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 Real-Time Data for Louisiana The USGS investigates
the occurrence, quantity, quality,
distribution, and movement of surface and
underground waters and disseminates the data
to the public, State and local governments,
public and private utilities, and other
Federal agencies involved with managing our
water resources.
Center
for Operational Oceanographic Products and
Services
National Weather Service Forecast Office -
New Orleans/Baton
Tide Predictions: Southeast and South
Central Louisiana Daily Coastal
Tide Data; Tidal Station Locations and
Ranges, for Mississippi Coast for
Louisiana Coast.
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Leaning About
Tides
Tide -The
periodic rise and fall of a body of water
resulting from
gravitational interactions between Sun,
Moon, and Earth. The vertical component of
the particulate motion of a tidal wave.
Although the accompanying horizontal
movement of the water is part of the same
phenomenon, it is preferable to designate
this motion as survey area. |
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High Tide
-The maximum height reached by a rising
tide. The high water is due to the periodic
tidal forces and the effects of
meteorological, hydrologic, and/or
oceanographic conditions. For tidal datum
computational purposes, the maximum height
is not considered a high water unless it
contains a tidal high. |
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Low
Tide -The
minimum height reached by a falling tide.
The low water is due to the periodic tidal
forces and the effects of meteorological,
hydrologic, and/or oceanographic conditions.
For tidal datum com-putational purposes, the
minimum height is not considered a low water
unless it contains a tidal low water |
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Introduction
The word "tides" is a generic term used to
define the alternating rise and fall in sea
level with respect to the land, produced by
the gravitational attraction of the moon and
the sun. To a much smaller extent, tides
also occur in large lakes, the atmosphere,
and within the solid crust of the earth,
acted upon by these same gravitational
forces of the moon and sun. Additional
nonastronomical factors such as
configuration of the coastline, local depth
of the water, ocean-floor topography, and
other hydrographic and meteorological
influences may play an important role in
altering the range, interval between high
and low water, an times of arrival of the
tides.
The most familiar evidence of the tides
along our seashores is the observed
recurrence of high and low water - usually,
but not always, twice daily. The term tide
correctly refers only to such a relatively
short-period, astronomically induced
vertical change in the height of the sea
surface (exclusive of wind-actuated waves
and swell); the expression tidal current
relates to accompanying periodic horizontal
movement of the ocean water, both near the
coast and offshore (but as distinct from the
continuous, stream-flow type of ocean
current).
Knowledge of the
times, heights, and extent of inflow and
outflow of tidal waters is of importance in
a wide range of practical applications such
as the following: Navigation through
intracoastal waterways, and within
estuaries, bays, and harbors; work on harbor
engineering projects, such as the
construction of bridges, docks, breakwaters,
and deep-water channels; the establishment
of standard chart datums for hydrography and
for demarcation of a base line or "legal
coastline" for fixing offshore territorial
limits both on the sea surface and on the
submerged lands of the Continental Shelf;
provision of information necessary for
underwater demolition activities and other
military engineering uses; and the
furnishing of data indispensable to fishing,
boating, surfing, and a considerable variety
of related water sport activities.
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The Astronomical Tide-Producing
Forces
General Considerations
At the surface of the earth, the earth's
force of gravitational attraction acts in a
direction inward toward its center of mass,
and thus holds the ocean water confined to
this surface. However, the gravitational
forces of the moon and sun also act
externally upon the earth's ocean waters.
These external forces are exerted as
tide-producing, or so-called "tractive"
forces. Their effects are superimposed upon
the earth's gravitational force and act to
draw the ocean waters to positions on the
earth's surface directly beneath these
respective celestial bodies (i.e., towards
the "sublunar" and "subsolar" points).
High tides are produced in the ocean waters
by the "heaping" action resulting from the
horizontal flow of water toward two regions
of the earth representing positions of
maximum attraction of combined lunar and
solar gravitational forces. Low tides are
created by a compensating maximum withdrawal
of water from regions around the earth
midway between these two humps. The
alternation of high and low tides is caused
by the daily (or diurnal) rotation of the
earth with respect to these two tidal humps
and two tidal depressions. The changing
arrival time of any two successive high or
low tides at any one location is the result
of numerous factors later to be discussed.
To all outward appearances, the moon
revolves around the earth, but in actuality,
the moon and earth revolve together around
their common center of mass, or gravity. The
two astronomical bodies are held together by
gravitational attraction, but are
simultaneously kept apart by an equal and
opposite centrifugal force produced by their
individual revolutions around the
center-of-mass of the earth-moon system.
This balance of forces in orbital revolution
applies to the center-of-mass of the
individual bodies only. At the earth's
surface, an imbalance between these two
forces results in the fact that there
exists, on the hemisphere of the earth
turned toward the moon, a net (or
differential) tide-producing force which
acts in the direction of the moon's
gravitational attraction, or toward the
center of the moon. On the side of the earth
directly opposite the moon, the net
tide-producing force is in the direction of
the greater centrifugal force, or away from
the moon.
Similar differential
forces exist as the result of the revolution
of the center-of-mass of the earth around
the center-of-mass of the earth-sun system. |
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Origin of the Tide-Raising Forces
To all outward appearances, the moon
revolves around the earth, but in actuality,
the moon and earth revolve together around
their common center of mass, or gravity. The
two astronomical bodies are held together by
gravitational attraction, but are
simultaneously kept apart by an equal and
opposite centrifugal force produced by their
individual revolutions around the
center-of-mass of the earth-moon system.
This balance of forces in orbital revolution
applies to the center-of-mass of the
individual bodies only. At the earth's
surface, an imbalance between these two
forces results in the fact that there
exists, on the hemisphere of the earth
turned toward the moon, a net (or
differential) tide-producing force which
acts in the direction of the moon's
gravitational attraction, or toward the
center of the moon. On the side of the earth
directly opposite the moon, the net
tide-producing force is in the direction of
the greater centrifugal force, or away from
the moon.
Similar differential
forces exist as the result of the revolution
of the center-of-mass of the earth around
the center-of-mass of the earth-sun system. |
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Nautical Twilight
is defined to begin in the morning, and to
end in the evening, when the center of the
sun is geometrically 12 degrees below the
horizon. At the beginning or end of nautical
twilight, under good atmospheric conditions
and in the absence of other illumination,
general outlines of ground objects may be
distinguishable, but detailed outdoor
operations are not possible, and the horizon
is indistinct. |
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Civil Twilight
is defined to begin in the morning, and to
end in the evening when the center of the
Sun is geometrically 6 degrees below the
horizon. This is the limit at which twilight
illumination is sufficient, under good
weather conditions, for terrestrial objects
to be clearly distinguished; at the
beginning of morning civil twilight, or end
of evening civil twilight, the horizon is
clearly defined and the brightest stars are
visible under good atmospheric conditions in
the absence of moonlight or other
illumination. In the morning before the
beginning of civil twilight and in the
evening after the end of civil twilight,
artificial illumination is normally required
to carry on ordinary outdoor activities.
Complete darkness, however, ends sometime
prior to the beginning of morning civil
twilight and begins sometime after the end
of evening civil twilight. |
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Sunrise and Sunset
conventionally refer to the times when the
upper edge of the disk of the Sun is on the
horizon, considered unobstructed relative to
the location of interest. Atmospheric
conditions are assumed to be average, and
the location is in a level region on the
Earth's surface. |
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Moonrise and Moonset
times are computed for exactly the same
circumstances as for sunrise and sunset.
However, moonrise and moonset may occur at
any time during a 24 hour period and,
consequently, it is often possible for the
Moon to be seen during daylight, and to have
moonless nights. It is also possible that a
moonrise or moonset does not occur relative
to a specific place on a given date.
Although Full Moon occurs each month at a
specific date and time, the Moon's disk may
appear to be full for several nights in a
row if it is clear. This is because the
percentage of the Moon's disk that appears
illuminated changes very slowly around the
time of Full Moon (also around New Moon, but
the Moon is not visible at all then). The
Moon may appear 100% illuminated only on the
night closest to the time of exact Full
Moon, but on the night before and night
after will appear 97-99% illuminated; most
people would not notice the difference. Even
two days from Full Moon the Moon's disk is
93-97% illuminated.
New Moon, First Quarter, Full Moon, and Last
Quarter phases are considered to be primary
phases and their dates and times are
published in almanacs and on calendars.
(Click here for a list.) The two crescent and two gibbous phases are
intermediate phases, each of which lasts for
about a week between the primary phases,
during which time the exact fraction of the
Moon's disk that is illuminated gradually
changes.
The phases of the Moon are related to
(actually, caused by) the relative positions
of the Moon and Sun in the sky. For example,
New Moon occurs when the Sun and Moon are
quite close together in the sky. Full Moon
occurs when the Sun and Moon are at nearly
opposite positions in the sky - which is why
a Full Moon rises about the time of sunset,
and sets about the time of sunrise, for most
places on Earth. First and Last Quarters
occur when the Sun and Moon are about 90
degrees apart in the sky. In fact, the two
"half Moon" phases are called First Quarter
and Last Quarter because they occur when the
Moon is, respectively, one- and
three-quarters of the way around the sky
(i.e., along its orbit) from New Moon.
The relationship of
the Moon's phase to its angular distance in
the sky from the Sun allows us to establish
very exact definitions of when the primary
phases occur, independent of how they
appear. Technically, the phases New Moon,
First Quarter, Full Moon, and Last Quarter
are defined to occur when the excess of the
apparent ecliptic (celestial) longitude of
the Moon over that of the Sun is 0, 90, 180,
and 270 degrees, respectively. These
definitions are used when the dates and
times of the phases are computed for
almanacs, calendars, etc. Because the
difference between the ecliptic longitudes
of the Moon and Sun is a monotonically and
rapidly increasing quantity, the dates and
times of the phases of the Moon computed
this way are instantaneous and well defined. |
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The percent of the Moon's surface
illuminated is a more refined, quantitative description
of the Moon's appearance than is the phase.
Considering the Moon as a circular disk, the
ratio of the area illuminated by direct
sunlight to its total area is the fraction
of the Moon's surface illuminated;
multiplied by 100, it is the percent
illuminated. At New Moon the percent
illuminated is 0; at First and Last Quarters
it is 50%; and at Full Moon it is 100%.
During the crescent phases the percent
illuminated is between 0 and 50% and during
gibbous phases it is between 50% and 100%.
For practical purposes, phases of the Moon
and the percent of the Moon illuminated are
independent of the location on the Earth
from where the Moon is observed. That is,
all the phases occur at the same time
regardless of the observer's position. |
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