10.10: Chapter Summary

Tide-Generating Force.

Bodies orbiting together do so around their common center of mass. A centripetal force acting toward the other body is needed to keep each body in orbit. The gravitational attraction between the bodies supplies this force. The centripetal force needed to maintain the Earth in its common orbit with the moon (or sun) is the same at all points within the Earth. However, the gravitational force of the moon (or sun) is inversely proportional to the distance from the moon’s (or sun’s) center of mass. The gravitational force exerted by the moon (or sun) is therefore slightly higher on the side of the Earth facing the moon (or sun) than on the other side. Hence, there is a slight excess of gravitational force over centripetal force at the Earth’s surface facing the moon (or sun), and a slight excess of centripetal force over gravitational force at the opposite side of the Earth. The slight imbalances are the tide-generating forces. Tide-generating forces exerted by the sun are only 46% as strong as those exerted by the moon because even though the sun is more massive than the moon, it is much farther away from the Earth than the moon is.

Characteristics of Tides.

Tides resemble patterns of oscillating sea surface elevation caused by simple progressive waves or by the addition of two or more progressive waves. Because the moon’s orbit is inclined to the equator, tides are usually predominantly one of three types. Diurnal tides have one high and one low tide per tidal day (24 h 49 min). Semidiurnal tides have two high and two low tides each tidal day, and the two lows (and two highs) are of equal height. Mixed tides have two high and two low tides each tidal day, but the two lows (and two highs) are not of equal height. Tidal range varies with location and from day to day within a lunar month. Spring tides (highest tidal range) and neap tides (lowest tidal range) occur twice each lunar month. Spring tides occur when the moon and sun are both on the same side of the Earth (new moon) or on opposite sides (full moon), so their tidal pulls are additive. Neap tides occur when the moon and sun are 90° apart (first and third quarters). Tidal range also varies from month to month. Tides are composed of numerous components with different periods, each caused by a regular periodic change in the Earth–moon and Earth–sun orbits, such as distance or declination.

Tide Waves.

Tide waves appear to progress from east to west as the Earth spins under the tide bulges. Continents interrupt the tides everywhere except around Antarctica and cause tide waves to be dispersed, refracted, and reflected in complex ways that affect tidal characteristics differently at different locations. Because its wavelength is so long, a tide wave is a shallow-water wave that is refracted by seafloor topography. The tide wave is a forced wave because it is too slow, even in deep oceans, to match the orbital velocity of the Earth’s surface as it spins, except near the poles. The Coriolis effect deflects tide waves. In ocean basins of suitable dimensions, this deflection can create tuned-oscillation standing waves called amphidromic systems. The crest of one tidal component enters the basin and passes counterclockwise (Northern Hemisphere) or clockwise (Southern Hemisphere) around the basin to arrive back at the entrance exactly when another crest arrives.

Tidal Currents.

Orbits within tide waves are so compressed, particularly in coastal and estuarine regions, that vertical tidal motion is very small in relation to the horizontal motions. The horizontal motions are tidal currents. Open-ocean tidal currents are weak, and their direction progresses in a rotary pattern during the tidal cycle. In coastal and estuarine areas, tidal currents generally reverse direction 180° as flood and ebb currents during the tidal cycle. Tides in such areas are usually a combination of progressive and standing waves, and the relationship between high or low tide and times of  slack water or maximum currents is different at each location. Tidal currents can be very strong, tidal range particularly high, and the progression of the tide wave complicated in estuaries where standing waves occur for some tidal components.

Tide Predictions.

Tides are too complex to be predicted without measurement at each location for which predictions are needed. Tidal-height measurements may now be made with satellite sensors. Tidal-height measurements recorded for several months or longer can be subjected to harmonic analysis. This analysis can accurately predict the tidal range and times of high and low tides at the studied location. Tidal currents are more difficult to predict.

Energy from Tides.

Electricity is generated from tidal energy in a few locations where turbines and dams have been placed across a bay or estuary. However, very few locations have a sufficiently large tidal range to make such projects feasible. Suitable locations have a unique ecology that depends on those tides.