■The name
Earth originates from the 8th century Anglo-Saxon word erda, which
means ground or soil. In Old English the word
became eorthe, then erthe in Middle English. Earth was first used as
the name of the sphere of the Earth around 1400. It is the only
planet whose name in English is not derived from Greco-Roman
mythology.
■In the
ancient past there were varying levels of belief in a flat Earth,
with the Mesopotamian culture portraying the world as a flat disk
afloat in an ocean.
■Ozone layer together with Earth's
magnetic field, blocks harmful radiation, permitting life on land.
■The
distance of the Earth from the Sun, as well as its orbital
eccentricity, rate of rotation, axial tilt, geological history,
sustaining atmosphere and protective magnetic field all contribute
to the conditions necessary to originate and sustain life on this
planet.
■The
Earth's axis of rotation is tilted 23.4° away from the
perpendicular to its orbital plane, producing seasonal
variations on the planet's surface with a period of one tropical
year (365.24 solar days).
■Outgassing
and volcanic activity produced the primordial atmosphere of the
Earth.
■Condensing
water vapor, augmented by ice and liquid water delivered by
asteroids and the larger proto-planets, comets, and trans-Neptunian
objects produced the oceans.
■The
highly energetic chemistry is believed to have produced a
self-replicating molecule around 4 billion years ago, and half a
billion years later, the last common ancestor of all life existed.
■Earth has
the highest density, the highest surface gravity and the strongest
magnetic field and It is the largest of the four solar terrestrial
planets, both in terms of size and mass.
■The
magnetic field forms the magnetosphere, which deflects particles in
the solar wind. The sunward edge of the bow shock is located at
about 13 times the radius of the Earth. The collision between the
magnetic field and the solar wind forms the Van Allen radiation
belts, a pair of concentric, torus-shaped regions of energetic
charged particles. When the plasma enters the Earth's atmosphere at
the magnetic poles, it forms the aurora.
■on
average it takes 24 hours—a solar day—for Earth to complete a full
rotation about its axis so that the Sun returns to the meridian. The
orbital speed of the Earth averages about 30 km/s (108,000 km/h),
which is fast enough to cover the planet's diameter (about 12,600
km) in seven minutes, and the distance to the Moon (384,000 km) in
four hours.
■The angle
of the Earth's tilt is relatively stable over long periods of time.
However, the tilt does undergo a slight, irregular motion (known as
nutation) with a main period of 18.6 years. The orientation (rather
than the angle) of the Earth's axis also changes over time,
precessing around in a complete circle over each 25,800 year cycle;
this caused by the varying attraction of the Sun and Moon on the
Earth's equatorial bulge.
■The
Hill sphere (gravitational sphere of influence)
of the Earth is about 1.5 Gm (or 1,500,000
kilometers) in radius. This is maximum distance
at which the Earth's gravitational influence is
stronger than the more distant Sun and planets.
Objects must orbit the Earth within this radius,
or they can become unbound by the gravitational
perturbation of the Sun.
■In
total, about 400 people have been outside the
Earth's atmosphere as of 2004, and, of these,
twelve have walked on the Moon. Normally the
only humans in space are those on the
International Space Station. The station's crew
of three people is usually replaced every six
months.
■Earth
was first photographed from space by Explorer 6
in 1959.
■Yuri
Gagarin became the first human to view Earth
from space in 1961.
■The
crew of the Apollo 8 was the first to view an
Earth-rise from lunar orbit in 1968. In 1972 the
crew of the Apollo 17 produced the famous "Blue
Marble" photograph of the planet Earth from
cislunar space(the most widely distributed image
in human history).
Source: Wikipedia
- NASA
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