Ocean Tides Background

Astronomical tides are the regular rise and fall of the sea surface caused by the gravitational attraction between the rotating Earth and the moon and sun. Tides can be thought of as progressive planetary-scale waves that propagate across ocean basins; wave crests are high tides and wave troughs are low tides. A tide wave's length is considerably greater than the depth of the ocean so it behaves as a shallow-water wave.

In the theoretical case of an ocean covered Earth of infinite depth with no continents, tides can be visualized as waves having lengths of one half of the circumference of the planet. Astronomical tides are forced waves in that they always follow the driving force of the moon and sun. Tide crests would be located directly below the celestial body (moon or sun) that is responsible for the tide-generating force. On our theoretical Earth, the speed of propagation of the tide crest depends, at least in part, on the rotation of the planet relative to the sun or moon. On the equator where the planet's circumference is about 38,700 km (24,000 mi), the tide crest would travel at about 1600 km (1000 mi) per hr. On the real Earth, continents break up the ocean into separate basins and the ocean has a finite depth. Tides are shallow water waves so that wave celerity depends on water depth. For an average ocean depth of 4000 m (13,000 ft), the tidal celerity is about 200 in per sec (444 mi per hr). Tides speed up where the ocean is relatively deep and slow over ridges where the ocean is shallower. At sea, the am­plitude of the tide wave is well under a meter but increases as it enters shallower coastal waters. Tides are measured mostly at coastal locations as local changes in sea level through time. The vertical difference in height, between water levels at high and low tides, is called the tidal range and generally varies between less than a meter to several meters. The time between successive high tides is the tidal period.