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The Planets .... brought to you by QuasArt Web Designs.
E A R T H (3rd planet from the Sun)
The Earth was not named after anything.
CHAPTERS
I II III IV V VI VII VIII
IX X XI XII XIII XIV XV XVI
INTRODUCTION
I
Earth, one of the planets in the solar system, the third in distance
from the sun and the fifth largest of the planets in diameter. The
mean distance of the earth from the sun is 149,503,000 km (92,897,000
mi). It is the only planet known to support life, although some of
the other planets have atmospheres and contain water.
The earth is not a perfect sphere but is slightly oblate, or flattened
at the poles. The diameter of the earth measured around the North
Pole and the South Pole is about 42 km (26 mi) less than the diameter
of the earth measured at the equator.
Earth, one of the planets in the solar system, the third in distance
from the sun and the fifth largest of the planets in diameter. The
mean distance of the earth from the sun is 149,503,000 km (92,897,000
mi). It is the only planet known to support life, although some of
the other planets have atmospheres and contain water.
The earth is not a perfect sphere but is slightly oblate, or flattened
at the poles. The diameter of the earth measured around the North
Pole and the South Pole is about 42 km (26 mi) less than the diameter
of the earth measured at the equator.
TOP OF PAGE
MOTION
II
In common with the entire solar system, the earth is moving through
space at the rate of approximately 20.1 km/sec or 72,360 km/h (approximately
12.5 mi/sec or 45,000 mph) toward the constellation of Hercules. The
Milky Way galaxy as a whole, however, is moving toward the constellation
Leo at about 600 km/sec (about 375 mi/sec). The earth and its satellite,
the moon, also move together in an elliptical orbit about the sun.
The eccentricity of the orbit is slight, so that the orbit is virtually
a circle. The approximate length of the earth's orbit is 938,900,000
km (583,400,000 mi), and the earth travels along it at a velocity
of about 106,000 km/h (about 66,000 mph). The earth rotates on its
axis once every 23 hr 56 min 4.1 sec (based on the solar year). A
point on the equator therefore rotates at a rate of a little more
than 1600 km/h (about 1000 mph), and a point on the earth
at the latitude of Portland, Oregon (45° north), rotates at about
1073 km/h (about 667 mph). In addition
to these primary motions, three other components of the total motion
of the earth exist: the precession of the equinoxes (see Ecliptic),
nutation (periodic variation in the inclination of the earth's axis
caused by the gravitational pulls of the sun and moon), and variation
of latitude (see Latitude and Longitude).
TOP OF PAGE
COMPOSITION
III
The earth is made up of a series of layers that formed early in the
planet's history, as heavier material gravitated toward the center
and lighter material floated to the surface. The dense, solid, inner
core of iron is surrounded by a liquid, iron, outer core. The lower
mantle consists of molten rock, which is surrounded by partially molten
rock in the asthenosphere and solid rock in the upper mantle and crust.
Between some of the layers, there are chemical or structural changes
that form discontinuities. Lighter elements, such as silicon, aluminum,
calcium, potassium, sodium, and oxygen, compose the outer crust.
The earth consists of five parts: the first, the atmosphere, is gaseous;
the second, the hydrosphere, is liquid; the third, fourth, and fifth,
the lithosphere, mantle, and core, are largely solid. The atmosphere
is the gaseous envelope that surrounds the solid body of the planet.
Although it has a thickness of more than 1100 km (more than 700 mi),
about half its mass is concentrated in the lower 5.6 km (3.5 mi).
The lithosphere, consisting mainly of the cold, rigid, rocky crust
of the earth, extends to depths of 100 km (60 mi). The hydrosphere
is the layer of water that, in the form of the oceans, covers approximately
70.8 percent of the surface of the earth. The mantle and core are
the heavy interior of the earth, making up most of the earth's mass.
The hydrosphere consists chiefly of the oceans, but technically includes
all water surfaces in the world, including inland seas, lakes, rivers,
and underground waters. The average depth of the oceans is 3794 m
(12,447 ft), more than five times the average height of the continents.
The mass of the oceans is approximately 1.35 quintillion (1.35 × 1018)
metric tons, or about 1/4400 of the total mass of the earth. The rocks
of the lithosphere have an average density of 2.7 and are almost entirely
made up of 11 elements, which together account for about 99.5 percent
of its mass. The most abundant is oxygen (about 46.60 percent of the
total), followed by silicon (about 27.72 percent), aluminum (8.13
percent), iron (5.0 percent), calcium (3.63 percent), sodium (2.83
percent), potassium (2.59 percent), magnesium (2.09 percent) and titanium,
hydrogen, and phosphorus (totaling less than 1 percent). In addition,
11 other elements are present in trace amounts of 0.1 to 0.02 percent.
These elements, in order of abundance, are carbon, manganese, sulfur,
barium, chlorine, chromium, fluorine, zirconium, nickel, strontium,
and vanadium. The elements are present in the lithosphere almost entirely
in the form of compounds rather than in their free state.
These compounds exist almost entirely in the crystalline state, so
they are, by definition, minerals. The lithosphere comprises two shells-the
crust and upper mantle-that are divided into a dozen or so rigid tectonic
plates (see Plate Tectonics). The crust itself is divided in two.
The sialic or upper crust, of which
the continents consist, is made up of igneous and sedimentary rocks
whose average chemical composition is similar to that of granite and
whose density is about 2.7. The simatic or lower crust, which forms
the floors of the ocean basins, is made of darker, heavier igneous
rocks such as gabbro and basalt, with an average density of about
3. The lithosphere also includes the upper mantle. Rocks at these
depths have a density of about 3.3. The upper mantle is separated
from the crust above by a seismic discontinuity, called the Moho,
and from the lower mantle below by a zone of weakness known as the
asthenosphere. Shearing of the plastic, partially molten rocks of
the asthenosphere, 100 km (60 mi) thick, enables the continents to
drift across the earth's surface and oceans to open and close. The
dense, heavy interior of the earth is divided into a thick shell,
the mantle, surrounding an innermost sphere, the core. The mantle
extends from the base of the crust to a depth of about 2900 km (1800
mi). Except for the zone known as the asthenosphere, it is solid,
and its density, increasing with depth, ranges from 3.3 to 6. The
upper mantle is composed of iron and magnesium silicates, as typified
by the mineral olivine. The lower part may consist of a mixture of
oxides of magnesium, silicon, and iron. Seismological research has
shown that the core has an outer shell about 2225 km (1380 mi) thick
with an average density of 10. This shell is probably rigid, and studies
show that its outer surface has depressions and peaks, the latter
forming where warm material rises. In contrast, the inner core, which
has a radius of about 1275 km (795 mi), is solid. Both core layers
are thought to consist largely of iron, with a small percentage of
nickel and other elements. Temperatures in the inner core may be as
high as 6650°C (12,000°F), and the average density is estimated to
be 13.
TOP OF PAGE
INTERNAL HEAT FLOW
IV
Intense heat from the inner core is continually radiated outward,
through the several concentric shells that form the solid portion
of the planet. The source of this heat is thought to be energy released
by the radioactive decay of uranium and other radioactive elements.
Convection currents within the mantle transfer most of this heat energy
from deep within the earth to the surface and are the driving force
behind continental drift. Convective flow supplies hot, molten rock
to the worldwide system of midocean ridges (see Ocean and Oceanography)
and feeds the lava that erupts from volcanoes on land.
TOP OF PAGE
AGE AND ORIGIN OF THE EARTH
V
Radiometric dating has enabled scientists to arrive at an estimate
of 4.65 billion years for the age of the earth (see Dating Methods).
Although the oldest earth rocks dated this way are not quite 4 billion
years old, meteorites, which correlate geologically with the earth's
core, give dates of about 4.5 billion years, and crystallization of
the core and meteorites is considered to have occurred at the same
time, some 150 million years after the earth and solar system first
formed (see Solar System: Theories of Origin). After originally condensing,
by gravitational attraction of cosmic dust and gas, the earth would
have
been almost homogeneous and relatively cool. But continued contraction
of these materials caused them to heat, as did the radioactivity of
some of the heavier elements. In the next stage of its formation,
as the earth became hotter, it began melting under the influence of
gravity. This caused the differentiation into crust, mantle, and core,
with the lighter silicates moving up and outward to form the mantle
and crust and the heavier elements, mainly iron and nickel, sinking
downward toward the center of the earth to form the core. Meanwhile,
by volcanic eruption, light, volatile gases and vapors continually
escaped from the mantle and crust. Some of these, mainly carbon dioxide
and nitrogen, were held by the earth's gravity and formed the primitive
atmosphere, while water vapor condensed to form the world's first
oceans.
TOP OF PAGE
TERRESSTRIAL MAGNETISM
VI
The phenomenon of terrestrial magnetism results from the fact that
the entire earth behaves as an enormous magnet. The English physician
and natural philosopher William Gilbert was the first to demonstrate
this similarity in about 1600, although the effects of terrestrial
magnetism had been utilized much earlier in primitive compasses.
TOP OF PAGE
MAGNETIC POLES
VII
A powerful magnetic field surrounds the earth, as if the planet has
an enormous bar magnet embedded within its interior. The S and N on
the magnet indicate the orientation of the earth's magnetic field.
Because the opposite ends of magnets attract, the northern end of
magnets on the earth are attracted to the southern end of the earth's
magnetic field, which is called magnetic north. Scientists believe
that convection currents of charged, molten metal circulating in the
earth's core are the source of the planet's magnetic field.
The
magnetic poles of the earth do not correspond with the geographic
poles of its axis. The north magnetic pole is presently located off
the western coast of Bathurst Island, in the Canadian Northwest Territories,
almost 1290 km (almost 800 mi) northwest of Hudson Bay. The south
magnetic pole is presently situated at the edge of the Antarctic continent
in Adélie Coast about 1930 km (about 1200 mi) northeast of Little
America. The position of the magnetic poles is not constant and shows
an appreciable change from year to year. Variations in the magnetic
field of the earth include secular variation, the change in the direction
of the field caused by the shifting of the poles. This is a periodic
variation that repeats itself after 960 years. A smaller annual variation
also exists, as does a diurnal, or daily, variation that can be detected
only by sensitive instruments.
TOP OF PAGE
DYNAMO THEORY
VIII
Measurements of the secular variation show that the entire magnetic
field has a tendency to drift westward at the rate of 19 to 24 km
(12 to 15 mi) per year. Clearly the magnetism of the earth is the
result of a dynamic rather than a passive condition, which would be
the case if the iron core of the earth were solid and passively magnetized.
Iron does not retain a permanent magnetism at temperatures above 540°
C (1000° F), however, and the temperature at the center of the earth
may be as high as 6650° C (12,000° F). The dynamo theory suggests
that the iron core is liquid (except at the very center of the earth
where the pressure solidifies the core), and that convection currents
within the liquid core behave like the individual wires in a dynamo,
thus setting up a gigantic magnetic field. The solid inner core rotates
more slowly than the outer core, thus accounting for the secular westward
drift. The irregular surface of the outer core may help to account
for some of the more irregular changes in the field.
TOP OF PAGE
INNER CORE STRUCTURE
IX
Another theory that may explain some variations in the earth's magnetic
field concerns the structure of the very inner core of the earth.
In 1995 scientists at the Carnegie Institute of Washington announced
that computer models of the earth's inner core appear to show one
huge, remarkably aligned iron crystal. Scientists think that the atoms
in the core are arranged so that each atom is packed with 12 neighboring
atoms in a tightly packed hexagonal structure (see Crystal (mineral)).
The molten outer core still provides the earth's magnetic field in
this theory, but the inner core would have some effect, probably causing
the magnetic field to warp slightly and causing especially large variations
in the position of the magnetic poles during times when the outer
core's effect is weaker, such as during a magnetic reversal. A crystalline
inner core would also explain why shock waves caused by earthquakes
take about four seconds longer to go from east to west through the
earth than from north to south, because the waves would travel more
quickly with the "grain" than across the grain of the crystal. VIIFIELD
INTENSITY The study of the intensity of the earth's magnetic field
is valuable from the points of view of pure science and of engineering,
and also for geological prospecting for mineral and energy resources.
Intensity measurements are made with instruments called magnetometers,
which determine the total intensity of the field and the intensities
in the horizontal and vertical directions. The intensity of the magnetic
field of the earth varies in different places on its surface. In the
temperate zones it amounts to about 0.6 oersted (the oersted is a
unit of measurement of a magnetic field; see Electrical Units), of
which 0.2 oersted is in a horizontal direction.
TOP OF PAGE
PALEOMAGNETISM
X
Studies of ancient volcanic rocks show that as they cooled, they "froze"
with their minerals oriented in the magnetic field existing at that
time. Worldwide measurements of such mineral deposits show that through
geological time the orientation of the magnetic field has shifted
with respect to the continents. The north magnetic pole 500 million
years ago, for example, lay south of Hawaii, and for the next 300
million years the magnetic equator lay across the United States. To
account for this, geologists believe that the outer crust of the earth
has gradually shifted around, even though the axis on which the earth
spins has remained the same. If this were the case, the climatic belts
would have remained the same, but the continents would have drifted
slowly through different "paleolatitudes".
TOP OF PAGE
TERRESTRIAL ELECTRICITY
XI
Three electrical systems generated in the earth and in the atmosphere
by natural geophysical processes are known. One of them is in the
atmosphere, and one is within the earth, flowing parallel to the surface
of the earth. The third, which transfers an electric charge continuously
between the atmosphere and the earth, flows vertically. See Electricity.
Atmospheric electricity, except for that associated with charges within
a cloud and lightning, results from the ionization of the atmosphere
by solar radiation and from the movement of clouds of ions carried
by atmospheric tides (see Ion; Ionization). Atmospheric tides result
from the gravitational attraction of the sun and the moon on the earth's
atmosphere (see Gravitation; Tide), and, like the oceanic tides, they
rise and fall daily. The ionization, and consequently the electrical
conductivity, of the atmosphere close to the surface of the earth
is low, but it increases rapidly with increasing altitude. Between
40 and 400 km (25 to 250 mi) above the earth, the ionosphere forms
an almost perfectly conducting spherical shell. The shell reflects
radio signals back to earth and absorbs electromagnetic radiations
approaching the earth from space. The ionization of the atmosphere
varies greatly, not only with altitude but with the time of day and
the latitude.
TOP OF PAGE
MAGNETIC REVERSALS
XII
Recent studies of remanent (residual) magnetism in rocks and of magnetic
anomalies on the floors of the oceans have shown that the magnetic
field of the earth has reversed its polarity at least 170 times in
the past 100 million years. Knowledge of these reversals, which can
be dated from radioactive isotopes in the rocks, has had a great influence
on theories of continental drift and the spreading of ocean floors.
TOP OF PAGE
EARTH CURRENTS
XIII
Earth currents constitute a worldwide system of eight loops of electric
current rather evenly distributed on both sides of the equator, plus
a series of smaller loops near the poles. Although it has been contended
that this system is induced entirely by the daily changes in atmospheric
electricity (and this may be true for short-term variations), it is
likely that the origins of the system are more complex. The core of
the earth, which consists of molten iron and nickel, is capable of
conducting electricity and can be likened to the armature of a huge
electric generator. Thermal convection currents in the core are believed
to move the molten metal in loop patterns relative to the magnetic
field of the earth, producing the system of earth currents that mirror
the pattern of convection currents within the core.
TOP OF PAGE
THE SURFACE CHARGE OF THE EARTH
XIV
The surface of the earth has a negative charge of electricity. Although
the conductivity of air near the earth is small, air is not a perfect
insulator, and the negative charge would drain off quickly if it were
not being continuously replenished in some way. In all places in which
measurements have been made in fair weather, a flow of positive electricity
has been observed to move downward from the atmosphere to the earth.
The negative charge of the earth is the cause, attracting positive
ions from the atmosphere to the earth. Although it has been suggested
that this downward current may be balanced by upward positive currents
in the polar regions, the preferred hypothesis today is that the negative
charge is transferred to the earth during storms and that the downward
flow of positive current during fair weather is balanced by a return
flow of positive current from areas of the earth experiencing stormy
weather. It has been proved that a negative charge is transferred
to earth from thunderclouds, and the rate at which storms develop
electric energy is sufficient to replenish the surface charge. In
addition, the frequency of storms appears to be greatest during the
time of day when the negative charge of the earth increases most rapidly.
TOP OF PAGE
MOONS AND SATELLITES
XVI
THE
MOON
Called Luna by the Romans, Selene and Artemis
by the Greeks,
and many other names in other mythologies.
The Moon is the only natural satellite of Earth:
orbit: 384,400 km from Earth
diameter: 3476 km
mass: 7.35e22 kg
The Moon, of course, has been known since prehistoric times. It is
the second brightest object in the sky after the Sun. As the Moon
orbits around the Earth once per month, the angle between the Earth,
the Moon and the Sun changes; we see this as the cycle of the Moon's
phases. The time between successive new moons is 29.5 days (709 hours),
slightly different from the Moon's orbital period (measured against
the stars) since the Earth moves a significant distance in its orbit
around the Sun in that time.
Due to its size and composition, the Moon is sometimes classified
as a terrestrial "planet" along with Mercury, Venus, Earth and Mars.
 |
The Moon was first visited by the Soviet spacecraft Luna 2 in 1959. It is the only extraterrestrial body to have been visited by humans. The first landing by americans was on July 20, 1969 (do you remember where you were?); the last was in December 1972. The Moon is also the only body from which samples have been returned to Earth. In the summer of 1994, the Moon was very extensively mapped by the little spacecraft Clementine and again in 1999 by Lunar Prospector. |
The gravitational forces between the Earth and the Moon cause some
interesting effects. The most obvious is the ocean tides. The Moon's
gravitational attraction is stronger on the side of the Earth nearest
to the Moon and weaker on the opposite side. Since the Earth, and
particularly the oceans, is not perfectly rigid it is stretched out
along the line toward the Moon. From our perspective on the Earth's
surface we see two small bulges, one in the direction of the Moon
and one directly opposite. The effect is much stronger in the ocean
water than in the solid crust so the water bulges are higher. And
because the Earth rotates much faster than the Moon moves in its orbit,
the bulges move around the Earth about once a day giving two high
tides per day. (This is a greatly simplified model; actual tides,
especialy near the coasts, are much more complicated.) But the Earth
is not completely fluid, either. The Earth's rotation carries the
Earth's bulges slightly ahead of the point directly beneath the Moon.
This means that the force between the Earth and the Moon is not exactly
along the line between their centers producing a torque on the Earth
and an accelerating force on the Moon. This causes a net transfer
of rotational energy from the Earth to the Moon, slowing down the
Earth's rotation by about 1.5 milliseconds/century and raising the
Moon into a higher orbit by about 3.8 centimeters per year. (The opposite
effect happens to satellites with unusual orbits such as Phobos
and Triton). The asymmetric nature
of this gravitational interaction is also responsible for the fact
that the Moon rotates synchronously, i.e. it is locked in phase with
its orbit so that the same side is always facing toward the Earth.
Just as the Earth's rotation is now being slowed by the Moon's influence
so in the distant past the Moon's rotation was slowed by the action
of the Earth, but in that case the effect was much stronger. When
the Moon's rotation rate was slowed to match its orbital period (such
that the bulge always faced toward the Earth) there was no longer
an off-center torque on the Moon and a stable situation was achieved.
The same thing has happened to most of the other satellites in the
solar system. Eventually, the Earth's rotation will be slowed to match
the Moon's period, too, as is the case with Pluto
and Charon.
 |
Actually, the Moon appears to wobble a bit (due to its slightly non-circular orbit) so that a few degrees of the far side can be seen from time to time, but the majority of the far side (left) was completely unknown until the Soviet spacecraft Luna 3 photographed it in 1959. (Note: there is no "dark side" of the Moon; all parts of the Moon get sunlight half the time (except for a few deep craters near the poles). Some uses of the term "dark side" in the past may have referred to the far side as "dark" in the sense of "unknown" (eg "darkest Africa; but even that meaning is no longer valid today!) |
The
Moon has no atmosphere. But evidence from Clementine suggested that
there may be water ice in some deep craters near the Moon's south
pole which are permanently shaded. This has now been confirmed by
Lunar Prospector. There is apparently ice at the north pole as well.
The cost of future lunar exploration just got a lot cheaper!
The Moon's crust averages 68 km thick and varies from essentially
0 under Mare Crisium to 107 km north of the crater Korolev on the
lunar far side. Below the crust is a mantle and probably a small core
(roughly 340 km radius and 2% of the Moon's mass). Unlike the Earth's
mantle, however, the Moon's is only partially molten. Curiously, the
Moon's center of mass is offset from its geometric center by about
2 km in the direction toward the Earth. Also, the crust is thinner
on the near side.
There are two primary types of terrain on the Moon: the heavily cratered
and very old highlands and the relatively smooth and younger maria.
The maria (which comprise about 16% of the Moon's surface) are huge
impact craters that were later flooded by molten lava. Most of the
surface is covered with regolith, a mixture of fine dust and rocky
debris produced by meteor impacts. For some unknown reason, the maria
are concentrated on the near side.
 |
A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs. These provide most of our detailed knowledge of the Moon. They are particularly valuable in that they can be dated. Even today, 20 years after the last Moon landing, scientists still study these precious samples. |
Most
rocks on the surface of the Moon seem to be between 4.6 and 3 billion
years old. This is a fortuitous match with the oldest terrestrial
rocks which are rarely more than 3 billion years old. Thus the Moon
provides evidence about the early history of the Solar System not
available on the Earth.
Most of the craters on the near side are named for famous figures
in the history of science such as Tycho,
Copernicus,
and Ptolemaeus. Features on the far have more modern references such
as Apollo, Gagarin and Korolev (with a distinctly Russian bias since
the first images were obtained by Luna 3). In addition to the familiar
features on the near side, the Moon also has the huge craters South
Pole-Aitken on the far side which is 2250 km in diameter and 12 km
deep making it the the largest impact basin in the solar system and
Orientale on the western limb (as seen from Earth; in the center of
the image at left) which is a splendid example of a multi-ring crater.
A total of 382 kg of rock samples were returned to the Earth by the
Apollo and Luna programs. These provide most of our detailed knowledge
of the Moon. They are particularly valuable in that they can be dated.
Even today, 20 years after the last Moon landing, scientists still
study these precious samples.
Prior to the study of the Apollo samples, there was no consensus about
the origin of the Moon. There were three principal theories: co-accretion
which asserted that the Moon and the Earth formed at the same time
from the Solar Nebula; fission which asserted that the Moon split
off of the Earth; and capture which held that the Moon formed elsewhere
and was subsequently captured by the Earth. None of these work very
well. But the new and detailed information from the Moon rocks led
to the impact theory: that the Earth collided with a very large object
(as big as Mars or more) and that the Moon formed from the ejected
material. There are still details to be worked out, but the impact
theory is now widely accepted.
The Moon has no global magnetic field. But some of its surface rocks
exhibit remanent magnetism indicating that there may have been a global
magnetic field early in the Moon's history.
With no atmosphere and no magnetic field, the Moon's surface is exposed
directly to the solar wind. Over its 4 billion year lifetime many
hydrogen ions from the solar wind have become embedded in the Moon's
regolith. Thus samples of regolith returned by the Apollo missions
proved valuable in studies of the solar wind. This lunar hydrogen
may also be of use someday as rocket fuel.
TOP OF PAGE
RECOMMENDED READING
XVI
Bascom, Willard. Waves and Beaches: The Dynamics of the Ocean Surface.
Doubleday, rev., 1980. Interesting description of conflict between
waves and beaches. Couper, Alastair, ed. The Times Atlas of the Oceans.
Van Nostrand, 1983. All aspects, with stunning maps and photos. Cousteau,
Jacques-Yves. The Ocean World of Jacques Cousteau. 20v. Danbury, 1975.
Comprehensive accounts of the sea-its legends, pollution, exploration,
resources, movements, and animal life. Duxbury, Alun C. and Duxbury,
Alison. The World Oceans: An Introduction. Addison-Wesley, 1984. New
approach at college level. Fodor, R. V. The Strange World of Deep
Sea Vents. Enslow, 1991. A brief, easy guide to the most significant
new discovery in deep-sea physical oceanography. Gross, M. Grant.
Oceanography. Merrill, 1989. Reliable brief treatment. Hamilton-Patterson,
James. The Great Deep. Random, 1992. A biography involving both recreational
and scientific oceanographic experiences. McGraw-Hill Encyclopedia
of Ocean and Atmospheric Sciences. McGraw, 1980. Drawn mostly from
the McGraw-Hill Encyclopedia of Science and Technology. Marx, Wesley.
The Oceans: Our Last Resource. Sierra Club, 1981. Thoughtful approach
to management of ocean resources. Melchior, Paul. The Tides of the
Planet Earth. Pergamon, 2d ed., 1983. Technical approach. Sea Frontiers:
Magazine of the International Oceanographic Foundation , 1954- . Oceans,
coastal areas, aquatic animals, plants, pollution, reviews. Singer,
S. Fred. The Ocean in Human Affairs. Paragon, 1987. Considers ocean
resources for an expanding world population. Wylie, Francis E. Tides
and the Pull of the Moon. S. Greene, 1979. The science and lore of
tides and influence on human affairs.
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WEB LINKS
XVI
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