Astronomical Tide

The astronomical tide is the periodic rise and fall of sea level produced solely by the gravitational attraction of the Moon and Sun on a rotating Earth, excluding wind, pressure, and other meteorological effects. It is the part of the water level that harmonic analysis reproduces from a sum of constituents such as M2 (12.4206 h), S2 (12.000 h), K1 (23.9345 h), and O1 (25.8193 h), each at a frequency fixed by lunar and solar orbital mechanics. Tide tables predict the astronomical tide; the difference between it and the observed level is the meteorological residual or surge that navigators must add for under-keel clearance.