Astronomical tides are observable as the periodic rising and falling of the surface of major water bodies on the earth. Tides are produced in response to the gravitational attraction of the sun, moon, and (to a considerably smaller extent) all other celestial bodies.
Because of its relative closeness to the earth, the moon induces the strongest effect on the tides. Tidal currents are produced in response to differences in the water surface elevation.
Because the earth is not covered by a uniform body of water, tidal ranges and periods vary from place to place and are dependent upon the natural period of oscillation for each water basin. Tidal periods are characterized as diurnal (one high and one low per day), semidiurnal (two highs and two lows per day), and mixed (two highs and two lows with unequal heights).
In the coastal zone, variations in topography, depth, seafloor sediment type, and lateral boundaries also affect the tide.