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Tides

2013 Tidal Calendar (pdf format)


"The Mighty Tides"

There is not one drop of water in the ocean, not even in the deepest parts of the abyss, that does not respond to the mysterious forces that create the tide. No other force that affects the sea is so strong. Compared with the tide, the wind-created waves are surface movements which are felt, at the most, no more than a hundred fathoms below the surface.To help you comprehend the enormous masses of water affected by tidal movement, consider the following example. Into one small bay on the east coast of North America - Passamaquoddy - two billion tons of water are carried by the tidal current twice each day; into the whole Bay of Fundy, 100 billion tons!

Response to 'Pull' of Moon

The tides are a response of the waters of the ocean to the gravitational "pull" of the moon and the more distant sun. Folks who live along the Alabama coast know that the moon, far more than the sun, controls the tides. Why is this? Should not the sun, with a mass 27 million times that of the moon, have the greater influence? No. Because in the mechanics of the universe, nearness counts far more than distant mass, and when all the mathematical calculations have been made, we find that the moon's power over the tides is more than twice that of the sun.

Moon Deciding Factor

Here's how it works -Twice each month, when the moon is a mere thread of silver in the sky, and again when it is full, we have the strongest tidal movements-the highest flood tides and lowest ebb tides of the lunar month. These are called the spring tides. At these times the sun, moon, and earth are directly in line and the pull of the two heavenly bodies is added together to bring the water high on the beaches and draw a brimming tide into the harbors so that boats float high beside their wharfs.

Likewise, twice each month at the quarters of the moon, when sun, moon, and earth lie at the apexes of a triangle and the pull of the sun and the moon are opposed, we have the moderate tidal movements called the neap tides. At this time the difference between high and low water is less than at any other time during the month. A simple way to explain or remember tidal rhythm is to think of it as twice-daily waves that move like the hour hand of a clock about a central point - mid-ocean. These waves run 12 hours and 25 minutes apart; their crests are high tides and the troughs are low tides.

Tides More Complicated

However, the tides are enormously more complicated than all this would suggest. The influence of sun and moon is constantly changing with the phases of the moon, with the distance of moon and sun from the earth, and with the position of each to the north or south of the equator.

The declination of the moon (its position north or south of the equator) is one of the most important factors affecting Alabama tides. As the moon revolves around the earth from east to west, it also has a north-south movement. The declination is the distance in degrees of latitude that the moon is north or south of the equator. The plane of the moon's orbit is not in the same plane as the equator, therefore, the declination of the moon is constantly changing. In the moon's fort-nightly change from maximum northerly to maximum southerly declination, the differences between morning and afternoon tides are greatest near the times the moon is over the equator.

Power Of Oscillation

Tides are further implicated by the fact that every body of water, whether natural or artificial, has its own period of oscillation. Disturb its waters and they will move with a seesaw or rocking motion with the most pronounced movement at the ends of the container and the least motion at the center. The truth of the matter is that local topography is all important in determining the features that, to our minds, make "the tide." The attractive force of the heavenly bodies sets the water in motion, but how, and how far, and how strongly it will rise depends on such things as the slope of the bottom, the depth of a channel, or the width of a bay's entrance.

When we spend a holiday or weekend at Gulf Shores or Dauphin Island, the ebbing and flowing of the tide may not leave much of an impression on our minds because in that great inland sea of the Atlantic-the Gulf of Mexico-the tidal rise is but a slight movement of no more than a foot or two.

On the shores of Alabama the tide is a long, deliberate undulation-one rise and one fall in the lunar day of 24 hours plus 50 minutes-resembling the untroubled breathing of that earth monster to whom the ancient Indians attributed all tides. In contrast, if you ever have the chance to summer on the Gulf of Maine around the Bay of Fundy, the rise and fall of the tide would be something you would never forget. You would have to accommodate your boating and swimming activities to a tide that rises and falls 40 to 50 feet a day! Although all earth lies under the same moon and sun, the above contrast shows us how much the topography, or shape of a basin in a certain location, affects the tide.

Tides Affect People, Fish

The lives of coastal Alabamians are affected daily by the tides and of even greater importance is the affect the rise and fall of the tide has on the life of seafood. The tidal flooding of the coastal marshes- the estuaries- is a vital factor that accounts for the fertility that enables us to enjoy the fishing and fine seafood that we sometimes take for granted.

Tides Growing Weaker

Let me leave you with the following fact: the tides are growing weaker and weaker. Tidal friction is constantly pushing the moon further and further away. As the moon recedes, it will have less power over the tides and it will also take the moon longer to complete its orbit around the earth. When finally the length of the day and month coincide, the moon will no longer rotate relatively to the earth and there will be no lunar tides.

If the history of the earth's tides should one day be written by some observer of the universe, it would no doubt be said that they reached their greatest grandeur and power in the younger days of Earth, and that they slowly grew feebler and less imposing until one day they ceased to be. As with all that is earthly, their days are numbered. All this, of course, will require time on a scale the mind finds difficult to conceive, and before it happens, it is quite probable that the human race will have vanished from the earth.


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