Earth (or
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, most
massive, and densest of the Solar System's four
terrestrial (or
rocky) planets. It is sometimes referred to
as
the World, the
Blue
Planet, or
Terra.
Home to millions of
species, including
humans, Earth is 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 radiation, permitting life on land. The physical properties
of the Earth, as well as its geological history and orbit, allowed
life to persist during this period. The world is expected to
continue supporting life for another 1.5 billion years, after which
the rising luminosity of the Sun will eliminate the
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; liquid water, necessary for
all known life, is not known to exist on any other planet's
surface. Earth'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.
Earth interacts with other objects in
outer
space, including the Sun and the
Moon. At
present, Earth orbits the Sun once for every roughly 366.26 times
it rotates about its axis. This length of time is a
sidereal year, which is equal to 365.26
solar days. 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). 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
4.1 and 3.8 billion years ago,
asteroid
impacts during the
Late Heavy
Bombardment caused significant changes to the 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. The 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 being the center of the universe, and
a modern perspective of the world as an integrated environment that
requires stewardship.
Chronology
Scientists have been able to reconstruct detailed information about
the planet's past. The earliest dated Solar System material is
dated to 4.5672 ± 0.0006 billion years ago, and
by 4.54 billion years ago (within an uncertainty of 1%)
theEarth and the other planets in the Solar System formed out of
the
solar nebula—a disk-shaped mass of
dust and gas left over from the formation of the Sun. This assembly
of the Earth through accretion was largely completed within
10–20 million years. Initially
molten, the outer layer of the planet Earth cooled to
form a solid crust when water began accumulating in the atmosphere.
The Moon formed shortly thereafter, 4.53 billion years
ago, most likely as the result of a Mars-sized object (sometimes
called
Theia) with about 10%
of the Earth's mass impacting the Earth in a glancing blow. Some of
this object's mass would have merged with the Earth and a portion
would have been ejected into space, but enough material would have
been sent into orbit to form the Moon.
Outgassing and
volcanic activity produced
the primordial atmosphere. 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
newly-formed Sun was only 70% of its present
luminosity, yet evidence shows that the
early oceans remained liquid—a contradiction dubbed the
faint young Sun paradox. A
combination of
greenhouse gases and
higher levels of
solar activity
served to raise the Earth's surface temperature, preventing the
oceans from freezing over.
Two major models have been proposed for the rate of continental
growth: steady growth to the present-day and rapid growth early in
Earth history.Current research shows that the second option is most
likely, with rapid initial growth of continental crust followed by
a long-term steady continental area. On
time scales lasting hundreds of millions
of years, the surface continually reshaped itself as continents
formed and broke up. The continents migrated across the surface,
occasionally combining to form a
supercontinent. Roughly
750 million years ago (
Ma), one of
the earliest known supercontinents,
Rodinia,
began to break apart. The continents later recombined to form
Pannotia, 600–540 Ma, then finally
Pangaea, which broke apart
180 Ma.
Evolution of life
At present, Earth provides the only example of an environment that
has given rise to the
evolution of life.
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. The development of
photosynthesis allowed the Sun's energy to be
harvested directly by life forms; the resultant oxygen accumulated
in the atmosphere and formed in a layer of
ozone (a form of
molecular
oxygen [O
3]) in the upper atmosphere. The
incorporation of smaller cells within larger ones resulted in the
development of complex cells
called
eukaryotes. True multicellular
organisms formed as cells within
colonies became increasingly specialized.
Aided by the absorption of harmful
ultraviolet radiation by the
ozone layer, life colonized the surface of
Earth.
Since the 1960s, it has been hypothesized that severe
glacial action between 750 and 580 Ma, during
the
Neoproterozoic, covered much of
the planet in a sheet of ice. This hypothesis has been termed
"
Snowball Earth", and is of
particular interest because it preceded the
Cambrian explosion, when multicellular
life forms began to proliferate.
Following the Cambrian explosion, about 535 Ma, there have
been five
mass extinctions. The
last
extinction event was 65 Ma, when a meteorite collision
probably triggered the extinction of the (non-avian)
dinosaurs and other large reptiles, but spared
small animals such as
mammals, which then
resembled shrews. Over the past 65 million years, mammalian
life has diversified, and several million years ago, an African
ape-like animal such as
Orrorin
tugenensis gained the ability to stand upright. This
enabled tool use and encouraged communication that provided the
nutrition and stimulation needed for a larger brain. The
development of agriculture, and then civilization, allowed humans
to influence the Earth in a short time span as no other life form
had, affecting both the nature and quantity of other life
forms.
The present pattern of
ice ages began about
40 Ma and then intensified during the
Pleistocene about 3 Ma. The polar regions
have since undergone repeated cycles of glaciation and thaw,
repeating every 40–100,000 years. The last ice age ended
10,000 years ago.
Future
The future of the planet is closely
tied to that of the Sun. As a result of the steady accumulation of
helium at the Sun's core, the
star's
total luminosity will slowly increase. The luminosity of the
Sun will grow by 10% over the next 1.1
Gyr (1.1 billion years) and by 40% over the
next 3.5 Gyr. Climate models indicate that the rise in
radiation reaching the Earth is likely to have dire consequences,
including the possible loss of the planet's oceans.
The Earth's increasing surface temperature will accelerate the
inorganic CO2 cycle, reducing its
concentration to lethal levels for plants (10
ppm for
C4
photosynthesis) in an estimated 900 million years. The lack of
vegetation will result in the loss of oxygen in the atmosphere, so
animal life will become extinct within several million more
years.Ward and Brownlee (2002). After another billion years all
surface water will have disappeared and the mean global temperature
will reach 70 °C(158 °F). The Earth is expected to be
effectively habitable for about another 500 million years, although
this may be extended up to if the nitrogen is removed from the
atmosphere. Even if the Sun were eternal and stable, the continued
internal cooling of the Earth would result in a loss of much of its
CO
2 due to reduced
volcanism,
and 35% of the water in the oceans would descend to the
mantle due to reduced steam venting from
mid-ocean ridges.
The Sun, as part of its
evolution,
will become a
red giant in about 5 Gyr.
Models predict that the Sun will expand out to about 250 times its
present radius, roughly .
See also Earth's fate is less clear. As a red giant, the Sun will
lose roughly 30% of its mass, so, without tidal effects, the Earth
will move to an orbit from the Sun when the star reaches it maximum
radius. Therefore, the planet is expected to escape envelopment by
the expanded Sun's sparse outer atmosphere, though most, if not
all, remaining life will be destroyed because of the Sun's
increased luminosity. However, a more recent simulation indicates
that Earth's orbit will decay due to tidal effects and drag,
causing it to enter the red giant Sun's atmosphere and be
destroyed.
Composition and structure
Earth is a terrestrial planet, meaning that it is a rocky body,
rather than a
gas giant like
Jupiter. It is the largest of the four solar
terrestrial planets, both in terms of size and mass. Of these four
planets, Earth also has the highest density, the highest
surface gravity, the strongest magnetic
field, and fastest rotation. It also is the only terrestrial planet
with active
plate tectonics.
Shape
The shape of the Earth is very close to that of an
oblate spheroid, a sphere squished along the
orientation from pole to pole such that there is a
bulge around the
equator.This bulge results from the
rotation of the Earth, and causes the diameter at
the equator to be 43 km larger than the
pole to pole diameter.
The average diameter
of the reference spheroid is about 12,742 km, which is
approximately 40,000 km/π, as the meter was originally defined as 1/10,000,000 of the
distance from the equator to the North Pole
through Paris
,
France.
Local
topography deviates from this
idealized spheroid, though on a global scale, these deviations are
very small: Earth has a
tolerance of about one part in about
584, or 0.17%, from the reference spheroid, which is less than the
0.22% tolerance allowed in
billiard
balls.
The largest local deviations in the rocky
surface of the Earth are Mount Everest
(8,848 m above local sea level) and the
Mariana
Trench
(10,911 m below local sea level).
Because of
the equatorial bulge, the feature farthest from the center of the
Earth is actually Mount Chimborazo
in Ecuador
.
Chemical composition
The mass of the Earth is approximately 5.98 kg. It is
composed mostly of
iron (32.1%), oxygen
(30.1%),
silicon (15.1%),
magnesium (13.9%),
sulfur
(2.9%),
nickel (1.8%),
calcium (1.5%), and
aluminium (1.4%); with the remaining 1.2%
consisting of trace amounts of other elements. Due to
mass segregation, the core region is
believed to be primarily composed of iron (88.8%), with smaller
amounts of nickel (5.8%), sulfur (4.5%),and less than 1% trace
elements.
The geochemist
F. W. Clarke calculated that a little
more than 47% of the Earth's crust consists of oxygen. The more
common rock constituents of the Earth's crust are nearly all
oxides; chlorine, sulfur and fluorine are the only important
exceptions to this and their total amount in any rock is usually
much less than 1%. The principal oxides are silica, alumina, iron
oxides, lime, magnesia, potash and soda. The silica functions
principally as an acid, forming silicates, and all the commonest
minerals of igneous rocks are of this nature. From a computation
based on 1,672 analyses of all kinds of rocks, Clarke deduced that
99.22% were composed of 11 oxides (see the table at right.) All the
other constituents occur only in very small quantities.
Internal structure
The interior of the Earth, like that of the other terrestrial
planets, is divided into layers by their
chemical or physical (
rheological) properties. The outer layer of the
Earth is a chemically distinct
silicate solid
crust,
which is underlain by a highly viscous solid mantle. The crust is
separated from the mantle by the
Mohorovičić
discontinuity, and the thickness of the crust varies: averaging
6 km under the oceans and 30–50 km on the continents. The
crust and the cold, rigid, top of the
upper
mantle are collectively known as the
lithosphere, and it is of the lithosphere that
the
tectonic plates are comprised.
Beneath the lithosphere is the
asthenosphere, a relatively low-viscosity
layer on which the lithosphere rides. Important changes in crystal
structure within the mantle occur at 410 and 660 kilometers
below the surface, spanning a
transition
zone that separates the upper and lower mantle. Beneath the
mantle, an extremely low viscosity liquid
outer core lies above a solid
inner core. The inner core may rotate at a
slightly higher
angular velocity
than the remainder of the planet, advancing by 0.1–0.5° per
year.
Geologic layers of the Earth
Earth cutaway from core to exosphere. Not to scale. |
Depth
km |
Component Layer |
Density
g/cm3 |
| 0–60 |
Lithosphere |
— |
| 0–35 |
... Crust |
2.2–2.9 |
| 35–60 |
... Upper mantle |
3.4–4.4 |
| 35–2890 |
Mantle |
3.4–5.6 |
| 100–700 |
... Asthenosphere |
— |
| 2890–5100 |
Outer core |
9.9–12.2 |
| 5100–6378 |
Inner core |
12.8–13.1 |
Heat
Earth's
internal heat comes from a
combination of
residual
heat from planetary accretion (about 20%) and heat produced
through
radioactive decay (80%).
The major heat-producing isotopes in the Earth are
potassium-40,
uranium-238,
uranium-235, and
thorium-232. At the center of the planet, the
temperature may be up to 7,000 K and the pressure could reach
360
GPa. Because much of the heat is
provided by radioactive decay, scientists believe that early in
Earth history, before isotopes with short half-lives had been
depleted, Earth's heat production would have been much higher. This
extra heat production, twice present-day at approximately
3 billion years ago, would have increased temperature
gradients within the Earth, increasing the rates of
mantle convection and
plate tectonics, and allowing the production
of igneous rocks such as
komatiites that
are not formed today.
Present-day major heat-producing isotopes
| Isotope |
Heat release
W/kg
isotope
|
Half-life
years
|
Mean mantle concentration
kg isotope/kg
mantle
|
Heat release
W/kg mantle
|
| 238U |
|
|
|
|
| 235U |
|
|
|
|
| 232Th |
|
|
|
|
| 40K |
|
|
|
|
Total heat loss from the earth is . A portion of the core's thermal
energy is transported toward the crust by
Mantle plumes; a form of convection consisting
of upwellings of higher-temperature rock. These plumes can produce
hotspots and
flood basalts.More of the heat in the Earth is
lost through plate tectonics, by mantle upwelling associated with
mid-ocean ridges. The final major mode of heat loss is through
conduction through the lithosphere, majority of which occurs in the
oceans due to the crust there being much thinner than that of the
continents.
Tectonic plates
The mechanically rigid outer layer of the Earth, the lithosphere,
is broken into pieces called
tectonic plates. These plates are
rigid segments that move in relation to one another at one of three
types of plate boundaries:
Convergent boundaries, at which two
plates come together,
Divergent
boundaries, at which two plates are pulled apart, and
Transform boundaries, in which two plates
slide past one another laterally.
Earthquakes, volcanic activity,
mountain-building, and
oceanic trench formation can occur along
these plate boundaries.The tectonic plates ride on top of the
asthenosphere, the solid but less-viscous part of the upper mantle
that can flow and move along with the plates,and their motion is
strongly coupled with patterns convection inside the
Earth's mantle.
As the tectonic plates migrate across the planet, the ocean floor
is
subducted under the leading edges of
the plates at convergent boundaries. At the same time, the
upwelling of mantle material at divergent boundaries creates
mid-ocean ridges. The combination of
these processes continually recycles the
oceanic crust back into the mantle. Because of
this recycling, most of the ocean floor is less than 100 million
years in age. The oldest oceanic crust is located in the Western
Pacific, and has an estimated age of about 200 million years. By
comparison, the oldest dated continental crust is 4030 million
years old.
Other
notable plates include the Indian
Plate, the Arabian Plate, the
Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean
. The Australian Plate actually fused with
Indian Plate between 50 and 55 million years ago. The
fastest-moving plates are the oceanic plates, with the
Cocos Plate advancing at a rate of 75 mm/yr
and the Pacific Plate moving 52–69 mm/yr. At the other
extreme, the slowest-moving plate is the Eurasian Plate,
progressing at a typical rate of about 21 mm/yr.
Surface
The Earth's
terrain varies greatly from
place to place. About 70.8% of the surface is covered by water,
with much of the
continental shelf
below sea level. The submerged surface has mountainous features,
including a globe-spanning
mid-ocean
ridge system, as well as undersea
volcanoes,
oceanic
trenches,
submarine canyons,
oceanic plateaus and
abyssal plains. The remaining 29.2% not
covered by water consists of
mountains,
deserts,
plains,
plateaus, and other
geomorphologies.
The planetary surface undergoes reshaping over geological time
periods due to the effects of
tectonics and erosion. The surface
features built up or deformed through plate tectonics are subject
to steady
weathering from
precipitation, thermal cycles,
and chemical effects.
Glaciation,
coastal erosion, the build-up of
coral reefs, and large meteorite impacts
also act to reshape the landscape.
The
continental crust consists of
lower density material such as the
igneous
rocks granite and
andesite. Less common is
basalt, a denser volcanic rock that is the primary
constituent of the ocean floors.
Sedimentary rockis formed from the
accumulation of sediment that becomes compacted together. Nearly
75% of the continental surfaces are covered by sedimentary rocks,
although they form only about 5% of the crust. The third form of
rock material found on Earth is
metamorphic rock, which is created from the
transformation of pre-existing rock types through high pressures,
high temperatures, or both. The most abundant silicate minerals on
the Earth's surface include
quartz, the
feldspars,
amphibole,
mica,
pyroxene and
olivine. Common
carbonate minerals include
calcite (found in
limestone),
aragonite and
dolomite.
The
pedosphere is the outermost layer of
the Earth that is composed of
soil and subject
to
soil formation processes. It exists
at the interface of the
lithosphere,
atmosphere,
hydrosphere and biosphere.
Currently the total arable land is 13.31% of the land surface, with
only 4.71% supporting permanent crops. Close to 40% of the Earth's
land surface is presently used for cropland and pasture, or an
estimated 1.3 km² of cropland and 3.4 km² of
pastureland.
The
elevation of the land surface of the Earth varies from the low
point of −418 m at the Dead Sea
, to a
2005-estimated maximum altitude of 8,848 m at the top of
Mount
Everest. The mean height of land above sea level is
840 m.
Hydrosphere
The abundance of water on Earth's surface is a unique feature that
distinguishes the "Blue Planet" from others in the Solar System.
The Earth's hydrosphere consists chiefly of the oceans, but
technically includes all water surfaces in the world, including
inland seas, lakes, rivers, and underground waters down to a depth
of 2,000 m.
The deepest underwater location is Challenger Deep
of the Mariana Trench in the Pacific Ocean
with a depth of −10,911.4 m. The
average depth of the oceans is 3,800 m, more than four times
the average height of the continents.
The mass of the oceans is approximately 1.35
metric tons, or about 1/4400 of the total mass of
the Earth, and occupies a volume of 1.386 km
3. If
all of the land on Earth were spread evenly, water would rise to an
altitude of more than 2.7 km.The total volume of the Earth's
oceans is: 1.4 km
3. The total surface area of the
Earth is 5.1 km². So, to first approximation, the average
depth would be the ratio of the two, or 2.7 km. About 97.5% of
the water is saline, while the remaining 2.5% is fresh water. The
majority of the fresh water, about 68.7%, is currently in the form
of ice.
About 3.5% of the total mass of the oceans consists of
salt. Most of this salt was released from volcanic
activity or extracted from cool, igneous rocks. The oceans are also
a reservoir of dissolved atmospheric gases, which are essential for
the survival of many aquatic life forms. Sea water has an important
influence on the world'sclimate, with the oceans acting as a large
heat reservoir. Shifts in the oceanic
temperature distributioncan cause significant weather shifts, such
as the
El Niño-Southern
Oscillation.
Atmosphere
The
atmospheric pressure on the
surface of the Earth averages 101.325
kPa,
with a
scale height of about
8.5 km. It is 78% nitrogen and 21% oxygen, with trace amounts
of water vapor, carbon dioxide and other gaseous molecules. The
height of the
troposphere varies with
latitude, ranging between 8 km at the
poles to 17 km at the equator, with some variation due to
weather and seasonal factors.
Earth's biosphere has significantly altered its
atmosphere.
Oxygenic
photosynthesis evolved 2.7 billion years ago,
forming the primarily nitrogen-oxygen
atmosphere that exists today. This change
enabled the proliferation of
aerobic
organisms as well as the formation of the ozone layer which,
together with Earth's magnetic field, blocks
ultraviolet solar
radiation, permitting life on land. Other atmospheric functions
important to life on Earth include transporting water vapor,
providing useful gases, causing small
meteors
to burn up before they strike the surface, and moderating
temperature. This last phenomenon is known as the
greenhouse effect: trace molecules within
the atmosphere serve to capture thermal energy emitted from the
ground, thereby raising the average temperature. Carbon dioxide,
water vapor, methane and ozone are the primary
greenhouse gases in the Earth's atmosphere.
Without this heat-retention effect, the average surface temperature
would be −18 °C and life would likely not exist.
Weather and climate
The Earth's atmosphere has no definite boundary, slowly becoming
thinner and fading into outer space. Three-quarters of the
atmosphere's mass is contained within the first 11 km of the
planet's surface. This lowest layer is called the
troposphere. Energy from the Sun heats this
layer, and the surface below, causing expansion of the air. This
lower density air then rises, and is replaced by cooler, higher
density air. The result is
atmospheric circulation that drives
the weather and climate through redistribution of heat
energy.
The primary atmospheric circulation bands consist of the
trade winds in the equatorial region below 30°
latitude and the
westerlies in the
mid-latitudes between30° and 60°. Ocean currents are also important
factors in determining climate, particularly the
thermohaline circulation that
distributes heat energy from the equatorial oceans to the polar
regions.
Water vapor generated through surface evaporation is transported by
circulatory patterns in the atmosphere.When atmospheric conditions
permit an uplift of warm, humid air, this water condenses and
settles to the surface as
precipitation. Most of the water
is then transported back to lower elevations by river systems,
usually returning to the oceans or being deposited into
lakes. This
water cycle is a
vital mechanism for supporting life on land, and is a primary
factor in the erosion of surface features over geological periods.
Precipitation patterns vary widely, ranging from several meters of
water per year to less than a millimeter.
Atmospheric circulation, topological
features and temperature differences determine the average
precipitation that falls in each region.
The Earth can be sub-divided into specific latitudinal belts of
approximately homogeneous climate. Ranging from the equator to the
polar regions, these are the
tropical (or
equatorial),
subtropical,
temperate and
polar
climates. Climate can also be classified based on the temperature
and precipitation, with the climate regions characterized by fairly
uniform air masses. The commonly used
Köppen climate
classification system (as modified by
Wladimir Köppen's student Rudolph
Geiger) has five broad groups (humid tropics,
arid, humid middle latitudes,
continental and cold polar), which are
further divided into more specific subtypes.
Upper atmosphere
Above the troposphere, the atmosphere is usually divided into the
stratosphere,
mesosphere, and
thermosphere. Each of these layers has a
different
lapse rate, defining the rate
of change in temperature with height. Beyond these, the
exosphere thins out into the
magnetosphere. This is where the Earth's
magnetic fields interact with the
solar
wind. An important part of the atmosphere for life on Earth is
the ozone layer, a component of the stratosphere that partially
shields the surface from ultraviolet light. The
Kármán line, defined as 100 km
above the Earth's surface, is a working definition for the boundary
between atmosphere and space.
Due to thermal energy, some of the molecules at the outer edge of
the Earth's atmosphere have their velocity increased to the point
where they can
escape from the
planet's gravity. This results in a slow but steady
leakage of the atmosphere into space.
Because unfixed
hydrogen has a low
molecular weight, it can achieve
escape
velocity more readily and it leaks into outer space at a
greater rate than other gasses. The leakage of hydrogen into space
is a contributing factor in pushing the Earth from an initially
reducing state to its current
oxidizing one. Photosynthesis provided a source of
free oxygen, but the loss of reducing agents such as hydrogen is
believed to have been a necessary precondition for the widespread
accumulation of oxygen in the atmosphere.Hence the ability of
hydrogen to escape from the Earth's atmosphere may have influenced
the nature of life that developed on the planet. In the current,
oxygen-rich atmosphere most hydrogen is converted into water before
it has an opportunity to escape. Instead, most of the hydrogen loss
comes from the destruction of
methane in the
upper atmosphere.
Magnetic field
The
Earth's magnetic field is
shaped roughly as a
magnetic dipole,
with the poles currently located proximate to the planet's
geographic poles. According to
dynamo
theory, the field is generated within the molten outer core
region where heat creates convection motions of conducting
materials, generating electric currents. These in turn produce the
Earth's magnetic field. The convection movements in the core are
chaotic in nature, and periodically change alignment. This results
in
field reversals at irregular
intervals averaging a few times every million years. The most
recent reversal occurred approximately 700,000 years ago.
The 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.
Orbit and rotation
Rotation
Earth's rotation period relative to the Sun—its mean solar day—is
86,400 seconds of mean solar time. Each of these seconds is
slightly longer than an
SI second because Earth's
solar day is now slightly longer than it was during the 19th
century due to
tidal
acceleration.
Earth's rotation period relative to the
fixed
stars, called its
stellar day by the
International
Earth Rotation and Reference Systems Service (IERS), is of mean
solar time (UT1), or Earth's rotation period relative to the
precessing or moving mean
vernal
equinox, misnamed its
sidereal day, is of mean solar time (UT1)
. Thus the sidereal day is shorter than the stellar day by about
8.4 ms. The length of the mean solar day in SI seconds is
available from the IERS for the periods 1623–2005 and
1962–2005.
Apart from
meteors within the atmosphere and
low-orbiting satellites, the main apparent motion of celestial
bodies in the Earth's sky is to the west at a rate of 15°/h =
15'/min. This is equivalent to an apparent diameter of the Sun or
Moon every two minutes; the apparent sizes of the Sun and the Moon
are approximately the same.
Orbit
Earth orbits the Sun at an average distance of about
150 million kilometers every
365.2564 mean solar days, or one
sidereal year. From Earth, this gives an
apparent movement of the Sun eastward with respect to the stars at
a rate of about 1°/day, or a Sun or Moon diameter every
12 hours. Because of this motion, 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 Moon revolves with the Earth around a common
barycenter every 27.32 days relative to the
background stars. When combined with the Earth–Moon system's common
revolution around the Sun, the period of the
synodic month, from new moon to new moon, is
29.53 days. Viewed from the
celestial north pole, the motion of Earth,
the Moon and their axial rotations are all
counter-clockwise. Viewed from a vantage
point above the north poles of both the Sun and the Earth, the
Earth appears to revolve in a counterclockwise direction about the
Sun. The orbital and axial planes are not precisely aligned:
Earth's
axis is tilted some
23.5 degrees from the perpendicular to the Earth–Sun plane,
and the Earth–Moon plane is tilted about 5 degrees against the
Earth-Sun plane. Without this tilt, there would be an eclipse every
two weeks, alternating between
lunar
eclipses and
solar eclipses.
The
Hill sphere, or
gravitational sphere of influence, of the Earth is
about 1.5 Gm (or 1,500,000
kilometers) in radius.For the Earth, the
Hill radius is
- \begin{smallmatrix} R_H = a\left ( \frac{m}{3M} \right
)^{\frac{1}{3}} \end{smallmatrix},
where
m is the mass of the Earth,
a is an
Astronomical Unit, and
M is the mass of the Sun. So the
radius in A.U. is about:
\begin{smallmatrix} \left ( \frac{1}{3 \cdot 332,946} \right
)^{\frac{1}{3}} = 0.01 \end{smallmatrix}. 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.
Earth, along with the Solar System, is situated in the
Milky Way galaxy, orbiting
about 28,000
light years from the
center of the galaxy. It is currently about 20 light years
above the galaxy's
equatorial plane
in the
Orion spiral arm.
Axial tilt and seasons
Because of the axial tilt of the Earth, the amount of sunlight
reaching any given point on the surface varies over the course of
the year. This results in
seasonal change in
climate, with summer in the northern hemisphere occurring when the
North Pole is pointing toward the Sun, and winter taking place when
the pole is pointed away. During the summer, the day lasts longer
and the Sun climbs higher in the sky. In winter, the climate
becomes generally cooler and the days shorter. Above the
Arctic Circle, an extreme case is reached
where there is no daylight at all for part of the year—a
polar night.
In the southern hemisphere the situation is
exactly reversed, with the South Pole
oriented opposite the direction of the North
Pole.
By astronomical convention, the four seasons are determined by the
solstices—the point in the orbit of maximum
axial tilt toward or away from the Sun—and the
equinoxes, when the direction of the tilt and the
direction to the Sun are perpendicular. Winter solstice occurs on
about December 21, summer solstice is near June 21, spring equinox
is around March 20 and autumnal equinox is about September
23.
The angle of the Earth's tilt is relatively stable over long
periods of time. However, the tilt does undergo
nutation; a slight, irregular motion 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 precession is the reason for the
difference between a sidereal year and a
tropical year. Both of these motions are
caused by the varying attraction of the Sun and Moon on the Earth's
equatorial bulge. From the perspective of the Earth, the poles also
migrate a few meters across the surface. This
polar motion has multiple, cyclical components,
which collectively are termed
quasiperiodic motion. In addition to an
annual component to this motion, there is a 14-month cycle called
the
Chandler wobble. The rotational
velocity of the Earth also varies in a phenomenon known as length
of day variation.
In modern times, Earth's
perihelion
occurs around January 3, and the
aphelion
around July 4. However, these dates change over time due to
precession and other orbital
factors, which follow cyclical patterns known as
Milankovitch cycles. The changing
Earth-Sun distance results in an increase of about 6.9% in solar
energy reaching the Earth at perihelion relative to aphelion. Since
the southern hemisphere is tilted toward the Sun at about the same
time that the Earth reaches the closest approach to the Sun, the
southern hemisphere receives slightly more energy from the Sun than
does the northern over the course of a year. However, this effect
is much less significant than the total energy change due to the
axial tilt, and most of the excess energy is absorbed by the higher
proportion of water in the southern hemisphere.
Moon
Characteristics
| Diameter |
3,474.8 km
2,159.2 mi |
| Mass |
7.349 kg
8.1 (short) tons |
| Semi-major axis |
384,400 km
238,700 mi |
| Orbital period |
27 d 7 h 43.7 m |
The Moon is a relatively large,
terrestrial, planet-like satellite, with
a diameter about one-quarter of the Earth's. It is the largest moon
in the Solar System relative to the size of its planet. (
Charon is larger relative to the
dwarf planet Pluto.) The
natural satellites orbiting other planets are called "moons" after
Earth's Moon.
The gravitational attraction between the Earth and Moon causes
tides on Earth. The same effect on the Moon
has led to its
tidal locking: its
rotation period is the same as the time it takes to orbit the
Earth. As a result, it always presents the same face to the planet.
As the Moon orbits Earth, different parts of its face are
illuminated by the Sun, leading to the
lunar
phases; the dark part of the face is separated from the light
part by the
solar
terminator.
Because of their
tidal
interaction, the Moon recedes from Earth at the rate of
approximately 38 mm a year. Over millions of years, these tiny
modifications—and the lengthening of Earth's day by about 23
µs a year—add up to significant changes.
During the
Devonian period, for example,
(approximately 410 million years ago) there were 400 days in a
year, with each day lasting 21.8 hours.
The Moon may have dramatically affected the development of life by
moderating the planet's climate.
Paleontological evidence and computer
simulations show that Earth's axial tilt is stabilized by tidal
interactions with the Moon. Some theorists believe that without
this stabilization against the
torques
applied by the Sun and planets to the Earth's equatorial bulge, the
rotational axis might be chaotically unstable, exhibiting chaotic
changes over millions of years, as appears to be the case for Mars.
If Earth's axis of rotation were to approach the
plane of the ecliptic, extremely severe weather
could result from the resulting extreme seasonal differences. One
pole would be pointed directly toward the Sun during
summer and directly away during
winter.
Planetary scientists who have studied the
effect claim that this might kill all large animal and higher plant
life. However, this is a controversial subject, and further studies
of Mars—which has a similar
rotation
period and axial tilt as Earth, but not its large Moon or
liquid core—may settle the matter.
Viewed from Earth, the Moon is just far enough away to have very
nearly the same apparent-sized disk as the Sun. The
angular size (or
solid
angle) of these two bodies match because, although the Sun's
diameter is about 400 times as large as the Moon's, it is also 400
times more distant. This allows total and annular
eclipses to occur on Earth.
A scale representation of the relative
sizes of, and distance between, Earth and Moon.
The most widely accepted theory of the Moon's origin, the
giant impact theory, states that it
formed from the collision of a Mars-size
protoplanet called Theia with the early Earth.
This hypothesis explains (among other things) the Moon's relative
lack of iron and volatile elements, and the fact that its
composition is nearly identical to that of the Earth's crust.
Earth has at least two
co-orbital
asteroids,
3753 Cruithne and
2002 AA29.
Habitability
A planet that can sustain life is termed habitable, even if life
did not originate there. The Earth provides the (currently
understood) requisite conditions of liquid water, an environment
where complex organic molecules can assemble and sufficient energy
to sustain
metabolism. 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.
Biosphere
The planet's life forms are sometimes said to form a "biosphere".
This biosphere is generally believed to have begun
evolving about 3.5 billion years ago. Earth
is the only place in the universe where life is known to exist.
Some scientists believe that Earth-like biospheres might be
rare.
The biosphere is divided into a number of
biomes, inhabited by broadly similar plants and
animals. On land primarily
latitude and
height above the sea level separates biomes. Terrestrial biomes
lying within the
Arctic,
Antarctic Circle or in high altitudes are
relatively barren of plant and animal life, while the greatest
latitudinal
diversity of species is found at the Equator.
Natural resources and land use
The Earth provides resources that are exploitable by humans for
useful purposes. Some of these are
non-renewable resources, such as
mineral fuels, that are difficult to
replenish on a short time scale.
Large deposits of
fossil fuels are
obtained from the Earth's crust, consisting of coal, petroleum,
natural gas and
methane clathrate. These deposits are used
by humans both for energy production and as feedstock for chemical
production. Mineral ore bodies have also been formed in Earth's
crust through a process of
Ore genesis,
resulting from actions of erosion and plate tectonics. These bodies
form concentrated sources for many metals and other useful
elements.
The Earth's biosphere produces many useful biological products for
humans, including (but far from limited to) food, wood,
pharmaceuticals, oxygen, and the recycling of
many organic wastes. The land-based
ecosystem depends upon topsoil and fresh water,
and the oceanic ecosystem depends upon dissolved nutrients washed
down from the land. Humans also live on the land by using
building materials to construct shelters.
In 1993, human use of land is approximately:
| Land use |
Percentage |
| Arable land |
13.13% |
| Permanent crops |
4.71% |
| Permanent pastures |
26% |
| Forests and woodland |
32% |
| Urban areas |
1.5% |
| Other |
30% |
The estimated amount of irrigated land in 1993 was
2,481,250 km².
Natural and environmental hazards
Large areas are subject to extreme weather such as tropical
cyclones,
hurricanes, or
typhoons
that dominate life in those areas. Many places are subject to
earthquakes,
landslides,
tsunamis,
volcanic eruptions,
tornadoes,
sinkholes,
blizzards, floods, droughts, and other
calamities and disasters.
Many localized areas are subject to human-made
pollution of the air and water,
acid rain and toxic substances, loss of vegetation
(
overgrazing,
deforestation,
desertification), loss of wildlife, species
extinction,
soil
degradation, soil depletion, erosion, and introduction of
invasive species.
A scientific consensus exists linking human activities to
global warming due to industrial carbon
dioxide emissions. This is predicted to produce changes such as the
melting of glaciers and ice sheets, more extreme temperature
ranges, significant changes in weather conditions and a
global rise in average sea levels.
Human geography
Cartography, the study and practice of
map making, and vicariously geography, have historically been the
disciplines devoted to depicting the Earth.
Surveying, the determination of locations and
distances, and to a lesser extent
navigation, the determination of position and
direction, have developed alongside cartography and geography,
providing and suitably quantifying the requisite information.
Earth has approximately 6,740,000,000 human inhabitants as of
November 2008. Projections indicate that the
world's human population will reach seven
billion in 2013 and 9.2 billion in 2050. Most of the growth is
expected to take place in
developing
nations. Human
population
density varies widely around the world, but a majority live in
Asia. By 2020, 60% of the world's population is
expected to be living in urban, rather than rural, areas.
It is estimated that only one eighth of the surface of the Earth is
suitable for humans to live on—three-quarters is covered by oceans,
and half of the land area is either desert (14%), high mountains
(27%), or other less suitable terrain.
The northernmost
permanent settlement in the world is Alert
, on Ellesmere
Island
in Nunavut
,
Canada. (82°28′N) The southernmost is the Amundsen-Scott
South Pole Station
, in Antarctica, almost exactly at the South
Pole. (90°S)
Independent sovereign
nations claim the
planet's entire land surface, with the exception of some parts of
Antarctica. As of 2007 there are
201 sovereign states, including the
192
United Nations member
states. In addition, there are 59
dependent territories, and a number of
autonomous
areas,
territories
under dispute and other entities. Historically, Earth has never
had a
sovereign government with
authority over the entire globe, although a number of nation-states
have striven for
world domination
and failed.
The
United Nations is a worldwide
intergovernmental
organization that was created with the goal of intervening in
the disputes between nations, thereby avoiding armed conflict. It
is not, however, a world government. While the U.N. provides a
mechanism for
international law
and, when the consensus of the membership permits, armed
intervention, it serves primarily as a forum for international
diplomacy.
The first human to orbit the Earth was
Yuri
Gagarin on April 12, 1961. In total, about 400 people visited
outer space and reached Earth orbit 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, currently six people, is usually
replaced every six months. Humans traveled the farthest from the
planet in 1970, when
Apollo 13 crew was
400,171 km away from Earth.
Cultural viewpoint
The name "Earth" was derived from the
Anglo-Saxon word
erda, which means
ground or soil. It became
eorthe in
Old English, then
erthe in
Middle English. The standard astronomical
symbol of the Earth consists of a cross circumscribed by a
circle.
Earth has often been personified as a
deity,
in particular a
goddess. In many cultures
the
mother goddess, also called the
Mother Earth, is also portrayed as a
fertility deity.
Creation myths in many religions recall a
story involving the creation of the Earth by a supernatural deity
or deities. A variety of religious groups, often associated with
fundamentalist branches of
Protestantism or
Islam,
assert that their
interpretations of
these creation myths in
sacred texts
are
literal truth and should be
considered alongside or replace conventional scientific accounts of
the formation of the Earth and the origin and development of life.
Such assertions are opposed by the
scientific community and other
religious groups. A prominent example is the
creation-evolution
controversy.
In the past there were varying levels of belief in a
flat Earth, but this was displaced by the concept
of a
spherical Earth due to
observation and circumnavigation. The human perspective regarding
the Earth has changed following the advent of spaceflight, and the
biosphere is now widely viewed from a globally integrated
perspective.This is reflected in a growing
environmental movement that is
concerned about humankind's effects on the planet.
See also
Notes
Other planets in the Solar System are either too hot or too cold to
support liquid water. However, it is confirmed to have existed on
the surface of Mars in the past, and may still appear today.
See:
References
Bibliography
Further reading
External links
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