The periodic rises and falls of the level of the sea constitute what are known as tides. The rise or fall does not occur simultaneously over the surface of an ocean or sea, but appears to radiate outwards some central part in the ocean or sea, as does the ripple caused by a stone thrown into a pond.
The periodic rises and falls of tides are fairly regular, and therefore the height of the tide at any particular time and place can be predicted with a reasonable degree of accuracy.
Causes of tides
The Earth and the Moon rotates about a common centre of gravity in the state of equilibrium: in other words, the speed is always exactly sufficient to maintain the Moon in its orbit around the Earth and to counteract the gravitational force is drawing the two masses together.
The gravitational forces act on the Earth as a whole, on the waters on or near the surface, and on the atmosphere. Since the gravitational force varies with the square of the distance, there will be a greater force acting upon the waters that are closer to the Moon than upon the Earth, and also a greater force acting upon the Earth than upon the waters are further from the Moon.
Therefore there will be a raising of the water immediately under the Moon and a similar "raising" on the other side of the Earth, where the waters tend to be left behind. A similar, but smaller, tide-raising force is produced by the rotation of the Earth around the Sun.
But for the effects the shapes of the of the oceans, the position of islands therein and the shape of the land masses, the rise and fall of the tides would closely follow the movement of the Moon round the Earth, varied to some extent in accordance with the positions of the Moon, Earth and Sun in relation to each other.
In fact, the manner in which the tides rise and fall in different seas and oceans varies considerably; the period of oscillation of the tide in one area differs from that in another and varies from about 6 to 24 hours.
Oscillation of about 24 hours, called a diurnal (or daily) tide, result in both one high water and one low water a day; one of about 12 hours.
A semi-diurnal (or half daily) tide would result in two high waters or two low waters a day (most of the UK)
A six hour period would result in four high and four low waters a day.
(The term ' tide ' includes a complete cycle of one high water and the succeeding low water. It is therefore incorrect to refer to the state of a particular tide as ' high tide ' or ' low tide ' , when what is really meant is high water or low water. The description " High Tide" would compare the height of that particular tide with that of some previous one.)
Adapted from the Admiralty Manual of Seamanship, Vol II, 1951