
Tides and Water Levels

Frequency of Tides  The Lunar Day 
Most coastal areas, with some exceptions, experience two high tides and two low tides every lunar day (Ross, D.A., 1995). Almost everyone is familiar with the concept of a 24hour solar day, which is the time that it takes for a specific site on the Earth to rotate from an exact point under the sun to the same point under the sun. Similarly, a lunar day is the time it takes for a specific site on the Earth to rotate from an exact point under the moon to the same point under the moon. Unlike a solar day, however, a lunar day is 24 hours and 50 minutes. The lunar day is 50 minutes longer than a solar day because the moon revolves around the Earth in the same direction that the Earth rotates around its axis. So, it takes the Earth an extra 50 minutes to “catch up” to the moon (Sumich, J.L., 1996; Thurman, H.V., 1994).
Because the Earth rotates through two tidal “bulges” every lunar day, coastal areas experience two high and two low tides every 24 hours and 50 minutes. High tides occur 12 hours and 25 minutes apart. It takes six hours and 12.5 minutes for the water at the shore to go from high to low, or from low to high.
