Planet
Earth is the third planet from the Sun and the only
astronomical object known to harbor life. This is enabled by Earth being
a water world, the only one in the Solar System sustaining liquid
surface
water. Almost all of Earth's water
is contained in its global ocean, covering 70.8% of Earth's crust. The
remaining 29.2% of Earth's crust is land, most of which is located in
the form of continental landmasses within Earth's land hemisphere. Most
of Earth's land is somewhat humid and covered by vegetation, while large
sheets of ice at Earth's polar deserts retain more water than Earth's
groundwater,
lakes,
rivers
and atmospheric water combined. Earth's crust consists of slowly moving
tectonic plates, which interact to produce mountain ranges, volcanoes,
and earthquakes. Earth has a liquid outer core that generates a
magnetosphere capable of deflecting most of the destructive solar winds
and cosmic radiation.
Earth has a dynamic atmosphere, which sustains Earth's surface
conditions and protects it from most meteoroids and UV-light at entry.
It has a composition of primarily nitrogen and oxygen. Water vapor is
widely present in the atmosphere, forming clouds that cover most of the
planet. The water vapor acts as a
greenhouse gas
and, together with other greenhouse gases in the atmosphere,
particularly carbon dioxide (CO2), creates the conditions for both
liquid surface
water
and water vapor to persist via the capturing of energy from the Sun's
light. This process maintains the current average surface temperature of
14.76 °C, at which
water
is liquid under atmospheric pressure. Differences in the amount of
captured energy between geographic regions (as with the equatorial
region receiving more sunlight than the polar regions) drive atmospheric
and ocean currents, producing a global climate system with different
climate regions, and a range of weather phenomena such as precipitation,
allowing components such as
nitrogen to cycle.
Earth is rounded into an ellipsoid with a circumference of about
40,000 km. It is the densest planet in the Solar System. Of the four
rocky planets, it is the largest and most massive. Earth is about eight
light-minutes away from the Sun and orbits it, taking a year (about
365.25 days) to complete one revolution. Earth rotates around its own
axis in slightly less than a day (in about 23 hours and 56 minutes).
Earth's axis of rotation is tilted with respect to the perpendicular to
its orbital plane around the Sun, producing seasons. Earth is orbited by
one permanent natural satellite, the Moon, which orbits Earth at
384,400 km (1.28 light seconds) and is roughly a quarter as wide as
Earth. The Moon's gravity helps stabilize Earth's axis, causes tides and
gradually slows Earth's rotation. Tidal locking has made the Moon
always face Earth with the same side.
Earth, like most other bodies in the Solar System, formed 4.5
billion years ago from gas in the early Solar System. During the first
billion years of Earth's history, the
ocean
formed and then life developed within it. Life spread globally and has
been altering Earth's atmosphere and surface, leading to the Great
Oxidation Event two billion years ago.
Humans emerged 300,000 years ago in
Africa
and have spread across every continent on Earth. Humans depend on
Earth's biosphere and natural resources for their survival, but have
increasingly impacted the planet's environment. Humanity's current
impact on Earth's climate and biosphere is unsustainable, threatening
the livelihood of humans and many other forms of life, and causing
widespread extinctions.
ETYMOLOGY
The Modern English word Earth developed, via Middle English,
from an Old English noun most often spelled eorðe. It has cognates in
every Germanic language, and their ancestral root has been reconstructed
as *erþō. In its earliest attestation, the word eorðe was used to
translate the many senses of Latin terra and Greek γῆ gē: the ground,
its soil, dry land, the human world, the surface of the world (including
the sea), and the globe itself. As with Roman Terra/Tellūs and Greek
Gaia, Earth may have been a personified goddess in Germanic paganism:
late Norse mythology included Jörð ("Earth"), a giantess often given as
the mother of Thor.
Historically, "Earth" has been written in lowercase. Beginning
with the use of Early Middle English, its definite sense as "the globe"
was expressed as "the earth". By the era of Early Modern English,
capitalization of nouns began to prevail, and the earth was also written
the Earth, particularly when referenced along with other heavenly
bodies. More recently, the name is sometimes simply given as Earth, by
analogy with the names of the other planets, though "earth" and forms
with "the earth" remain
common. House styles now vary: Oxford spelling recognizes the
lowercase form as the more common, with the capitalized form an
acceptable variant. Another convention capitalizes "Earth" when
appearing as a name, such as a description of the "Earth's atmosphere",
but employs the lowercase when it is preceded by "the", such as "the
atmosphere of the earth"). It almost always appears in lowercase in
colloquial expressions such as "what on earth are you doing?"
The name Terra /ˈtɛrə/ occasionally is used in scientific
writing and especially in science fiction to distinguish humanity's
inhabited planet from others, while in poetry Tellus /ˈtɛləs/ has been
used to denote personification of the
Earth. Terra is also the name of the planet in some Romance
languages, languages that evolved from Latin, like Italian and
Portuguese, while in other Romance languages the word gave rise to names
with slightly altered spellings, like the Spanish Tierra and the French
Terre. The Latinate form Gæa or Gaea (English: /ˈdʒiː.ə/) of the Greek
poetic name Gaia (Γαῖα; Ancient Greek: [ɡâi̯.a] or [ɡâj.ja]) is rare,
though the alternative spelling Gaia has become common due to the Gaia
hypothesis, in which case its pronunciation is /ˈɡaɪ.ə/ rather than the
more classical English /ˈɡeɪ.ə/.
There are a number of adjectives for the planet Earth. The word
"earthly" is derived from "Earth". From the Latin Terra comes terran
/ˈtɛrən/, terrestrial /təˈrɛstriəl/, and (via French) terrene /təˈriːn/,
and from the Latin Tellus comes tellurian /tɛˈlʊəriən/ and telluric.
NATURAL HISTORY
FORMATION
The
oldest material found in the Solar System is dated to 4.5682+0.0002
−0.0004 Ga (billion years) ago. By 4.54±0.04 Ga the primordial
Earth had formed. The bodies in the Solar System formed and evolved with
the Sun. In theory, a solar nebula partitions a volume out of a
molecular cloud by gravitational collapse, which begins to spin and
flatten into a circumstellar disk, and then the planets grow out of that
disk with the Sun. A nebula contains gas, ice grains, and dust
(including primordial nuclides). According to nebular theory,
planetesimals formed by accretion, with the primordial Earth being
estimated as likely taking anywhere from 70 to 100 million years to
form.
Estimates of the age of the Moon range from 4.5 Ga to
significantly younger. A leading hypothesis is that it was formed by
accretion from material loosed from Earth after a Mars-sized object with
about 10% of Earth's mass, named Theia, collided with Earth. It hit
Earth with a glancing blow and some of its mass merged with Earth.
Between approximately 4.1 and 3.8 Ga, numerous asteroid impacts during
the Late Heavy Bombardment caused significant changes to the greater
surface environment of the Moon and, by inference, to that of Earth.
AFTER FORMATION
Earth's atmosphere and oceans were formed by volcanic activity
and outgassing. Water vapor from these sources condensed into the
oceans, augmented by water and ice from asteroids, protoplanets, and
comets. Sufficient water to fill the oceans may have been on Earth since
it formed. In this model, atmospheric greenhouse gases kept the oceans
from freezing when the newly forming Sun had only 70% of its current
luminosity. By 3.5 Ga, Earth's magnetic field was established, which
helped prevent the atmosphere from being stripped away by the solar
wind.
As the molten outer layer of Earth cooled it formed the first
solid crust, which is thought to have been mafic in composition. The
first continental crust, which was more felsic in composition, formed by
the partial melting of this mafic crust. The presence of grains of the
mineral zircon of Hadean age in Eoarchean sedimentary rocks suggests
that at least some felsic crust existed as early as 4.4 Ga, only 140 Ma
after Earth's formation. There are two main models of how this initial
small volume of continental crust evolved to reach its current
abundance: (1) a relatively steady growth up to the present day, which
is supported by the radiometric dating of continental crust globally and
(2) an initial rapid growth in the volume of continental crust during
the Archean, forming the bulk of the continental crust that now exists,
which is supported by isotopic evidence from hafnium in zircons and
neodymium in sedimentary rocks. The two models and the data that support
them can be reconciled by large-scale recycling of the continental
crust, particularly during the early stages of Earth's history.
New continental crust forms as a result of plate tectonics, a
process ultimately driven by the continuous loss of heat from Earth's
interior. Over the period of hundreds of millions of years, tectonic
forces have caused areas of continental crust to group together to form
supercontinents that have subsequently broken apart. At approximately
750 Ma, one of the earliest known supercontinents, Rodinia, began to
break apart. The continents later recombined to form Pannotia at 600–540
Ma, then finally Pangaea, which also began to break apart at 180 Ma.
The most recent pattern of ice ages began about 40 Ma, and then
intensified during the Pleistocene about 3 Ma. High- and middle-latitude
regions have since undergone repeated cycles of glaciation and thaw,
repeating about every 21,000, 41,000 and 100,000 years. The Last Glacial
Period, colloquially called the "last ice age", covered large parts of
the continents, to the middle latitudes, in ice and ended about 11,700
years ago.
ORIGIN OF LIFE AND EVOLUTION
Chemical reactions led to the first self-replicating molecules
about four billion years ago. A half billion years later, the last
common ancestor of all current life arose. The evolution of
photosynthesis allowed the Sun's energy to be harvested directly by life
forms. The resultant molecular oxygen (O2) accumulated in the
atmosphere and due to interaction with ultraviolet solar radiation,
formed a protective ozone layer (O3) 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 Earth's surface. Among the earliest
fossil evidence for life is microbial mat fossils found in 3.48
billion-year-old sandstone in Western Australia, biogenic graphite found
in 3.7 billion-year-old metasedimentary rocks in Western Greenland, and
remains of biotic material found in 4.1 billion-year-old rocks in
Western Australia. The earliest direct evidence of life on Earth is
contained in 3.45 billion-year-old Australian rocks showing fossils of
microorganisms.
During the Neoproterozoic, 1000 to 539 Ma, much of Earth might
have been covered in ice. This hypothesis has been termed "Snowball
Earth", and it is of particular interest because it preceded the
Cambrian explosion, when multicellular life forms significantly
increased in complexity. Following the Cambrian explosion, 535 Ma, there
have been at least five major mass extinctions and many minor ones.
Apart from the proposed current Holocene extinction event, the most
recent was 66 Ma, when an asteroid impact triggered the extinction of
the non-avian dinosaurs and other large
reptiles, but largely spared
small animals such as insects, mammals, lizards and birds. Mammalian
life has diversified over the past 66 Mys, and several million years ago
an African ape species gained the ability to stand upright. This
facilitated tool use and encouraged communication that provided the
nutrition and stimulation needed for a larger brain, which led to the
evolution of humans. The development of agriculture, and then
civilization, led to humans having an influence on Earth and the nature
and quantity of other life forms that continues to this day.
FUTURE
Earth's expected long-term future is tied to that of the Sun.
Over the next 1.1 billion years, solar luminosity will increase by 10%,
and over the next 3.5 billion years by 40%. Earth's increasing surface
temperature will accelerate the inorganic carbon cycle, reducing CO2
concentration to levels lethally low for plants (10 ppm for C4
photosynthesis) in approximately 100–900 million years. The lack of
vegetation will result in the loss of oxygen in the atmosphere, making
animal life impossible. Due to the increased luminosity, Earth's mean
temperature may reach 100 °C (212 °F) in 1.5 billion years, and all
ocean water will evaporate and be lost to space, which may trigger a
runaway greenhouse effect, within an estimated 1.6 to 3 billion years.
Even if the Sun were stable, a fraction of the water in the modern
oceans will descend to the mantle, due to reduced steam venting from
mid-ocean ridges.
The Sun will evolve to become a red giant in about 5 billion
years. Models predict that the Sun will expand to roughly 1 AU (150
million km; 93 million mi), about 250 times its present radius. Earth's
fate is less clear. As a red giant, the Sun will lose roughly 30% of its
mass, so, without tidal effects, Earth will move to an orbit 1.7 AU
(250 million km; 160 million mi) from the Sun when the star reaches its
maximum radius, otherwise, with tidal effects, it may enter the Sun's
atmosphere and be vaporized.
PHYSICAL CHARACTERISTICS
SIZE & SHAPE
Earth has a rounded shape, through hydrostatic equilibrium, with
an average diameter of 12,742 kilometers (7,918 mi), making it the
fifth largest planetary sized and largest terrestrial object of the
Solar System.
Due to Earth's rotation it has the shape of an ellipsoid,
bulging at its Equator; its diameter is 43 kilometers (27 mi) longer
there than at its poles. Earth's shape furthermore has local topographic
variations. Though the largest local variations, like the Mariana
Trench (10,925 meters or 35,843 feet below local sea level), only
shortens Earth's average radius by 0.17% and Mount Everest (8,848 meters
or 29,029 feet above local sea level) lengthens it by only 0.14%. Since
Earth's surface is farthest out from Earth's center of mass at its
equatorial bulge, the summit of the volcano Chimborazo in Ecuador
(6,384.4 km or 3,967.1 mi) is its farthest point out. Parallel to the
rigid land topography the Ocean exhibits a more dynamic topography.
To measure the local variation of Earth's topography, geodesy
employs an idealized Earth producing a shape called a geoid. Such a
geoid shape is gained if the ocean is idealized, covering Earth
completely and without any perturbations such as tides and winds. The
result is a smooth but gravitational irregular geoid surface, providing a
mean sea level (MSL) as a reference level for topographic measurements.
SURFACE
Earth's surface is the boundary between the atmosphere, and the
solid Earth and oceans. Defined in this way, it has an area of about 510
million km2 (197 million sq mi). Earth can be divided into two
hemispheres: by latitude into the polar Northern and Southern
hemispheres; or by longitude into the continental Eastern and Western
hemispheres.
Most of Earth's surface is ocean water: 70.8% or 361 million km2
(139 million sq mi). This vast pool of salty water is often called the
world ocean, and makes Earth with its dynamic hydrosphere a water world
or ocean world. Indeed, in Earth's early history the ocean may have
covered Earth completely. The world ocean is commonly divided into the
Pacific Ocean, Atlantic
Ocean, Indian
Ocean, Antarctic or Southern
Ocean, and Arctic
Ocean, from largest to smallest. The ocean covers
Earth's oceanic crust, but to a lesser extent with shelf seas also
shelves of the continental crust. The oceanic crust forms large oceanic
basins with features like abyssal plains, seamounts, submarine
volcanoes, oceanic trenches, submarine canyons, oceanic plateaus, and a
globe-spanning mid-ocean ridge system.
At Earth's polar regions, the ocean surface is covered by
seasonally variable amounts of sea ice that often connects with polar
land, permafrost and ice sheets, forming polar ice caps.
Earth's land covers 29.2%, or 149 million km2 (58 million sq mi)
of Earth's surface. The land surface includes many islands around the
globe, but most of the land surface is taken by the four continental
landmasses, which are (in descending order): Africa-Eurasia, America
(landmass), Antarctica, and Australia (landmass). These landmasses are
further broken down and grouped into the continents. The terrain of the
land surface varies greatly and consists of mountains, deserts, plains,
plateaus, and other landforms. The elevation of the land surface varies
from a low point of −418 m (−1,371 ft) at the Dead Sea, to a maximum
altitude of 8,848 m (29,029 ft) at the top of Mount Everest. The mean
height of land above sea level is about 797 m (2,615 ft).
Land can be covered by surface water, snow, ice, artificial structures or vegetation. Most of Earth's land hosts
vegetation, but ice sheets (10%, not including the equally large land under
permafrost) or cold as well as hot deserts (33%) occupy also considerable amounts of it.
The pedosphere is the outermost layer of Earth's land surface
and is composed of soil and subject to soil formation processes. Soil is
crucial for land to be arable. Earth's total arable land is 10.7% of
the land surface, with 1.3% being permanent cropland. Earth has an
estimated 16.7 million km2 (6.4 million sq mi) of cropland and 33.5
million km2 (12.9 million sq mi) of pastureland.
The land surface and the ocean floor form the top of Earth's
crust, which together with parts of the upper mantle form Earth's
lithosphere. Earth's crust may be divided into oceanic and continental
crust. Beneath the ocean-floor sediments, the oceanic crust is
predominantly basaltic, while the continental crust may include lower
density materials such as granite, sediments and metamorphic rocks.
Nearly 75% of the continental surfaces are covered by sedimentary rocks,
although they form about 5% of the mass of the crust.
Earth's surface topography comprises both the topography of the
ocean surface, and the shape of Earth's land surface. The submarine
terrain of the ocean floor has an average bathymetric depth of 4 km, and
is as varied as the terrain above sea level.
Earth's surface is continually being shaped by internal plate
tectonic processes including earthquakes and volcanism; by weathering
and erosion driven by ice, water, wind and temperature; and by
biological processes including the growth and decomposition of biomass
into soil.
TECTONIC PLATES
Earth's mechanically rigid outer layer of Earth's crust and
upper mantle, the lithosphere, is divided into tectonic plates. These
plates are rigid segments that move relative to each other at one of
three boundaries types: at convergent boundaries, two plates come
together; at divergent boundaries, two plates are pulled apart; and at
transform boundaries, two plates slide past one another laterally. Along
these plate boundaries,
earthquakes,
volcanic activity, mountain-building, and oceanic trench formation can
occur. 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.
As the tectonic plates migrate, oceanic crust 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 recycles the
oceanic crust back into the mantle. Due to this recycling, most of the
ocean floor is less than 100 Ma old. The oldest oceanic crust is located
in the Western Pacific and is estimated to be 200 Ma old. By
comparison, the oldest dated continental crust is 4,030 Ma, although
zircons have been found preserved as clasts within Eoarchean sedimentary
rocks that give ages up to 4,400 Ma, indicating that at least some
continental crust existed at that time.
The seven major plates are the Pacific, North American,
Eurasian, African, Antarctic, Indo-Australian, and South American. Other
notable plates include 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 fused with the Indian
Plate between 50 and 55 Ma. The fastest-moving plates are the oceanic
plates, with the Cocos Plate advancing at a rate of 75 mm/a (3.0
in/year) and the Pacific Plate moving 52–69 mm/a (2.0–2.7 in/year). At
the other extreme, the slowest-moving plate is the South American Plate,
progressing at a typical rate of 10.6 mm/a (0.42 in/year)'
INTERNAL STRUCTURE
Earth's interior, like that of the other terrestrial planets, is
divided into layers by their chemical or physical (rheological)
properties. The outer layer 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. The
thickness of the crust varies from about 6 kilometers (3.7 mi) under the
oceans to 30–50 km (19–31 mi) for the continents. The crust and the
cold, rigid, top of the upper mantle are collectively known as the
lithosphere, which is divided into independently moving tectonic plates.
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 km (250 and
410 mi) 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. Earth's inner core may
be rotating at a slightly higher angular velocity than the remainder of
the planet, advancing by 0.1–0.5° per year, although both somewhat
higher and much lower rates have also been proposed. The radius of the
inner core is about one-fifth of that of Earth. Density increases with
depth, as described in the table on the right.
Among the Solar System's planetary-sized objects Earth is the object with the highest density.
CHEMICAL COMPOSITION
Earth's mass is approximately 5.97×1024 kg (5,970 Yg). It is
composed mostly of iron (32.1% by mass), oxygen (30.1%), silicon
(15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium
(1.5%), and aluminum (1.4%), with the remaining 1.2% consisting of trace
amounts of other elements. Due to gravitational separation, the core is
primarily composed of the denser elements: iron (88.8%), with smaller
amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace
elements. The most common rock constituents of the crust are oxides.
Over 99% of the crust is composed of various oxides of eleven elements,
principally oxides containing silicon (the silicate minerals), aluminum,
iron, calcium, magnesium, potassium, or sodium.
INTERNAL HEAT (CENTRAL HEATING)
The major heat-producing isotopes within Earth are potassium-40,
uranium-238, and thorium-232. At the center, the temperature may be up
to 6,000 °C (10,830 °F), and the pressure could reach 360 GPa (52
million psi). Because much of the heat is provided by radioactive decay,
scientists postulate that early in Earth's history, before isotopes
with short half-lives were depleted, Earth's heat production was much
higher. At approximately 3 Gyr, twice the present-day heat would have
been produced, increasing the rates of mantle convection and plate
tectonics, and allowing the production of uncommon igneous rocks such as
komatiites that are rarely formed today.
The mean heat loss from Earth is 87 mW m−2, for a global heat
loss of 4.42×1013 W. 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 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, the majority of which occurs under the oceans because
the crust there is much thinner than that of the continents.
GRAVITATIONAL FIELD
The gravity of Earth is the acceleration that is imparted to
objects due to the distribution of mass within Earth. Near Earth's
surface, gravitational acceleration is approximately 9.8 m/s2 (32
ft/s2). Local differences in topography, geology, and deeper tectonic
structure cause local and broad regional differences in Earth's
gravitational field, known as gravity anomalies.
MAGNETIC FIELD (OUR SPACE SHIELD)
The main part of Earth's magnetic field is generated in the
core, the site of a dynamo process that converts the kinetic energy of
thermally and compositionally driven convection into
electrical
and magnetic field energy. The field extends outwards from the core,
through the mantle, and up to Earth's surface, where it is,
approximately, a dipole. The poles of the dipole are located close to
Earth's geographic poles. At the equator of the magnetic field, the
magnetic-field strength at the surface is 3.05×10−5 T, with a magnetic
dipole moment of 7.79×1022 Am2 at epoch 2000, decreasing nearly 6% per
century (although it still remains stronger than its long time average).
The convection movements in the core are chaotic; the magnetic poles
drift and periodically change alignment. This causes secular variation
of the main field and field reversals at irregular intervals averaging a
few times every million years. The most recent reversal occurred
approximately 700,000 years ago.
The extent of Earth's magnetic field in space defines the
magnetosphere. Ions and electrons of the solar wind are deflected by the
magnetosphere; solar wind pressure compresses the dayside of the
magnetosphere, to about 10 Earth radii, and extends the nightside
magnetosphere into a long
tail. Because the velocity of the solar wind is greater than the
speed at which waves propagate through the solar wind, a supersonic bow
shock precedes the dayside magnetosphere within the solar wind. Charged
particles are contained within the magnetosphere; the plasmasphere is
defined by low-energy particles that essentially follow magnetic field
lines as Earth rotates. The ring current is defined by medium-energy
particles that drift relative to the geomagnetic field, but with paths
that are still dominated by the magnetic field, and the Van Allen
radiation belts are formed by high-energy particles whose motion is
essentially random, but contained in the magnetosphere.
During magnetic storms and substorms, charged particles can be
deflected from the outer magnetosphere and especially the magnetotail,
directed along field lines into Earth's ionosphere, where atmospheric
atoms can be excited and ionized, causing 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 (86,400.0025 SI seconds).[154]
Because Earth's solar day is now slightly longer than it was during the
19th century due to tidal deceleration, each day varies between 0 and 2
ms longer than the mean solar day.
Earth's rotation period relative to the fixed stars, called its
stellar day by the International Earth Rotation and Reference Systems
Service (IERS), is 86,164.0989 seconds of mean solar time (UT1), or 23h
56m 4.0989s. Earth's rotation period relative to the precessing or
moving mean March equinox (when the Sun is at 90° on the equator), is
86,164.0905 seconds of mean solar time (UT1) (23h 56m 4.0905s). Thus the
sidereal day is shorter than the stellar day by about 8.4 ms.
Apart from meteors within the atmosphere and low-orbiting
satellites, the main apparent motion of celestial bodies in Earth's sky
is to the west at a rate of 15°/h = 15'/min. For bodies near the
celestial equator, this is equivalent to an apparent diameter of the Sun
or the Moon every two minutes; from Earth's surface, the apparent sizes
of the Sun and the Moon are approximately the same.
ORBIT
Earth orbits the Sun, making Earth the third-closest planet to
the Sun and part of the inner Solar System. Earth's average orbital
distance is about 150 million km (93 million mi), which is the basis for
the Astronomical Unit and is equal to roughly 8.3 light minutes or 380
times Earth's distance to the Moon.
Earth orbits the Sun every 365.2564 mean solar days, or one
sidereal year. With an apparent movement of the Sun in Earth's sky at a
rate of about 1°/day eastward, which is one apparent Sun or Moon
diameter every 12 hours. Due to 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 Earth averages about 29.78 km/s (107,200
km/h; 66,600 mph), which is fast enough to travel a distance equal to
Earth's diameter, about 12,742 km (7,918 mi), in seven minutes, and the
distance to the Moon, 384,000 km (239,000 mi), in about 3.5 hours.
The Moon and Earth orbit a common barycenter every 27.32 days
relative to the background stars. When combined with the Earth–Moon
system's common orbit 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 counterclockwise. Viewed from a vantage point above the Sun and
Earth's north poles, Earth orbits in a counterclockwise direction about
the Sun. The orbital and axial planes are not precisely aligned: Earth's
axis is tilted some 23.44 degrees from the perpendicular to the
Earth–Sun plane (the ecliptic), and the Earth-Moon plane is tilted up to
±5.1 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 the sphere of gravitational influence, of
Earth is about 1.5 million km (930,000 mi) in radius. This is the
maximum distance at which Earth's gravitational influence is stronger
than the more distant Sun and planets. Objects must orbit 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 and orbits about 28,000 light-years from its center.
It is about 20 light-years above the galactic plane in the Orion Arm.
AXIAL TILT AND SEASONS
The axial tilt of Earth is approximately 23.439281° with the
axis of its orbit plane, always pointing towards the Celestial Poles.
Due to Earth's axial tilt, the amount of sunlight reaching any given
point on the surface varies over the course of the year. This causes the
seasonal change in climate, with summer in the Northern Hemisphere
occurring when the Tropic of Cancer is facing the Sun, and in the
Southern Hemisphere when the Tropic of Capricorn faces the Sun. In each
instance, winter occurs simultaneously in the opposite hemisphere.
During the summer, the day lasts longer, and the Sun climbs
higher in the sky. In winter, the climate becomes cooler and the days
shorter. Above the Arctic Circle and below the Antarctic Circle there is
no daylight at all for part of the year, causing a polar night, and
this night extends for several months at the poles themselves. These
same latitudes also experience a midnight sun, where the sun remains
visible all day.
By astronomical convention, the four seasons can be determined
by the solstices — the points in the orbit of maximum axial tilt toward or
away from the Sun - and the equinoxes, when Earth's rotational axis is
aligned with its orbital axis. In the Northern Hemisphere, winter
solstice currently occurs around 21 December; summer solstice is near 21
June, spring equinox is around 20 March and autumnal equinox is about
22 or 23 September. In the Southern Hemisphere, the situation is
reversed, with the summer and winter solstices exchanged and the spring
and autumnal equinox dates swapped.
The angle of Earth's axial tilt is relatively stable over long
periods of time. Its axial tilt does undergo nutation; a slight,
irregular motion with a main period of 18.6 years. The orientation
(rather than the angle) of 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 the Moon on Earth's equatorial bulge. The
poles also migrate a few meters across Earth's 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. Earth's
rotational velocity also varies in a phenomenon known as length-of-day
variation.
In modern times, Earth's perihelion occurs around 3 January, and
its aphelion around 4 July. 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 causes an
increase of about 6.8% in solar energy reaching Earth at perihelion
relative to aphelion. Because the Southern Hemisphere is tilted toward
the Sun at about the same time that 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. 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.
EARTH-MOON SYSTEM
MOON
The Moon is a relatively large, terrestrial, planet-like natural
satellite, with a diameter about one-quarter of Earth's. It is the
largest moon in the Solar System relative to the size of its planet,
although Charon is larger relative to the dwarf planet Pluto. The
natural satellites of other planets are also referred to as "moons",
after Earth's. The most widely accepted theory of the Moon's origin, the
giant-impact hypothesis, states that it formed from the collision of a
Mars-size protoplanet called Theia with the early Earth. This hypothesis
explains the Moon's relative lack of iron and volatile elements and the
fact that its composition is nearly identical to that of Earth's crust.
The gravitational attraction between Earth and the 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
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. Due to their tidal interaction,
the Moon recedes from Earth at the rate of approximately 38 mm/a (1.5
in/year). Over millions of years, these tiny modifications—and the
lengthening of Earth's day by about 23 µs/yr—add up to significant
changes. During the Ediacaran period, for example, (approximately 620
Ma) there were 400±7 days in a year, with each day lasting 21.9±0.4
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 think that without this
stabilization against the torques applied by the Sun and planets to
Earth's equatorial bulge, the rotational axis might be chaotically
unstable, exhibiting large changes over millions of years, as is the
case for Mars, though this is disputed.
Viewed from Earth, the Moon is just far enough away to have
almost 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 solar eclipses to occur on
Earth.
On 1 November 2023, scientists reported that, according to
computer simulations, remnants of a protoplanet, named Theia, could be
inside the Earth, left over from a collision with the Earth in ancient
times, and afterwards becoming the Moon.
ASTEROIDS & ARTIFICIAL SATELLITES
Earth's co-orbital asteroids population consists of
quasi-satellites, objects with a horseshoe orbit and trojans. There are
at least five quasi-satellites, including 469219 Kamoʻoalewa. A trojan
asteroid companion, 2010 TK7, is librating around the leading Lagrange
triangular point, L4, in Earth's orbit around the Sun. The tiny
near-Earth asteroid 2006 RH120 makes close approaches to the Earth
- Moon system roughly every twenty years. During these
approaches, it can orbit Earth for brief periods of time.
As of September 2021, there are 4,550 operational, human-made
satellites orbiting Earth. There are also inoperative satellites,
including Vanguard 1, the oldest satellite currently in orbit, and over
16,000 pieces of tracked space debris. Earth's largest artificial
satellite is the International Space Station.
HYDROSPHERE
Earth's hydrosphere is the sum of Earth's water and its
distribution. Most of Earth's hydrosphere consists of Earth's global
ocean. Earth's hydrosphere also consists of water in the atmosphere and
on land, including clouds, inland seas, lakes, rivers, and underground
waters down to a depth of 2,000 m (6,600 ft).
The mass of the oceans is approximately 1.35×1018 metric tons or
about 1/4400 of Earth's total mass. The oceans cover an area of 361.8
million km2 (139.7 million sq mi) with a mean depth of 3,682 m (12,080
ft), resulting in an estimated volume of 1.332 billion km3 (320 million
cu mi). If all of Earth's crustal surface were at the same elevation as a
smooth sphere, the depth of the resulting world ocean would be 2.7 to
2.8 km (1.68 to 1.74 mi). About 97.5% of the water is saline; the
remaining 2.5% is fresh water. Most fresh water, about 68.7%, is present
as ice in ice caps and glaciers. The remaining 30% is ground water, 1%
surface water (covering only 2.8% of Earth's land) and other small forms
of fresh water deposits such as permafrost, water vapor in the
atmosphere, biological binding, etc.
In Earth's coldest regions, snow survives over the summer and
changes into ice. This accumulated snow and ice eventually forms into
glaciers, bodies of ice that flow under the influence of their own
gravity. Alpine glaciers form in mountainous areas, whereas vast ice
sheets form over land in polar regions. The flow of glaciers erodes the
surface changing it dramatically, with the formation of U-shaped valleys
and other
landforms. Sea ice in the Arctic covers an area about as big as the United States, although it is quickly retreating as a consequence of
climate
change.
The average salinity of Earth's oceans is about 35 grams of salt
per kilogram of seawater (3.5%
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's
climate, with the oceans acting as a large heat
reservoir. Shifts in the oceanic temperature distribution can
cause significant weather shifts, such as the El Niño–Southern
Oscillation.
The abundance of water, particularly liquid water, on Earth's
surface is a unique feature that distinguishes it from other planets in
the Solar System. Solar System planets with considerable atmospheres do
partly host atmospheric water vapor, but they lack surface conditions
for stable surface
water. Despite some moons showing signs of large reservoirs of
extraterrestrial liquid water, with possibly even more volume than
Earth's ocean, all of them are large bodies of water under a kilometers
thick frozen surface layer.
ATMOSPHERE
The atmospheric pressure at Earth's sea level averages 101.325
kPa (14.696 psi), with a scale height of about 8.5 km (5.3 mi). A dry
atmosphere is composed of 78.084% nitrogen, 20.946% oxygen, 0.934%
argon, and trace amounts of carbon dioxide and other gaseous
molecules. Water vapor content varies between 0.01% and 4% but
averages about 1%. Clouds cover around two-thirds of Earth's surface,
more so over oceans than land. The height of the troposphere varies with
latitude, ranging between 8 km (5 mi) at the poles to 17 km (11 mi) at
the equator, with some variation resulting from weather and seasonal
factors.
Earth's biosphere has significantly altered its atmosphere.
Oxygenic photosynthesis evolved 2.7 Gya, forming the primarily
nitrogen–oxygen atmosphere of today. This change enabled the
proliferation of aerobic organisms and, indirectly, the formation of the
ozone layer due to the subsequent conversion of atmospheric O2 into O3.
The ozone layer blocks ultraviolet solar radiation, permitting life on
land. Other atmospheric functions important to life 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 the greenhouse effect: trace molecules within the
atmosphere serve to capture thermal energy emitted from the surface,
thereby raising the average temperature. Water vapor, carbon dioxide,
methane, nitrous oxide, and ozone are the primary greenhouse gases in
the atmosphere. Without this heat-retention effect, the average surface
temperature would be −18 °C (0 °F), in contrast to the current +15 °C
(59 °F), and life on Earth probably would not exist in its current form.
WEATHER & CLIMATE
Earth's atmosphere has no definite boundary, gradually becoming
thinner and fading into outer space. Three-quarters of the atmosphere's
mass is contained within the first 11 km (6.8 mi) of the 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 thermal 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 between 30° and 60°. Ocean heat content and currents
are also important factors in determining climate, particularly the
thermohaline circulation that distributes thermal energy from the
equatorial oceans to the polar regions.
Earth receives 1361 W/m2 of incoming solar irradiance (insolation). The amount of
solar energy that reaches Earth's surface decreases with increasing
latitude. At higher latitudes, the sunlight reaches the surface at lower
angles, and it must pass through thicker columns of the atmosphere. As a
result, the mean annual air temperature at sea level decreases by about
0.4 °C (0.7 °F) per degree of latitude from the equator. Earth's
surface can be subdivided 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.
Further factors that affect a location's climates are its
proximity to oceans, the oceanic and atmospheric circulation, and
topology. Places close to oceans typically have colder summers and
warmer winters, due to the fact that oceans can store large amounts of
heat. The wind transports the cold or the heat of the ocean to the land.
Atmospheric circulation also plays an important role: San Francisco and
Washington DC are both coastal cities at about the same latitude. San
Francisco's climate is significantly more moderate as the prevailing
wind direction is from sea to land. Finally, temperatures decrease with
height causing mountainous areas to be colder than low-lying areas.
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 falls to
the surface as precipitation. Most of the water is then transported to
lower elevations by river systems and usually returned to the oceans or
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, topographic features, and
temperature differences determine the average precipitation that falls
in each region.
The commonly used Köppen climate classification system has five
broad groups (humid tropics, arid, humid middle latitudes, continental
and cold polar), which are further divided into more specific subtypes.
The Köppen system rates regions based on observed temperature and
precipitation. Surface air temperature can rise to around 55 °C (131 °F)
in hot deserts, such as Death Valley, and can fall as low as −89 °C
(−128 °F) in Antarctica.
UPPER ATMOSPHERE
The upper atmosphere, the atmosphere above the troposphere, is
usually divided into the stratosphere, mesosphere, and thermosphere.
Each layer has a different lapse rate, defining the rate of change in
temperature with height. Beyond these, the exosphere thins out into the
magnetosphere, where the geomagnetic fields interact with the solar
wind. Within the stratosphere is the ozone layer, a component that
partially shields the surface from ultraviolet light and thus is
important for life on Earth. The Kármán line, defined as 100 km (62 mi)
above Earth's surface, is a working definition for the boundary between
the atmosphere and outer space.
Thermal energy causes some of the molecules at the outer edge of
the atmosphere to increase their velocity to the point where they can
escape from Earth's gravity. This causes a slow but steady loss of the
atmosphere into space. Because unfixed
hydrogen
has a low molecular mass, it can achieve escape velocity more readily,
and it leaks into outer space at a greater rate than other gases. The
leakage of hydrogen into space contributes to the shifting of Earth's
atmosphere and surface 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 thought to have been a
necessary precondition for the widespread accumulation of oxygen in the
atmosphere. Hence the ability of
hydrogen
to escape from the atmosphere may have influenced the nature of life
that developed on Earth. 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.
LIFE ON EARTH
Earth is the only known place that has ever been habitable for life. Earth's life developed in Earth's early bodies of
water some hundred million years after Earth formed.
Earth's life has been shaping and inhabiting many particular
ecosystems on Earth and has eventually expanded globally forming an
overarching biosphere. Therefore, life has impacted Earth, significantly
altering Earth's atmosphere and surface over long periods of time,
causing changes like the Great Oxidation Event.
Earth's life has over time greatly diversified, allowing the
biosphere to have different biomes, which are inhabited by comparatively
similar plants and animals. The different biomes developed at distinct
elevations or
water
depths, planetary temperature latitudes and on land also with different
humidity. Earth's species diversity and biomass reaches a peak in
shallow waters and with forests, particularly in equatorial, warm and
humid conditions. While freezing polar regions and high altitudes, or
extremely arid areas are relatively barren of plant and animal life.
Earth provides liquid water
- an environment where complex organic molecules can assemble
and interact, and sufficient energy to sustain a metabolism. Plants and
other organisms take up nutrients from water, soils and the atmosphere.
These nutrients are constantly recycled between different species.
Extreme weather, such as tropical cyclones (including hurricanes
and typhoons), occurs over most of Earth's surface and has a large
impact on life in those areas. From 1980 to 2000, these events caused an
average of 11,800 human deaths per year. Many places are subject to
earthquakes, landslides, tsunamis, volcanic eruptions,
tornadoes,
blizzards, floods, droughts, wildfires, and other calamities and
disasters. Human impact is felt in many areas due to pollution of the
air and water, acid rain, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife,
species
extinction,
soil degradation, soil depletion and erosion. Human activities release
greenhouse gases into the atmosphere which cause global warming. This is
driving changes such as the melting of glaciers and ice sheets, a
global rise in average sea levels, increased risk of drought and
wildfires, and migration of species to colder areas.
HUMAN GEOGRAPHY
Originating from earlier primates in Eastern Africa 300,000
years ago humans have since been migrating and with the advent of
agriculture in the 10th millennium
BC increasingly settling Earth's land. In the 20th century
Antarctica had been the last continent to see a first and until today limited human presence.
Human population has since the 19th century grown
exponentially to seven billion in the early 2010s, and is projected to
peak at around ten billion in the second half of the 21st century. Most
of the growth is expected to take place in sub-Saharan Africa.
Distribution and density of human population varies greatly
around the world with the majority living in south to eastern Asia and
90% inhabiting only the Northern Hemisphere of Earth, partly due to the
hemispherical predominance of the world's land mass, with 68% of the
world's land mass being in the Northern Hemisphere. Furthermore, since
the 19th century humans have increasingly converged into urban areas
with the majority living in urban areas by the 21st century.
Beyond Earth's surface humans have lived on a temporary basis,
with only special purpose deep underground and underwater presence, and a
few space stations. Human population virtually completely remains on
Earth's surface, fully depending on Earth and the environment it
sustains. Since the second half of the 20th century, some hundreds of
humans have temporarily stayed beyond Earth, a tiny fraction of whom
have reached another celestial
body; the Moon.
Earth has been subject to extensive human settlement, and humans
have developed diverse societies and cultures. Most of Earth's land has
been territorially claimed since the 19th century by sovereign states
(countries) separated by political borders, and 205 such states exist
today, with only parts of Antarctica and a few small regions remaining
unclaimed. Most of these states together form the
United
Nations, the leading worldwide intergovernmental organization, which extends human governance over the ocean and
Antarctica, and therefore all of Earth.
NATURAL RESOURCES & LAND USE
Earth has resources that have been exploited by humans. Those
termed non-renewable resources, such as fossil fuels, are only
replenished over geological timescales. Large deposits of fossil fuels
are obtained from Earth's crust, consisting of coal, petroleum, and
natural gas. These deposits are used by humans both for energy
production and as feedstock for chemical production. Mineral ore bodies
have also been formed within the crust through a process of ore genesis,
resulting from actions of magmatism
(magnetism
in magam), erosion, and plate
tectonics. These metals and other elements are extracted by mining, a process which often brings environmental and health damage.
Earth's biosphere produces many useful biological products for humans, including
food,
wood, pharmaceuticals,
oxygen, and the recycling of organic waste. The land-based ecosystem depends upon topsoil and fresh
water, and
the oceanic ecosystem depends on dissolved nutrients washed down from
the land. In 2019, 39 million km2 (15 million sq mi) of Earth's land
surface consisted of
forest and
woodlands,
12 million km2 (4.6 million sq mi) was shrub and grassland, 40 million
km2 (15 million sq mi) were used for animal feed production and grazing,
and 11 million km2 (4.2 million sq mi) were cultivated as croplands. Of
the 12–14% of ice-free land that is used for croplands, 2 percentage
points were irrigated in 2015. Humans use building materials to
construct
shelters.
HUMANS & THE ENVIRONMENT
Human activities have impacted Earth's environments. Through activities such as the burning of
fossil
fuels, humans have been increasing the amount of greenhouse
gases in the atmosphere, altering Earth's energy budget and climate. It
is estimated that global temperatures in the year 2020 were 1.2 °C (2.2
°F) warmer than the
pre-industrial baseline. This increase in temperature, known as global
warming, has contributed to the melting of
glaciers, rising sea
levels, increased risk of drought and wildfires, and migration of species to colder areas.
Human activity is also turning arable land into deserts at an alarming
rate, known as desertification.
The United Nations host climate
change conferences almost every year, the latest as we write in Azerbaijan,
in November 2024, COP29.
The concept of planetary boundaries was introduced to quantify
humanity's impact on Earth. Of the nine identified boundaries, five have
been crossed: Biosphere integrity,
climate
change, chemical pollution, destruction of wild habitats and the nitrogen
cycle are thought to have passed the safe threshold. As of 2018, no
country meets the basic needs of its population without transgressing
planetary boundaries. It is thought possible to provide all basic
physical needs globally within
sustainable levels of resource
use.
CULTURAL & HISTORICAL VIEWPOINT
Human cultures have developed many views of the planet. The
standard astronomical symbols of Earth are a quartered circle,
representing the four corners the world
and a globus cruciger, ♁. Earth is sometimes personified as a
deity. In many cultures it is a mother goddess that is also the primary
fertility deity. Creation myths in many religions involve the creation
of Earth by a supernatural deity or deities. The Gaia hypothesis,
developed in the mid-20th century, compared Earth's environments and
life as a single self-regulating organism leading to broad stabilization
of the conditions of habitability.
Images of Earth taken from space,
particularly during the Apollo program, have been credited with
altering the way that people viewed the planet that they lived on,
called the overview effect, emphasizing its beauty, uniqueness and
apparent fragility. In particular, this caused a realization of the
scope of effects from
human
activity on Earth's environment. Enabled by science, particularly Earth
observation, humans have started to take action on environmental issues
globally, acknowledging the impact of humans and the interconnectedness
of Earth's environments.
Scientific investigation has resulted in several culturally
transformative shifts in people's view of the planet. Initial belief in a
flat Earth was gradually displaced in
Ancient Greece
by the idea of a spherical Earth, which was attributed to both the
philosophers Pythagoras and Parmenides. Earth was generally believed to
be the center of the universe until the 16th century, when scientists
first concluded that it was a moving object, one of the planets of the
Solar System.
It was only during the 19th century that geologists realized
Earth's age was at least many millions of years. Lord Kelvin used
thermodynamics to estimate the age of Earth to be between 20 million and
400 million years in 1864, sparking a vigorous debate on the subject;
it was only when
radioactivity
and radioactive dating were discovered in the late 19th and early 20th
centuries that a reliable mechanism for determining Earth's age was
established, proving the planet to be billions of years old.
The
burning question being: is
there other intelligent life in the Universe, or are we the only ones?
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