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MOON,
name given to the natural satellite of the Earth, and sometimes
applied to the satellites of the other planets in the solar system.
The diameter of the moon is about 3480 km (about 2160 mi), or about
one-fourth that of earth, and the moon’s volume is about
one-fiftieth that of the earth. The mass of the earth is 81 times
greater than the mass of the moon. Thus the average density of the
moon is only three-fifths, and the pull of gravity at the lunar
surface only one-sixth that of the earth. The moon has no free water
and essentially no atmosphere, and no weather exists to change its
surface; yet it is not totally inert.
| BRIEF SURVEY OF THE MOON |
| Mean Distance from earth |
384,403 km (238,856 mi) |
| Diameter |
3480 km (2160 mi) |
| Period of revolution |
27.322 earth days |
| Eccentricity of orbit |
0.055 |
| Inclination of orbit |
5°9´ |
| Rotation period (sidereal day) |
27.322 earth days |
| Period of phases |
29.53 earth days |
| Mass (earth = 1) |
0.012 |
| Mean density (water = 1) |
0.605 |
The moon moves about the earth at an average distance of 384,403
km (238,856 mi), and at an average speed of 3700 km per hr (about
2300 mph). It completes one revolution in an elliptical orbit about
the earth in 27 days, 7 hr, 43 min, and 11.5 sec with reference
to the stars. For the moon to go from one phase to the next similar
phase, or one lunar month, requires 29 days, 12 hr, 44 min, and
2.8 sec. The moon rotates on its axis once in about the same period
of time that elapses for its sidereal period of revolution, accounting
for the fact that virtually the same portion of the moon is always
turned toward the earth. Although the moon appears bright to the
eye, it reflects into space only 7 percent of the light that falls
on it. The reflectivity, or albedo, of 0.07 is similar to that of
coal dust.
At any one time, an observer can see only 50 percent of the moon’s
entire surface. However, an additional 9 percent can be seen from
time to time around the apparent edge because of the relative motion
called libration. This is caused by slightly different angles of
view from the earth at different relative positions of the moon
along its inclined elliptical orbit.
The moon shows progressively different phases as it moves
along its orbit around the earth. Half the moon is always in sunlight,
just as half the earth has day while the other half has night. The
phases of the moon depend on how much of the sunlit half can be
seen at any one time. In the phase called the new moon, the face
is completely in shadow. About a week later, the moon is in first
quarter, resembling a luminous half-circle; another week later,
the full moon shows its fully lighted surface; a week afterward,
in its last quarter, the moon appears as a half-circle again. The
entire cycle is repeated each lunar month. The moon is full when
it is farther away from the sun than the earth; it is new when it
is closer. When it is more than half-illuminated, it is said to
be in gibbous phase. The moon is said to be waning when it progresses
from full to new, and to be waxing as it proceeds again to full.
Temperatures on its surface are extreme, ranging from a maximum
of 127° C (261° F) at lunar noon to a minimum
of –173° C (–279° F) just before
lunar dawn.
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New
Moon |
Crescent
Moon |
Quarter
Moon |
Full
Moon |
Last
Quarter |
Crescent
Moon |
New
Moon |
The full moon nearest the autumnal equinox ushers in a period
of several successive days when it rises soon after sunset. Because
this phenomenon gives farmers in temperate latitudes extra hours
of light in which to harvest their crops before the first frost,
it is known as the harvest moon. The hunter’s moon, the
next full moon after the harvest moon, is accompanied by a similar
but less marked phenomenon.
Throughout the 19th and 20th centuries, visual exploration
through powerful telescopes has yielded a fairly comprehensive picture
of the visible side of the moon. The hitherto unseen far side of
the moon was first revealed to the world in October 1959 through
photographs made by the Soviet Lunik III spacecraft.
These photographs showed that the far side of the moon is similar
to the near side except that large lunar maria are absent. Craters
are now known to cover the entire moon, ranging in size from huge,
ringed maria to those of microscopic size. Photographs from U.S.
spacecraft—Ranger 7, 8, and 9 and Orbiter
1 and 2—launched by the National
Aeronautics and Space Administration (NASA) in 1964 and 1966 further
supported these conclusions. The entire moon has about 3 trillion
craters larger than about 1 m (3.32 ft) in diameter.
The successful landings of unmanned spacecraft of the Surveyer
series by the U.S. and the Luna series by the USSR in the 1960s,
and, finally, the manned landings on the lunar surface as part of
the U.S. Apollo program made direct measurement of the physical
and chemical properties of the moon a reality. The Apollo astronauts
collected rocks, took thousands of photographs, and set up instruments
on the moon that sent information back to earth by radio telemetry.
These instruments measured temperature and gas pressure at the lunar
surface; the heat flow from the moon’s interior; molecules
and ions of hot gases streaming out from the atmosphere of the sun,
called the solar wind; the magnetic field and gravity of the moon;
seismic vibrations of the lunar surface caused by so-called moonquakes,
landslides, and meteoroid impacts; and, through laser beams, the
precise distance between the earth and the moon.
All six manned landings on the moon—Apollo
11, 12, and 14–17—returned
samples of rock and soil to earth, weighing 384 kg (846 lb) in all.
It was not until the final mission, Apollo 17, that
the astronaut crew included a geologist, H. H. Schmitt (1935– ).
The scientist spent 22 hours exploring the Taurus-Littrow Valley
region, covering 35 km (22 mi) in a lunar roving vehicle.
In January 1998 the Lunar Prospector probe
sent into orbit around the moon—NASA’s first lunar
mission in 25 years—detected water ice at the moon’s
poles, confirming an indication by the U.S. Defense Department’s Clementine probe
in 1994. The amount estimated was seen as a potential resource for
future lunar research colonies.
Ancient observers of the moon believed that the dark regions
on its face were oceans, giving rise to the Latin name mare (“sea”),
which is still used today; the brighter regions were likewise held
to be continents. Modern observation and exploration of the moon has
yielded far more comprehensive and specific knowledge. Since the
Renaissance, telescopes have revealed a wealth of lunar detail,
and lunar spacecraft have contributed further to this knowledge.
Features discernible on the surface of the moon include craters,
mountain ranges, plains or maria, faults, domes, rilles, and rays.
The largest distinct crater, called Bailly, is about 295 km (about
183 mi) wide and 3960 m (about 13,000 ft) deep. The largest mare
or sea is Mare Imbrium (Sea of Rains), about 1200
km (about 750 mi) wide. The highest mountains, in the Leibnitz and
Doerfel ranges near the south pole of the moon, have peaks up to 6100
m (about 20,000 ft) in height, comparable to the Himalayas on earth.
Craters as small as 1.6 km (1 mi) across have been defined in telescopic
observations. The origin of lunar craters was long debated; the
latest evidence indicates that nearly all craters were formed by
explosive impacts of high-velocity meteorites or small asteroids,
mostly during the early part of lunar history, when the solar system
still contained many such fragments. Some craters, rilles, and domes,
however, display characteristics of indisputable volcanic origin.
It is now known, from measuring the ages of lunar rocks retrieved
during lunar missions, that the moon is about 4,600,000,000 years
old, or about the same age as the earth and probably the rest of
the solar system. Rocks from the lunar maria were formed when molten
rock solidified between 3,160,000,000 and 3,960,000,000 years ago.
These rocks most nearly resemble terrestrial basalts, a volcanic
rock type widely distributed on earth, but with certain important
differences. Evidence indicates that the lunar highlands, or continents,
may be formed of a less dense plutonic igneous
rock called anorthosite, which consists almost entirely of the mineral
plagioclase. Other important lunar sample types include glasses,
breccias (complex assemblages of rock fragments cemented together
by heat or pressure, or both), and the soils or regolith (finely
divided rock fragments produced by many millions of years of meteoritic
bombardment).
The moon’s magnetic field is not as strong or widespread
as that of the earth. Some lunar rocks are weakly magnetic, indicating
that they solidified in a somewhat stronger magnetic field. Magnetic
and other measurements indicate an internal temperature of the moon
as high as 1600° C (2912° F), which is above the melting
point of most lunar rocks. Evidence from seismic recordings suggests
that some regions near the lunar center may be liquid.
Seismometers operating on the lunar surface have also recorded
signals of between 70 and 150 meteorite impacts per year, with masses
from 100 g to 1000 kg (0.22 to 2200 lb). Hence the moon is still
being bombarded by meteoroids, although not as often as in the past,
and this may be a problem for engineers who design permanent bases
for the lunar surface. The surface is covered by a layer of rubble,
which may be several kilometers deep in the maria and of as yet
unknown depth in the highlands. The rubble zone is believed to have
been formed by the impacts of meteoroids.
Before the modern age of space exploration, scientists had
three major theories for the origin of the moon: fission from the
earth; formation in earth orbit; and formation far from earth. Then,
in 1975, having studied moon rocks and close-up pictures of the
moon, scientists proposed what has come to be regarded as the most
probable of the theories of formation, planetesimal impact.
The modern version of this theory proposes that the moon was
spun off from the earth when the earth was young and rotating rapidly
on its axis. This idea gained support partly because the density
of the moon is the same as that of the rocks just below the crust,
or upper mantle, of the earth. A major difficulty with this theory
is that the angular momentum of the earth, in order to achieve rotational
instability, would have to have been much greater than the angular
momentum of the present earth-moon system.
This theory proposes that the earth and moon, and all other
bodies of the solar system, condensed independently out of the huge
cloud of cold gases and solid particles that constituted the primordial
solar nebula. Much of this material finally collected at the center
to form the sun.
According to this theory, independent formation of the earth
and moon, as in the above theory, is assumed; but the moon is supposed
to have formed at a different place in the solar system, far from
earth. The orbits of the earth and moon then, it is surmised, carried
them near each other so that the moon was pulled into permanent
orbit about the earth.
First published in 1975, this theory proposes that early in
the earth’s history, well over 4 billion years ago, the
earth was struck by a large body called a planetesimal, about the
size of Mars. The catastrophic impact blasted portions of the earth
and the planetesimal into earth orbit, where debris from the impact
eventually coalesced to form the moon. This theory, after years
of research on moon rocks in the 1970s and ’80s, has became
the most widely accepted one for the moon’s origin. The major
problem with the theory is that it would seem to require that the
earth melted throughout, following the impact, whereas the earth’s
geochemistry does not indicate such a radical melting.