tides

Tides: Introduction, Spring and Neap Tides, The Effects of Perigee and Apogee, Beware the Proxigean Tides, Tides Great and Small, Ever Heard of an Air Tide?, Ever Heard of an Earth Tide?, Tidal Glossary.

Photo by Stephen O. Muskie / www.outtakes.com

I must down to the seas again,
for the call of the running tide
Is a wild call and a clear call
that may not be denied.

-- John Masefield
English poet, playwright, and fiction writer (1878-1967)

THAT THE TIDES rise and fall is something we take for granted, like the rising and setting of the Sun, the phases of the Moon, or the melting of winter into spring. But there's much more to the story of the ocean's movement than meets the eye. Astronomer Bob Berman tells that story here. If you'd like to delve deeper into the subject, visit the links below.

TIDE PREDICTIONS
If you'd like to know when it will be high tide in your area, visit our Tide Predictions page.

WHY DO WE HAVE TIDES?
By Bob Berman

Folklore often alludes to the "pull of the Moon" -- probably because coastal civilizations have always noticed that the ocean's rhythmic rise and fall mostly follows the Moon's, rather than the Sun's, motion and position. Although the Sun's gravitational influence has an immense effect on us (we orbit the Sun and not the Moon), tides move to the tune of the Moon. But why does the Sun have the greater gravitational pull and the Moon the greater tidal influence?

Tides are caused by the difference between the Moon's gravitational pull on one side of Earth compared to its pull on the other because of its proximity to Earth. The Sun is so distant -- 400 times farther away from us than the Moon -- that there isn't much difference in its gravitational pull at different points on Earth. In other words, the Sun's gravitational influence is almost the same in Russia as it is in Australia as it is in Canada, and so on. The Moon, however, is so close to us that there's a big difference between the gravitational pull on the side of Earth nearest the Moon and on the side of Earth farthest away. This difference in the Moon's gravitational effect is the tidal effect.

Since oceans are fluid and uncontained, they flow toward the side of Earth nearest the Moon. This creates a high-water bulge. A second high-water bulge occurs simultaneously on the side of Earth opposite the Moon. The second bulge exists because the Moon doesn't orbit the center of Earth. Rather, Earth and the Moon swing as a unit around the center of their combined mass. The midpoint of their weight is located much closer to Earth, which is 81 times heavier than the Moon. This center of mass, or barycenter, sits about a thousand miles beneath Earth's surface at whatever point is facing the Moon at any particular time.

As both Earth and the Moon complete their orbit around the barycenter every 27.32166 days, the side of Earth farthest from this point experiences the fastest motion. Whirled around as if on a carnival ride, the oceans there are whipped centrifugally and rise upward as if being partially hurled away, creating the second high-water bulge.

Because Earth rotates under these high-water bulges, high tides move around the world in a daily cycle: About every 12-1/2 hours, there's a new one, which gives every beach on Earth roughly 2 high tides a day. Since the Moon is also moving around Earth, the high tides arrive almost an hour later each day throughout the lunar cycle.

Thus we have two high-water bulges, and two high tides -- one on the side of Earth facing the Moon and the other on the opposite side. Neither is caused by the Moon actually pulling on water.

Now morn has come,
And with the morn the punctual tide again.

-- Susan Coolidge
American writer (1835-1905)

SPRING AND NEAP TIDES
By Bob Berman By Bob Berman By Bob Berman By Bob Berman By Bob Berman By Bob Berman By Bob Berman By Bob Berman

Since antiquity, people have noticed that oceans exhibit a much greater tidal range around the time of the full and new Moon. This is when the Moon and Sun are either together in the sky or are on opposite sides of the heavens. Higher tides occur during these Moon phases because the Sun also exerts a gravitational pull on our oceans, although it is only 46 percent as strong as the Moon's. When the gravitational effects of the Sun and the Moon combine, we get spring tides, which have nothing to do with the season of spring. The term refers to the action of the seas springing out and then springing back. These are times of high high tides and low low tides. A week later, during either of the two quarter Moon phases, when the Sun and Moon are at right angles to each other and their tidal influences partially cancel each other out, neap tides occur, and the tidal range is minimal. In fact, because the oceans take a bit of time to catch up to the geometry of the Moon, spring and neap tides usually occur about a day after the respective lunar cycles.

The punctual tide draws up the bay
With ripple of wave and hiss of spray.

-- Susan Coolidge
American writer (1835-1905)

THE EFFECTS OF PERIGEE AND APOGEE
By Bob Berman

The Moon's orbit periodically grows rounder and then more oval; it is never a perfect circle. The point at which it is nearest Earth each month is called its perigee (this varies throughout the year). The point at which the Moon is farthest from Earth each month is called its apogee (this varies throughout the year as well). During the final quarter of the 20th century, the Moon was as close to Earth as 216,500 miles (surface to surface) at its most extreme perigee and as far away as 247,700 miles at its most extreme apogee. At perigee, tidal ranges are increased; at apogee, they are decreased.

To a lesser degree, the Sun has a similar tidal effect, a consequence of its location just 91.5 million miles away in early January but some 94 million miles distant in July.

Rain is likely to commence on the turn of the tide.

-- Weather proverb

BEWARE THE PROXIGEAN TIDES
By Bob Berman

Earth experiences proxigean tides -- an extraordinary tidal range that, when combined with other factors (such as unusually low barometric pressure or onshore winds), can cause erosion on coastlines, among other problems. The proximity of the Sun to Earth in December or January will occasionally coincide with a new or full Moon that just happens to occur when the Moon is at its monthly perigee -- or perhaps even when it's at the year's most extreme perigee. This potent combination happened in 1990, 1992, and 2001.

">This series of events can result in interesting but not necessarily destructive ocean levels. The final devastating ingredient is a storm at sea. The onshore winds of an ocean storm can literally whip up the waters, typically raising tides by several feet and occasionally by much more. What's more, the low pressure characteristic of such storms lifts oceans: A 1-inch drop in barometric pressure raises the seas by 13.2 inches, which can cause a dangerous period of proxigean tides to boil over into a coastal catastrophe. (It's also possible for only one of any given day's two high tides to be catastrophic.)

Proxigean tidal conditions combined with disastrous storms to bring death and calamity to the eastern and southern United States in 1723, 1846, 1851, 1885, 1900, 1914, 1931, and 1978. The greatest loss of life -- 6,000 people -- occurred in Galveston, Texas, on September 8, 1900. Low-lying regions of Europe also have faced devastation when such astronomical and meteorological conditions coincided: One hundred thousand people died in Holland in 1099, and half that many in 1287.

The tide rises, the tide falls,
The twilight darkens, the curlew calls;
Along the sea-sands damp and brown
The traveller hastens toward the town,
And the tide rises, the tide falls.

-- Henry Wadsworth Longfellow
American poet (1807-1882)

TIDES GREAT AND SMALL
By Bob Berman

Some parts of the world, such as Tahiti, a Polynesian island in the South Pacific, have no lunar tides at all. There, a single daily tide caused by the Sun raises the seas just 1 foot. In other places, such as the famed Bay of Fundy in Nova Scotia, tides routinely rise and fall by 40 feet and can get much higher during extreme conditions that produce proxigean tides.

Throughout the world, extreme tidal variations are largely due to the shape of the ocean bottom and a bay's orientation and outline. High tides can be either exaggerated in height or reduced by the shape of the bay and the way the undersea surface rises at that location.

Tahiti (and all of Polynesia), however, experiences an entirely different effect. Think of it this way: Just as carrying a shallow pan of water sets up a swishing back-and-forth motion that soon makes the water spill over the pan's sides, there is a middle, or fulcrum, point where the water hardly moves at all. The islands of Polynesia are located right at the fulcrum point for the Pacific basin (this fulcrum point has nothing to do with the equator), resulting in almost no tidal movement.

These and many other local physical variations, some still poorly understood, produce the fascinating range of tides found around the world.

The tide keeps its course

-- James Howell
English author (c. 1594-1666)

EVER HEARD OF AN AIR TIDE?
By Bob BermanEVER HEARD OF AN AIR TIDE?
By Bob BermanEVER HEARD OF AN AIR TIDE?
By Bob Berman

In the past century, an "air tide," or shifting of the atmosphere, has been recognized and studied. Since 1918, it has actually been measured in the Northern Hemisphere. Although this Moon-linked barometric variation is small -- just 1/1,000 inch of mercury -- its effects are not. (Barometric pressure is measured as inches of mercury. When the weather forecaster says the barometer is 29.95 inches, he or she is talking about how many inches of mercury could be supported by the current pressure of the atmosphere.) Statistical studies show that more cloudiness, rainfall, and storms are generated during some lunar phases (such as the full Moon) than others.

Down beyond the haven the tide comes with a shout.

-- William Sharp
Scottish writer (1855-1905)

EVER HEARD OF AN EARTH TIDE?
By Bob Berman

Like the oceans, the solid crust of Earth also moves to a lunar rhythm. While the seas rise and fall by a worldwide average of three feet daily, the ground oscillates by about eight inches (scientists measure this with satellites). This mutation is not merely a surface phenomenon; the entire planet undergoes constant and complex rhythmic deformations. What's remarkable is that they're so small. It's evidence that our planet is incredibly well built. To have a body as massive and as near as the Moon, and to have our planet changed so little by its presence, means that Earth is one of the solar system's construction success stories.

TIDAL GLOSSARY

Apogean Tide: A monthly tide of decreased range that occurs when the Moon is farthest from Earth (at apogee).
Diurnal: Applies to a location that normally experiences one high water and one low water during a tidal day of approximately 24 hours.
Mean Lower Low Water: The arithmetic mean of the lesser of a daily pair of low waters, observed over a specific 19-year cycle called the National Tidal Datum Epoch.
Neap Tide: A tide of decreased range occurring twice a month, when the Moon is in quadrature (during the first and last quarter Moons, when the Sun and the Moon are at right angles to each other relative to Earth).
Perigean Tide: A monthly tide of increased range that occurs when the Moon is closest to Earth (at perigee).
Semidiurnal: Having a period of half a tidal day. East Coast tides, for example, are semidiurnal, with two highs and two lows in approximately 24 hours.
Spring Tide: Named not for the season of spring, but from the German springen (to leap up). This tide of increased range occurs at times of syzygy (q.v.) each month. A spring tide also brings a lower low water.
Syzygy: Occurs twice a month, when the Sun and the Moon are in conjunction (lined up on the same side of Earth at the new Moon) and when they are in opposition (on opposite sides of Earth at the full Moon, though usually not so directly in line as to produce an eclipse). In either case, the gravitational effects of the Sun and the Moon reinforce each other, and tidal range is increased.
Vanishing Tide: A mixed tide of considerable inequality in the two highs or two lows, so that the "high low" may become indistinguishable from the "low high." The result is a vanishing tide, where no significant difference is apparent.

Links to More Tide Information

The National Oceanographic and Atmospheric Administration (NOAA) maintains a much more detailed glossary of tide terms and tidal history data.

Very detailed current weather conditions for designated reporting stations is available from the National Weather Service. You can choose the area you want to know about by clicking on a map. The information, updated hourly, includes some or all of the following conditions: temperature, dew point, wind direction and speed, sea surface temperature, wave heights, and wave periods.

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