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Forces That Result in Daily Tides

Anyone who has spent a day at the beach has observed the differences between daily high and low tides. This rise and fall of the sea level is caused by tide-generating forces (TGFs). These TGFs are the result of the gravitational attraction between the Earth, the moon and the sun, the centrifugal forces associated with their orbits, and several other forces and factors. Although our tidal system appears simple at first glance, all these tide-generating forces and factors combine to form a very complex, dynamic system.

Lunar Forces
- The primary cause of oceanic tides is the gravitational pull of the moon. The daily tidal pattern is affected by both the orbital period of the moon and the spin of the Earth. As the Earth completes one 24-hour rotation, the moon also moves in its 28-day lunar orbit. This creates a lunar tide period that is slightly longer than 24 hours--50 minutes longer to be exact. During this period, a location will experience two high tides and two low tides, each high tide arriving approximately 12 hours and 25 minutes after the last high tide.
Solar Forces
- Just as the moon exerts a gravitational pull on the oceans, the sun, to a lesser degree, does the same. Because the Earth's relative motion around the sun is so slow, the solar tide period is 12 hours. Additionally, the position of the sun and moon relative to the Earth impacts oceanic tides. When these bodies are in syzygy, aligned in a straight line with the Earth, their gravitational forces combine to produce the highest tides, called spring tides. When these bodies are perpendicular to one another, their gravitational forces work against each other, producing the lowest tides, called neap tides.
Centrifugal Forces
- In addition to the gravitational forces that affect the tides, TGFs also include the centrifugal forces created by the orbit of the Earth around the sun. For an object in orbit, this force is equal to the gravitational force, but opposite in direction. There is also a centrifugal force in play between the Earth and moon. All of these centrifugal forces impact the oceanic tides, though to a lesser degree than the gravitational effects of the moon.
Other Forces and Factors
- The Earth's spin also creates a centrifugal force, strongest at the equator and weakest at the poles. The oceans are fluid and exhibit momentum, another force that impacts tides. Geographical areas that work to funnel this momentum create greater tidal ranges. Ocean currents provide a force that affects oceanic tides. When incoming high tides coincide with ocean currents, their forces will combine, creating higher tidal rises. The Bay of Fundy is an example of a geographic location that combines all these factors, producing an extreme range between high and low tides. Finally, there are other minimum forces, such as the gravitational pull of other planets, and variables, such as the minute changes in the distances of elliptical orbits, that also factor into the overall tidal equation on Earth.

Nature