Tide is a term applied to the alternate rising and falling of the sea, twice in each lunar day, to the attraction of the moon and the sun. The movement is most marked on shores which shelve gradually. The average interval between successive high tides is twelve hours and twenty-five minutes, half the time between successive passages of the moon across a given meridian. The height of the tide varies rhythmically. The highest, or spring, tides gradually change to the lowest, or neap, tides. The interval between successive spring tides is half a lunar month. Usually spring tides occur at or near the time when the moon is new or full, and neap tides when the moon is in the first or third quarter. This fact leading to an early realisation of the moon being the main cause of the tide.
The mass of the moon attracts the oceanic waters, which, being fluid, make a little peak pointing directly from the earth's centre to the centre of the moon; this peak is held on the line of centres while the earth rotates beneath it. To observers on the earth the peak of water appears to move. At the antipodes of this peak, on the side of the earth remote from the moon, a second peak occurs, because the distant water is again attracted to the line of centers. These peaks are the successive high tides.
The lunar attraction is coupled with a similar attraction due to the sun, but only of about just under one half the magnitude. When the line of centers of the earth and the moon approximates to the line of centers of the earth and the sun, i.e. at full and new moon, the combined solar and lunar tides produce the spring tides. When the two lines of centers are at right angles, at the first and third quarters, the solar attraction reduces the lunar effect, and produces the neap tides. The neap tides are, on average, 5:13 of the spring tides.
The above explanation of the outstanding tidal features merely explains the phenomena as they would occur upon an earth with a uniform film of water and without land mass. The interposition of the great land masses, and the differences in the oceanic depths, causes variations in the tides along the coasts. Research Tide
Abbreviations
General Information
Research Tide