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Geology LibreTexts

8.2: Tides

Tides have been studied and tracked for thousands of years since they are important to the livelihood of both merchants and fishermen.

The Moon & and the Sun

The force driving tides is a combination of the gravity fields of the sun and the moon, with water motion influenced then by the Coriolis effect from the Earth’s rotation. The gravitation influence of the moon is larger than the sun, since the moon is so (relatively) close to the Earth. 

While the moon is the main reason for tide occurrence, the sun also plays a vital role. The sun is responsible for causing high-high tide (HHT) during new moons and full moons. Meanwhile, during ¼ moons, the sun causes lower-high tide (LHT).

There are three classifications of tides;

  • Diurnal: One high tide and one low, per 24 hour
  • Semidiurnal: Two high and two low tides per 24 hours, slightly different in level.
  • Mixed Diurnal: Two high and two low tides per 24 hours, large difference in level. The result is what is referred to as High High Tide (HHT) and Low High Tide (LHT) as well as High Low Tide (HLT) and Low Low Tide (LLT). 

Within the classifications are further extremes. Spring tides are extreme high and low tides and happen when the Earth, moon and sun are all in alignment and the gravitational pull from the sun and the moon are added together. King tides are extreme high spring tides, and occur only a few times a year.

Amphidromic points or nodes add complexity that is needed for tides to function properly, or as we know them to function. This is the point in water that has no tidal influence, as the distance from this point increases, the tidal range (amplitude) also increases. These amphidromic points occur due to the Coriolis effect and the interference within ocean basins or bays that cause a wave pattern called an amphidromic system, which means the tidal crests rotates around this point. or (diagram 1)

Equilibrium Theory of Tides

Isaac Newton first conceived the Equilibrium Theory of Tides, also called “static” tides. This is the theory that a hypothetical global ocean exists at unchanging equilibrium with the tide producing forces. The Equilibrium Theory of Tides is highly idealized, even to the point of altering the globe’s shape to better align with the celestial body  (sun and moon), and the theory ignores the continents. (diagram 1)

Tides and Sea Level Rise

The California King Tides Project uses this opportunity to envision Sea Level Rise by the help of citizen scientists who photograph areas where the high tide causes flooding. Neap tides are unusually low tides and occur when the sun and the moon are at 90 degrees to each other and the gravitational pull from each is partially canceled.