Tide

Tide, periodic rise and fall of land and water on the earth. Tidal motions are primarily exhibited by water: the motion of the land is barely detectable. As the earth-moon system rotates about its center of gravity, which is within the earth, the earth bulges in the direction of the moon, as well as in exactly the opposite direction owing to the moon's gravitational attraction and the centrifugal forces resulting from the system's revolution. Since the moon orbits the earth in the same direction that the earth rotates, the bulge “travels” round the earth each lunar day (24.83 hr); hence most points on the earth have a high tide every 12.42 hr. The sun produces a similar though smaller tidal effect. Exceptionally high high tides occur at full and new moon (spring tides), particularly if the moon is at perigee; exceptionally low high tides (neap tides), at first and third quarter. The friction of the tides causes the day to lengthen 0.001 sec per century.