tide: one construct, many systems

In physical oceanography, the tide is the periodic rise and fall of the sea driven by the gravitational pull of the Moon and Sun, which acts unevenly across the Earth: because gravity weakens with distance, the near side is pulled more and the far side less than the planet's centre, raising two bulges that the Earth's rotation carries past any coast as high and low water. Doodson's 1921 The harmonic development of the tide-generating potential decomposed this forcing into the set of harmonic constituents whose beating also produces the fortnightly spring–neap cycle. The referent is the response to a differential gravitational field, the identical construct that, applied to other bodies, becomes astrophysical tides.[1]

Unit

The amplitude and phase of tidal constituents, and water height, in metres

Recovery means

Not applicable

Measured by

Resolved by harmonic analysis of tide-gauge and satellite-altimeter records into named constituents with amplitude and phase

In astrophysics, tides are the same differential-gravity effect at the scale of moons, stars, and planets: because a companion's pull varies across a body, it stretches that body along the line to the companion, and when the stretch changes over an eccentric orbit the repeated flexing dissipates energy as heat. Peale, Cassen, and Reynolds's 1979 Melting of Io by tidal dissipation predicted, just before the Voyager flyby, that such tidal heating should make Jupiter's moon Io intensely volcanic. It is the identical construct as the ocean tide, the response to a gravitational gradient across a body, realised here as orbital flexing and internal heating rather than a rising sea.[2]

Unit

The tidal heating rate and the amplitude of the tidal bulge or flexing

Recovery means

Not applicable

Measured by

Estimated from orbital eccentricity, the body's tidal dissipation factor and rigidity, and observed heat flow or volcanic activity