Q: What causes the rhythmic ebb and flow of the tides?

A: The tides are an effect of gravitation. The moon is the primary cause as it pulls on the waters of the oceans and holds them in lockstep while the earth rotates under them. The tidal force is due to the difference in distance between the moon and the opposite sides of the earth. The pull of gravity depends on distance and one side of the earth is closer to the moon than the other side. As a result the water is pulled away from the earth on one side and the earth is pulled away from the water on the other side. This results in two small tidal bulges about one foot high which line up with the moon, one on each side of the earth. As the earth rotates under them a high tide occurs directly under the bulges on both sides of the earth. The tides would be very regular and barely noticeable if this was the entire story, but the moon is not the only thing involved. For one thing, as the earth rotates it drags the bulges just slightly ahead of the moon causing the high and low tides to occur before the moon is overhead. Another factor is the sun. Its tidal force is about half of the moon's so that when the sun and moon are aligned their pulls add up to cause higher tides and a greater tidal range. At other times the sun and moon are pulling in different directions and produce a smaller tidal range. The tidal bulges are also modified by geography. The ocean basins and the coastline act together to magnify the slight bulges into tidal ranges as great as fifty feet in some places and to alter their arrival times at different shores. After being constricted between Africa and Antarctica, when the tidal bulge is released into the Atlantic Ocean it spreads northward in long, low ripples like water sloshing in a flat tray. A similar effect occurs in the Pacific Ocean but with a different result because of its greater width. Thus, the actual tides observed at a particular place depend on where it is in the ocean and the shape of the coastline and adjacent sea floor. The declination of the moon and sun north or south of equator also has an effect as does the periodically changing distance between the earth, sun and moon. The astronomical factors repeat in an eighteen year cycle, and so do the tides.

Richard Brill is assistant professor of science at Honolulu Community College where he teaches earth and physical science. Send questions to him at Honolulu Community College, 874 Dillingham Blvd., Honolulu, HI 96817 or email to rickb@hcc.hawaii.edu

The Tides ©1995 Richard C. Brill