 |
| Tides |
Tides are the alternating rise and fall of bodies of water, relative to land. Each 24-hour period, there are two high tides and two low tides. The arrival times and heights of the tides change every day and follow a pattern over days, months, and seasons.
The shape of a coastline, water depth, shape of the seafloor (bathymetry), weather, and other local factors affect the heights and arrival times of tides at specific locations. The daily tides bring ocean nutrients that nourish brackish-water (slightly salty) plants and wildlife that live in tidal wetlands.
Explaining the tides
Humans in maritime (sea-going and coastal) societies have always recognized and measured the daily, monthly, and yearly pattern of water level rise and fall along coastlines. Navigation, construction, and fishing in coastal areas require precise knowledge of the local tides, and tide prediction is an ancient science. The ancient Hawaiian “moon calendar” charts the tides and relates them to fishing and agricultural harvests.
    |
John, Abbot of Wallingford, who died in 1213 supposedly authored the oldest European tide chart. One entry predicts the hours of high water at London Bridge (“flod at london brigge”) on the Thames River. The scientific explanations for how the tides work and why they occur are, however, relatively new discoveries.
Ancient Chinese and European philosophers theorized that Earth inhaled and exhaled water. Most ancient scientists, including Greek philosopher Aristotle (384–322 B.C.E.), were silent on the subject of tides. (Ancient Egyptians, Greeks, and Romans lived on the Mediterranean Sea, which has relatively insignificant tides.)
Isaac Newton’s theory of gravity is the basis for understanding tides. Isaac Newton (1642–1727), a seventeenth century English mathematician and physicist, theorized that all objects exert an attractive force, called gravity, on other objects. The strength of the gravitational pull between objects depends on their relative sizes and the distance between them.
Earth, a very large object, pulls smaller objects, like people or apples, strongly toward its center. (Newton’s theory of gravity was supposedly inspired by his observation of an apple falling from a tree.) Earth’s gravitational pull keeps the Moon in orbit around the planet. Newton’s ideas were later applied to an explanation of tides by French mathematician Pierre Simon Laplace (1749–1827), and Irish physicist William Thomson (1824–1907), who is also known as Lord Kelvin.
How tides work
 |
| How tides work |
The gravitational pull of the Moon and Sun on Earth’s oceans, inland seas, and large lakes causes tides. The Moon’s pull on the surface of the oceans as Earth spins on its axis causes two high tides and two low tides during each 24-hour day. To visualize the tides, imagine Earth as a ball completely covered with water.
Earth’s gravity holds the water on the planet’s surface. The Moon’s gravity pulls a bulge of water toward it. Another force due to the spinning of the Earth and called the centrifugal force also bulges water at the equator in an outward direction, much like a fast-spinning amusement ride pushes your body toward one side of your seat.
Centrifugal force causes a second bulge to form on the direct opposite side of Earth to balance the bulge facing the Moon. As Earth rotates on its axis over 24 hours, the bulges remain stationary with respect to the Moon. Every location on Earth experiences the passing of both bulges in the form of two high tides each day. The low water moments between the bulges cause two daily low tides.
The relative positions of the Earth, Moon, and Sun constantly shift. The Moon’s monthly circuit around Earth causes the tides to occur slightly later each day. If the Moon were stationary over the spinning Earth, the high tides would be exactly 12 hours apart, and tides would occur exactly every six hours. As it is, the first high tide of a 24-hour day happens about 50 minutes later than the previous day.
The gravitational pull of the Sun also affects the height of the tides. Solar (sun) tides are much weaker than lunar (moon) tides because the Sun, although much larger than the Moon, is much farther away from Earth. The relative positions of the Earth, Moon, and Sun constantly change during Earth’s year-long trip around the Sun. Very high and very low tides, called spring tides, occur when the Sun and Moon are aligned and pulling at the tidal bulges from the same or exact opposite sides of Earth.
Spring tides happen twice a month (about every 15 days) during the new and full moons. The opposite conditions, when high tide is not very high and low tide is not very low, are called neap tides. These happen when the Moon and Sun are at right angles to each other so their gravitational forces cancel one another.
Tides vary around the world
 |
| Tides vary around the world |
Earth is obviously not a perfect, water-covered sphere. The continents, seafloor, ocean currents and (mass of air surrounding Earth) winds all affect the tidal bulges as they move around Earth each day. Some places, like the Bay of Fundy in Nova Scotia, Canada, and the English Channel between Great Britain and France, experience very large tides.
Other places, like the Mediterranean Sea, have barely noticeable tides. Sometimes the shape of an inlet (a narrow body of water between two islands or leading inland), bay, or harbor delays the tides; in the Gulf of Mexico there is only one high and one low tide each day.
A large storm like a hurricane can add to the tidal bulge as it approaches the shore. Along many coastlines, strong tides carry salt water and ocean sediment (particles of sand, gravel, and silt) far inland. Many rivers, bays, and estuaries (coastal wetlands) experience tides many miles from the ocean.