Tides

Lunar Tides The moon, being much nearer to the earth than the sun, is the principal cause of tides. When the moon is directly over a given point on the surface of the earth, it exerts a powerful pull on the water, which therefore rises above its normal level. Water covering the portion of the earth farthest from the moon is also subject to this pull, so that another distinct dome of water is formed …

…water at that point is called the lunitidal interval, or the high-water interval for that point. The low-water interval is the period between the time the moon crosses the meridian and the next low water. Average values for the high-water lunitidal intervals during periods of new and full moon are known as the establishment of the port. Values for the intervals during other periods of the month are often referred to as the corrected establishment.