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*other moons named after Earth's moon, some markup, xref calamities |
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''View of [[Africa]], [[Antarctica]] and [[Arabian Peninsula]] from [[Apollo 17]]. This picture was taken as Apollo 17 left Earth [[orbit]] en route to the [[Luna|Moon]]. Taken on [[December 7]], [[1972]]. [http://www.wikipedia.com/upload/earth-apollo17.jpg click here for larger image]''
*Average orbital radius: 149,600,000 km (1 [[AU]]) from Sun
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Some planetologists believe that the Earth is only marginally able to support life. The evidence is that life is practically nonexistent at the poles, and increases in density from the poles to the equator. The so-called "temperate" zones are unable to support life for 1/3 to 1/4 of an orbit. Life in these zones exhibits extreme cold-adaptations including anti-freeze blood, exothermic metabolisms, insulation and long-term estivation.
'''The Moon'''
Earth is unique in its solar system in having "[[Luna|the Moon]]", a huge terrestrial planet-like satellite that is about 1/4 of Earth's diameter. The natural [[Moon|moons]] orbiting other planets are so called after Earth's moon, which is called [[Luna]].
By coincidence, the Moon is just far enough away to have, when seen from the Earth, the same apparent angular size as the Sun. This allows a total [[eclipse]] to occur on Earth.
Also, the Moon's rotation relative to its orbit around the Earth means it always presents the same face to the planet, seeming to disappear and reappear as it moves in and out of the Earth's shadow.
The origin of the Moon is presently unknown, but one popular theory has it that it was formed from the collision of a [[Mars (planet)|Mars]]-sized protoplanet into the early Earth. This theory explains (among other things) the moon's lack of iron and volatile elements. See [[Giant impact theory]].
Paleontological evidence shows that Earth's axial tilt is stabilized by tidal interactions with its moon. Without this stabilization, the rotational axis would be chaotically unstable, as it is with any sphere. If Earth's axis of rotation where to approach the plane of the ecliptic, extremely severe weather could result as one pole was continually heated and the other cooled. Planetologists who have studied the effect claim that this might kill all large animal and higher plant life. This remains a controversial subject, however, and further studies of Mars - which shares Earth's rotation period and axial tilt, but not its large moon - may provide additional information.
Earth is the only planet in our [[solar system]], or the known universe whose surface has liquid [[water]]. Earth's solar orbit, vulcanism, gravity, greenhouse effect, magnetic field and oxygen atmosphere seem to combine to make Earth a water planet. Earth is actually beyond the outer edge of the orbits which would be warm enough to form liquid water. Without the greenhouse effect, the Earth's water would freeze.
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Without Lunar tides, some planetologists speculate that Earth's crust would have solidified. Its crust would have accreted all available carbon as limestone. This would remove the carbon needed by the biosphere. As the greenhouse effect failed, the result would be an ice planet similar to mars. The oxygen would all combine with surface minerals, and then as the water sublimed, the hydrogen would be cracked from the water and scoured away by the solar wind, creating a desert.
<b>Map references:</b>
World, Time Zones
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*<i>note:</i> boundary situations with neighboring states prevent many countries from extending their fishing or economic zones to a full 200 nm; 43 nations and other areas that are landlocked include [[Afghanistan]], [[Andorra]], [[Armenia]], [[Austria]], [[Azerbaijan]], [[Belarus]], [[Bhutan]], [[Bolivia]], [[Botswana]], [[Burkina Faso]], [[Burundi]], [[Central African Republic]], [[Chad]], [[Czech Republic]], [[Ethiopia]], [[Holy See]] (Vatican City), [[Hungary]], [[Kazakhstan]], [[Kyrgyzstan]], [[Laos]], [[Lesotho]], [[Liechtenstein]], [[Luxembourg]], [[Malawi]], [[Mali]], [[Moldova]], [[Mongolia]], [[Nepal]], [[Niger]], [[Paraguay]], [[Rwanda]], [[San Marino]], [[Slovakia]], [[Swaziland]], [[Switzerland]], [[Tajikistan]], [[Macedonia|The Former Yugoslav Republic of Macedonia]], [[Turkmenistan]], [[Uganda]], [[Uzbekistan]], [[West Bank]], [[Zambia]], [[Zimbabwe]]
two large areas of polar climates separated by two rather narrow temperate zones from a wide equatorial band of tropical to subtropical climates. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter.
the greatest ocean depth is the [[Mariana Trench]] at 10,924 m in the [[Pacific Ocean]]
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*''highest point:'' [[Mount Everest]] [[1e3 m|8,850 m]] (1999 est.)
*The Earth's crust contains large deposits of [[fossil fuel]]s ([[coal]], [[oil]], [[natural gas]], [[methane clathrate]]). These deposits are used both for energy production and as feedstock for chemical production.
*Mineral [[ore]] bodies have been formed in the Earth's crust by the action of [[erosion]] and [[plate tectonics]]. These ore bodies form concentrated sources for many metals and other useful elements.
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Some of these resources are difficult to replenish on a short time scale, called "non-renewable" resources. The exploitation of non-renewable resources by human civilization has become a subject of significant controversy in modern [[environmentalism]] movements.
*<i>arable land:</i> 10%
*<i>permanent crops:</i> 1%
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2,481,250 sq km (1993 est.)
large areas subject to overpopulation, industrial disasters, pollution (air, water, [[acid rain]], toxic substances), loss of vegetation ([[overgrazing]], [[deforestation]], [[desertification]]), loss of wildlife, soil degradation, soil depletion, [[erosion]]
6,080,671,215 (July 2000 est.)
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<b>Sex ratio:</b>
*
*<i>under 15 years:</i> 1.05 male(s)/female
*<i>15-64 years:</i> 1.02 male(s)/female
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2.8 children born/woman (2000 est.)
<b>Data code:</b>
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*<i>services:</i> NA%
*<i>lowest 10%:</i> NA%
*<i>highest 10%:</i> NA%
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