A tide is the alternate rising and falling of the sea, usually twice in each lunar day at a particular place, due to the attraction of the moon and sun. Tides are important for both surfers, boaters, and fishermen alike, and in the case of the grunion, important for their spawning.
What influences tides and how are they affected by the sun and moon? A great explanation comes from the “The Amazing Grunion“, a Marine Resources Leaflet No. 3 from the State of California Department of Fish and Game, written by Jerome D. Spratt from the Marine Resources Region and published in 1971. Diurnal cycles refers to the daily tidal rhythms.
Influence of Celestial Bodies
Tides are caused by forces exerted on the earth by celestial bodies in direct proportion to their mass. Theoretically all celestial bodies affect the tides but realistically only the sun and moon need be considered.
Since the sun has 26 million times the mass of the moon, one might expect it to be the dominant tide producing force. However, the fore exerted by a celestial body decreases rapidly as its distance from the earth increases (inversely proportional to the cube of the distance). consequently, the sun, being almost 400 times farther from earth than the moon, exerts less than half as much force as the moon.
Spring and Neap Tides
Tidal highs and lows vary according to the relative positions of the sun, earth, and moon. Highest and lowest tides occur when the sun, earth, and moon are most in line, such as during full moon (sun and moon on opposite sides of the earth) and new moon (sun and moon on the same side of the earth). These tides are know as “spring” tides. The tides occurring during the first and last quarters of the moon, when the sun and moon are least in line, are know as “neap” tides and are intermediate in range.
Diurnal, Semidiunal, and Mixed Tides
A diurnal tidal cycle happens if there is one high and one low tide every lunar day. A semidiurnal tidal cycle happens if there are two high and two low tides of about the same size every lunar day. A mixed tidal cycle, or a mixed semidiurnal tidal cycle, happens if there are two high and two low tides of different sizes every lunar day.
“NOAA’s National Ocean Service: Tide Cycle Variations.” NOAA Ocean Service Education. National Ocean Service, 25 Mar. 2008. Web. 13 Mar. 2014.
Spratt, Jerome D. The Amazing Grunion. Vol. 3. Sacramento: State of California; Resources Agency, 1971. Print. Marine Resources Leaflet.