The model disregards friction, inertia, and the irregular distribution of the land masses of the Earth. The theoretical tide formed under these conditions is known as the equilibrium tide. Also known as gravitational tide. The celestial equator. The vernal equinox is the point where the Sun crosses the Equator from south to north and it occurs about March 21.
Celestial longitude is reckoned eastward from the vernal equinox.
The autumnal equinox is the point where the Sun crosses the Equator from north to south and it occurs about September 23. Same as geopotential surface. Average high water interval on days of the new and full Moon. This interval is also sometimes called the common or vulgar establishment to distinguish it from the corrected establishment, the latter being the mean of all the high water intervals. The latter is usually 10 to 15 minutes less than the common establishment.