotation reshape the actual ocean response.
Phase lags are measured from the equilibrium tide, so a kappa of 230 means local M2 runs well behind the Moon.
Also heard as
- equilibrium theory
Related terms
Tide-generating force
The small difference between the Moon's or Sun's pull at a point on Earth and its pull at Earth's centre.
Harmonic constituent
One of the regular cosine waves, each with a period fixed by astronomy, that together make up the tide.
Phase lag
How far a constituent's local high water trails the corresponding astronomical forcing, given in degrees.
Amphidromic point
A point in the sea where a tidal constituent has zero range and around which its high water rotates.