the tides

These are periodic movements of the water level caused by the attraction of the moon and the sun. next to the oscillations, others of maximum or minimum magnitude are produced at intervals of about 14 days. they are due to the joint action of both stars, when they are in conjunction, or to the difference of their respective effects, when they are in position.

These movements are especially intense in some places, such as on the coasts of the North Sea.
where the difference in water level between high tide and low tide reaches up to 13 m. in the closed seas, on the contrary, those differences are minimal and are reduced to a few centimeters this is the case of the Mediterranean.

the displacement of these quantities of liquid causes currents, sometimes of great intensity that have a great influence on the coastal traffic of small vessels. the minimum fact that the level goes up and down determines that access to ports in regions with high tides is limited to certain hours, unless channels are built to allow traffic at all times.