Diurnal tide – once a day

Semi-diurnal – twice a day (two high and two low sea levels per day)

7.1. The forces that generate the tides are:

a. The gravity force of the moon. This force moves the water in the oceans toward the moon. Thus, the water retries from some places and concentrates opposite to the moon. Thus, some places experience low tide, water has moved away, while other places experience high tide

b. The centrifugal forces from the rotation of the Moon-Earth system. Moon and earth are rotating together around the barycenter, which is the center of gravity of the Earth-Moon system. Since the earth is much larger than the moon the barycenter is very close to the center of the earth and many times we think that the moon is rotating around the earth, however both the noon and the earth are rotating around the barycenter. This rotation introduces a centrifugal force that tents to move the water in the oceans of the surface of the earth. Of cource, gravity is much stronger and does not let the water leave from the earth. Nevertheless the centrifugal forces moves the water as shown in the graph, from the places with small centrifugal force to the places with larger centrifugal force.

Similar forces also exist from the sun.

c. The centrifugal force from the rotation of the Sun and the Earth as a system

d. The gravitational (tractive force) from the sun

From the figures it is obvious that two bulges are formed because the tractive and centrifugal forces act on opposite directions. To make it simpler, if the moon were positioned in the plane of the equator and there were no continents this balance would create two bulges, one under the Moon and one on the opposite side of the earth and there would always be low tide in the poles and high tide close to the equator.

7.2. Diurnal tide – Explanation

A point P at the rotating surface of the earth will experience a varying water depth with two maximum (two high tides). The first when it is in the moon’s side and one when it is in the opposite side. Also tow low tides as it passes through the belt of the low water. This is a semi-diurnal tide M2. If P is located at a high latitude such that the low water belt (center line of the belt) is not crossed by point P as it rotates, then the water depth will encounter a single maximum (high tide) and a single minimum (low tide) during the day. This is a diurnal tide M1.
The same types of tides occur due to the sun S1 S2

7.3. Spring tide – Neap tide

The sides generated by the Sun and Moon coincide of counteract depending on the position of the moon relative to the sun. When the Sun and the Moon are aligned (twice a month, either in the same or opposite sides) the amplitude of the total tide is at a maximum. That is a spring tide. When the position of the moon is perpendicular to the Sun, the tidal amplitude is minimum (neap tide)