Ambient Weather TIDECLOCK-21 5.5" Nautical Brass Quartz Tide Clock Tides are due to the gravitational pull of the sun and the moon on the earth's surface. Most areas of the world have semi-diurnal tides, a high tide approximately every 12 hours 25 minutes. This Ambient Weather tide clock utilizes a battery-operated movement that keeps lunar time relating to the movement of the tides-12 hours 25 minutes. An easy-to-read dial indicates time to high and low tides, as well as to the ebbing and flooding half tides. Set this clock to wherever you want to track the average tide cycle-for example, your favorite beach, harbor, or fishing area. Tide Clock Limitations: This tide schedule is geared to tides for the east coast of North America. They will not accurately depict tides for the west coast.