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The Earth is the third planet from the Sun, and the fifth-largest of the eight planets in the Solar
System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the World or
by its Latin name, Terra.
Home
to millions of species including humans, Earth is currently the only place in the universe where life is known to exist. The
planet formed 4.54 billion years ago, and life appeared on its surface within a billion years. Since then, Earth's biosphere
has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic
organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful solar radiation,
permitting life. The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist
during this period. Without intervention, the planet could be expected to continue supporting life for between 0.5 and 2.3
billion years, after which the rising luminosity and expansion of the Sun—as a result of the gradual but inexorable
depletion of its hydrogen fuel—would eventually eliminate the planet's biosphere.
Earth's
outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods
of many millions of years. About 71% of the surface is covered with salt water oceans, the remainder consisting of continents
and islands which together have many lakes and other sources of water contributing to the hydrosphere. Water, necessary
for all known life, is not known to exist on any other planet's surface. Earth's poles are mostly covered with solid ice (Antarctic
ice sheet) or sea ice (Arctic ice cap). The planet's interior remains active, with a thick layer of relatively solid mantle,
a liquid outer core that generates a magnetic field, and a solid iron inner core.
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Earth interacts with other objects in space, especially
the Sun and the Moon. At present, Earth orbits the Sun once for every 366.26 times it rotates about its axis. The
Earth's axis of rotation is tilted 23.4° away from the perpendicular orbital plane, producing seasonal variations
on the planet's surface within a period of one year (365.24 solar days). Earth's only known natural satellite, the Moon,
which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows
the planet's rotation. Between approximately 3.8 billion and 4.1 billion years ago, numerous asteroid impacts caused significant
changes to the greater surface environment.
Both
the mineral resources of the planet, as well as the products of the biosphere, contribute resources that are used to support
a global human population. These inhabitants are grouped into about 200 independent sovereign states, which interact through
diplomacy, travel, trade, and military action. Human cultures have developed many views of the planet, including personification
as a deity, a belief in a flat Earth or in Earth as the center of the universe, and a modern perspective of the world as an
integrated environment that requires stewardship.
EARTH (Click on photo's
to enlarge )
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