Because they depend on many factors including the geometry of your local coastline, and exactly where the Sun and Moon are located. Also, like the surface of a vibrating drum, the world oceans have vibratory modes that get stimulated in changing ways from minute to minute. Finally, there are storms at sea and elsewhere which move large quantities of water.
Detailed forecasts are available for high and low tides in all sea ports.
Copyright 1997 Dr. Sten Odenwald Return to Ask the Astronomer .