Ocean tides are moved by gravity.
There are several forces of gravity that cause ocean tides. Tides are periodic rises and falls of bodies of water. These forces of gravity balance out, on average, but the mismatches at the Earth's surface cause a horizontal force toward the points closest to and farthest from the moon. These are called the lunar tide-generating force and the solar tide-generating force. To an observer on the Earth, the tides appear to move east to west around the Earth each day.
Solar and Lunar Tides
The solar or diurnal tide lasts 12 hours and the lunar or semidiurnal tide runs 12 hours and 42 minutes. The lunar period is longer because the moon revolves around the Earth every 27 days. Ocean tides at any time consist of a mixture of solar and lunar tides. The spinning of the Earth on its axis and the Earth's revolution around the sun further weaken the diurnal or solar tides. The largest lunar tide in the world can be found in the Bay of Fundy, where it couples with the Gulf of Maine. The maximum high tide is about 39 to 49 feet.
Gravity
Gravity, or a force that attracts, is what causes the tides to react to the sun and moon. The mass of an object and the object's proximity to another object determine the gravitational pull that the two objects have on each other and any other object in the same proximity. These forces are what affect the tidal forces, as Sir Isaac Newton explained in 1687. Concerning the sun and moon, however, the proximity of the object plays a bigger role than its mass.
Shape of Shoreline
Tides also can be influenced by the shape of the shoreline, according to the National Oceanic and Atmospheric Administration (NOAA). Tides are impacted when tidal bulges hit a continental border, which increases the magnitude of the waves making them much higher. Islands that are not near a continental border have small tides, sometimes about 3 feet or less. The shape of the bay also can increase the size of tides. Funnel-shaped bays, such as the Bay of Fundy, have the most dramatic effect on the size of the waves.
Wind and Weather
NOAA points out that wind and weather also can cause tides to vary. Strong winds offshore can push the water away from the coastlines and make low tides lower while onshore winds can pile water onto the shoreline. Also, high-pressure weather systems depress sea levels and cause exceptionally low tides while low-pressure situations cause tides to become much higher.
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