The Earth’s Satelite (The Moon)
The study of the moon's surface increased with the
invention of the telescope by Galileo in 1610 and culminated in 1969 when the
first human actually set foot on the moon's surface. The physical
characteristics and surface of the moon thus have been studied telescopically,
photographically, and more recently by instruments carried by manned and
unmanned spacecraft (such as space probes and space rovers).
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The moon's diameter is about 3,476 km at the moon's
equator, a little more than 1/4 the earth's diameter.
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The moon has about 1/81 the mass of the earth and is
3/5 as dense.
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On the moon's surface the force of gravitation is
about 1/6 that on earth.
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It has been established that the moon completely lacks
an atmosphere, but several space probes have found evidence of water ice in the
soil.
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At its most extreme, the surface temperature can rise
to above 125°C (257°F) at lunar noon at the equator and can sink below - 245°C
( - 409°F) at night in the northern polar region.
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It is also more accurate to say that the earth and
moon together revolve about their common center of mass, rather than saying
that the moon revolves about the earth.
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The moon moves in a counterclockwise direction with an
average orbital speed of about 0.6 mi/sec (1 km/sec).
Characteristics of the Lunar surface
a)
The outer surface of the
moon:
The lunar
surface consists of two main zones;
i.
The mountainous
highlands which are the brighter regions on the moon, where the terrain is rough
and covered with rocky rubble.
The highlands
are densely scarred by thousands of craters (shallow circular depressions),
usually ringed by well-defined walls and often possessing a central peak.
Craters range in diameter from a few feet to many miles, and in some regions
there are so many that they overlap or several smaller craters lie within a
large crater.
ii.
The large, roughly circular lowlands or plains called Maria (sing. mare; from Lat., = sea) by
early astronomers, who erroneously believed them to be bodies of water.
The smooth floors of the Maria,
varying from flat to gently undulating, are covered by a thin layer of powdered
rock that darkens them.
Craters are also found on the Maria,
although there are nowhere near as many as in the lunar highlands.
Mare
and highland rocks differ in both appearance and chemical content. For example,
mare rocks are richer in iron and poorer in aluminum than highland rocks. The
maria consist largely of basalt, i.e., igneous rock formed from magma. In the
highlands the majority of the rocks are breccias—conglomerates formed from
basaltic rock.
Other
prominent surface features include the Rilles
and Rays.
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Rilles are sinuous, canyonlike clefts found near the
edges of mountain ranges.
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Rays are bright streaks radiating outward from certain
craters, such as Tycho.
b)
The internal structure of
the moon
Diffraction
of seismic waves provided the first clear-cut evidence for a lunar crust,
mantle, and core analogous (similar) to those of the earth.
The
lunar crust is about 70 km thick and the inner core has a radius of about 1,000
km, about 2/3 of the radius of the moon itself.
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