Tides

Introduction

Tides are the variation of sea level at coastal locations. This variation is caused by periodic vertical movements— alternating rising and falling of sea level—of ocean waters, such as those within the open sea, bays, and gulfs, with respect to land masses. More specifically, tides result from the distortion in the shape of Earth (specifically, its large water bodies) caused by the force of gravity exerted by the Moon and, to a lesser extent, by the sun, and the centrifugal forces produced by the revolution of the Moon-Earth system and, to a lesser extent, by the revolution of the Sun-Earth system around their common centers of gravity.

Tides, to a smaller degree, are also found in large lakes, such as the Great Lakes of North America, and other bodies of waters, such as rivers, that are raised and lowered by ocean waters. The highest tides on Earth occur at the Bay of Fundy in Nova Scotia, Canada. The range of tides there can exceed 40 ft (12 m).

Historical Background and Scientific Foundations

Tides are produced due to the force of gravity exerted by the Moon and, to a lesser degree, by the sun onto Earth,

and centrifugal forces produced by the revolution of the Earth and Moon (and Earth and sun) around their common centers of gravity. These two forces are balanced (zero) at the center of mass of Earth. However, at points on the surface, these forces are not zero, which is what produces tides.

The centrifugal force always has a constant magnitude (size) and is directed away from the center of revolution of two bodies. Consequently, the centrifugal force is continuously directed away from the Moon in the Moon-Earth system, and away from the sun in the sun-Earth system.

The gravitational force produced by the Moon or sun is of a different magnitude at different points on Earth. This variation occurs because the magnitude of gravity changes with distance between bodies based on the Universal Law of Gravity developed by British physicist Isaac Newton (1642–1727). It states that gravity is much weaker the farther away two massive bodies are to each other.

When ocean waters face the Moon, the magnitude of Moon's gravitational force on these waters is larger than for the waters farthest away because the near-waters are about 8,000 mi (12,870 km) closer to the Moon— the approximate diameter of Earth. These near-waters have a large amount of gravitational force pulling at them but with a smaller centrifugal force in the opposite direction. The subtraction of the two forces still produces a positive tide-raising force.

Conversely, when waters are facing away from the Moon, the gravitational force is smaller than felt by the waters on the near side, but the centrifugal force—which is still of the same magnitude—is in the same direction as the gravitational force. Thus, the two forces are added together to, again, produce another tide-raising force.

When a particular side of Earth is facing the Moon, its water has a tendency to be pulled toward it. This action produces high tides—also called direct tides. The water away from the Moon, because it feels less gravity, tends to move away from the lunar body, rather than to rotate with Earth. This action also produces high tides—called opposite tides. The water that is perpendicular from these points, conversely, experiences low tides. The cycle of two high tides and two low tides is called a lunar day, which lasts about 24 hours, 50 minutes.

The sun also produces tides on Earth. However, because it is so far away—approximately 93 million mi (150 million km)—its gravitational force on tides is less than the gravitational force from the Moon, which is only about 238,000 mi (383,000 km) away.

WORDS TO KNOW

CENTRIFUGAL FORCE: The inertial reaction that causes a body to move away from a center about which it revolves.

ECLIPSE: The partial or total apparent darkening of the sun when the moon comes between the sun and Earth (solar eclipse), or the darkening of the moon when the full moon is in Earth's shadow (lunar eclipse).

GRAVITATIONAL FORCE: The mutual force of attraction that exists between all particles or bodies with mass. A force dependent upon mass and the distance between objects. The English physicist and mathematician Sir Isaac Newton (1642–1727) set out the classical theory of gravity in his Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). According to classical theory, gravitational force, always attractive between two objects, increases directly and proportionately with mass of the objects but is inversely proportional to the square of the distance between the objects.

LAW OF GRAVITY: Mathematical statement of the force of attraction between all material objects. The law of gravity states that the force pulling two objects together is proportional to the product of their masses divided by the square of the distance between them. Gravity keeps Earth's atmosphere from drifting away and is a crucial force in all circulations of wind and water.

LUNAR DAY: The length of time between one sunrise on the surface of Earth's moon and the next: the length of a day on the moon. A lunar day is approximately 28 days long; that is, the moon rotates once on its axis in the time it takes it to circle Earth, which means that one face of the moon is always kept facing Earth.

TSUNAMI: Ocean wave caused by a large displacement of mass under the surface of the water, such as an earthquake or volcanic eruption.

Impacts and Issues

The relative positions of these three bodies (Earth, Moon, and sun) do not change appreciably over time, so the variation of tides based on their interactions is fairly regular. However, scientists have found that the 1,500-year climate cycle on Earth could be affected by tides. Certain alignments of the Moon, sun, and Earth— such as during an eclipse—make for strong tides. Thus, the warming and cooling patterns experienced on Earth could be affected by these strong tides.

Tides are not caused only by the gravitational tugging from the Moon and sun, but also by physical processes within Earth. Earthquakes, hurricanes, undersea volcanic eruptions, and underwater landslides can cause what are called tidal waves or tsunamis, which can cause severe damage to shorelines and loss of life to coastal residents.

In addition, the circulation of the atmosphere creates weather phenomena that dramatically change the amplitude (height) of tides. Such phenomena include hurricanes and cyclones.

Scientists estimate that sea level is roughly 4 in (102 mm) higher than it was about one hundred years ago. Recent satellite measurements show sea level is going up about .08 in (2 mm) each year. As sea levels continue to rise, larger tides are created that potentially could cause more destruction to coastal areas and displacement, injury, and death to people living and working nearby.

Scientists are not sure whether rising sea levels are related to global warming or just a part of normal weather fluctuations. Either way, rising sea levels and increased tides have created adverse problems for coastal areas.

On the positive side, tides can be used to produce electricity, which is considered an alternative energy source to petroleum. To harness electricity from their motion, tides are allowed to flow through a dam that drives a turbine (a machine with rotating blades) to produce electricity. After flowing through, the water is trapped. After the tides on the other side retreat, the trapped water is then released. This action causes it to reverse direction back through the dam, again driving the turbines to produce electricity. The first tide-producing electrical power plant in the world is the 2,500-ft (750-m) Rance tidal power plant, which is in operation on the Rance River in northwestern France.

See Also Coastlines, Changing; Hurricanes; Ocean Circulation and Currents; Oceans and Seas; Sea Level Rise; Sea Temperatures and Storm Intensity.

BIBLIOGRAPHY

Books

Cartwright, David Edgar. Tides: A Scientific History. Cambridge and New York: Cambridge University Press, 2000.

McCully, James Greig. Beyond the Moon: A Conversational Common Sense Guide to Understanding the Tides. Hackensack, NJ: World Scientific, 2006.

Pugh, David T. Changing Sea Levels: Effects of Tides, Weather, and Climate. Cambridge and New York: Cambridge University Press, 2004.

Web Sites

“It's High Tide for Ice Age Climate Change.” Science News, April 5, 2000. <http://www.sciencenews.org/pages/pdfs/data/2000/157-16/15716-08.pdf> (accessed November 9, 2007).

William Arthur Atkins