1.6 The Tides

“The winds, the sea, and the moving tides are what they are. If there is wonder and beauty and majesty in them, science will discover these qualities… If there is poetry in my book about the sea, it is not because I deliberately put it there, but because no one could write truthfully about the sea and leave out the poetry.” Rachel Carson

Tides are periodic variations in the sea level generated by the gravitational attraction that the celestial bodies Earth, Moon and Sun exert on each other (FIG1_SES1.6).

FIG1_SES1.6 High and low tides.
By W3C® The source code of this SVG is valid. This vector image was created with Inkscape. – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15500578

The rhythmic variation of the sea level, with a movement both vertical and horizontal, is indicated as a “tide” and is determined by the gravitational attraction between the Earth and the Moon and by the centrifugal effect of the rotation of the Earth-Moon system; also the solar attraction influences but in a lesser way. The Moon rotates around the Earth every 24 hours and 50 minutes and this 50-minute gap with respect to the Earth’s rotation time is the so-called “tide period” that passes between the minimum and maximum tides.

We can say that when the sea rises and covers the beach, there is high tide and usually the greatest height occurs when there is a full moon and a new moon; when the sea is very far behind the beach, instead, there is low tide.

For example:
low tide 5:07 a.m
hight tide 10:30 a.m
low tide 17:21 p.m
hight tide 22:48 p.m

The widest tides known are those of the Bay of Fundy in Nova Scotia (FIG2_SES1.6). These tides can have 15 m difference in level. In the Tyrrhenian Sea the tide excursions are about few cm, while in the Adriatic Sea can be more than 1 m.

FIG2_SES1.6 Bay of Fundy.
By Tttrung – Created by the uploader, from two separated GFDL photographs (en:Image:Bay_of_Fundy_Low_Tide.jpg and en:Image:Bay_of_Fundy_High_Tide.jpg) taken in 1972 by Samuel Wantman., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=451805

The small variations in sea level near the coast can affect the life of marine anemones, for example, which are attached to the rocks (FIG3_SES1.6); these animals have evolved solutions for the short lack of water during the low tide, they close themself and keep inside them the water necessary for survival.

FIG3_SES1.6 Actinia equina.
By Esculapio – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3990652

Tidal movements can give rise to strong currents, which, thanks to the transport of food and sexual products, larvae and spores, favor the development of sessile and filtering benthic organisms.