Image the Earth. Credit: NASA

Earth is an s planet. Our house world’s abundance of water — and life — renders it unique in our solar system. Various other planets, plus a couple of moons, have actually ice, atmospheres, seasons and also even weather, yet only on planet does the whole facility mix come with each other in a method that motivates life — and lots the it.

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Earth is the 3rd planet from the Sun, and also the densest and also fifth-largest that the eight planets in the Solar System. That is additionally the largest of the Solar System’s four terrestrial planets. The is occasionally referred to together the people or the Blue Planet.

Earth formed about 4.54 billion years ago, and also life appeared on its surface within its an initial billion years. Earth’s biosphere then significantly changed the atmospheric and also other simple physical conditions, which enabled the proliferation that organisms and also the development of the ozone layer, which in addition to Earth’s magnetic field blocked harmful solar radiation, and also permitted previously ocean-confined life to move safely come land. The physics properties of the Earth, and its geological history and orbit, have enabled life come persist. Estimates on exactly how much much longer the earth will be able to continue to assistance life range from 500 million year (myr), come as lengthy as 2.3 billion years (byr).

Earth’s lithosphere is separated into number of rigid segments, or tectonic plates, the migrate across the surface ar over durations of numerous millions of years. About 71% of the surface is spanned by salt water oceans, through the remainder consist of of continents and also islands which together have countless lakes and also other resources of water that add to the hydrosphere. Earth’s poles are mostly covered with ice that is the solid ice cream of the Antarctic ice sheet and also the sea ice the is the polar ice packs. The planet’s internal remains active, with a solid iron inner core, a liquid outer core the generates the magnetic field, and a special layer of relatively solid mantle.

Earth gravitationally interacts with other objects in space, particularly the Sun and also the Moon. Throughout one orbit about the Sun, the planet rotates around its very own axis 366.26 times, producing 365.26 solar days, or one sidereal year. The Earth’s axis the rotation is tilted 23.4° away from the perpendicular that its orbit plane, producing seasonal sport on the planet’s surface with a duration of one tropical year (365.24 solar days).

The Moon is Earth’s only natural satellite. It started orbiting the Earth around 4.53 exchange rate years earlier (bya). The Moon’s gravitational communication with planet stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet’s rotation. The planet is home to millions of species of life, consisting of humans. Both the mineral resources of the planet and also the assets of the biosphere add resources that are provided to support a global human population. These occupants are grouped into around 200 independent sovereign states, which communicate through diplomacy, travel, trade, and military action. Human societies have developed many views of the planet, consisting of its personification together a planetary deity, its form as flat, its place as the facility of the universe, and also in the modern-day Gaia Principle, together a single, self-regulating biology in its own right.

Formation that Earth

The faster material uncovered in the Solar device is dated to 4.5672±0.0006 bya; therefore, that is inferred the the planet must have actually been developed by accretion roughly this time. By 4.54±0.04 bya the primordial earth had formed. The formation and also evolution of the Solar device bodies emerged in tandem with the Sun. In concept a solar nebula partitions a volume the end of a molecule cloud by gravitational collapse, which starts to spin and flatten into a circumstellar disk, and then the planets grow out of the in tandem through the star. A nebula contains gas, ice grains and dust (including primordial nuclides). In nebular concept planetesimals commence developing as particulate accrues through cohesive clumping and then by gravity. The assembly the the primordial planet proceeded for 10–20 myr. The Moon developed shortly thereafter, about 4.53 bya.

The Moon’s development remains debated. The functioning hypothesis is the it formed by accretion from product loosed indigenous the earth after a Mars-sized object dubbed Theia influenced with Earth. The model, however, is no self-consistent. In this scenario, the massive of Theia is 10% the the Earth’s mass, it impacts with the planet in a glancing blow, and some that its mass merges with the Earth. Between about 3.8 and also 4.1 bya, plenty of asteroid impacts throughout the Late hefty Bombardment caused far-reaching changes come the higher surface setting of the Moon, and also by inference, to the Earth. Earth’s atmosphere and oceans developed by volcanic task and outgassing that included water vapor. The beginning of the world’s oceans was condensation augmented through water and ice delivered by asteroids, proto-planets, and comets. In this model, atmospheric “greenhouse gases” preserved the seas from freeze while the newly creating Sun was just at 70% luminosity. By 3.5 bya, the Earth’s magnetic field was established, which helped prevent the setting from gift stripped far by the solar wind.

A crust formed when the molten outer layer that the planet earth cooled to kind a solid as the collected water vapor started to act in the atmosphere. The two models that explain land mass propose either a steady expansion to the present-day forms or, an ext likely, a quick growth early on in Earth history followed by a permanent steady continent area. Continents developed by plate tectonics, a procedure ultimately driven by the consistent loss of warm from the earth’s interior. ~ above time scales lasting hundreds of millions that years, the supercontinents have actually formed and broken up three times. Approximately 750 mya (million year ago), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to kind Pannotia, 600–540 mya, then ultimately Pangaea, which additionally broke apart 180 mya.


The form of the earth approximates one oblate spheroid, a sphere flattened follow me the axis native pole come pole such the there is a bulge approximately the equator. This bulge outcomes from the rotation of the Earth, and also causes the diameter in ~ the equator to be 43 kilometres (kilometer) larger than the pole-to-pole diameter. For this reason the furthest allude on the surface ar from the Earth’s facility of fixed is the Chimborazo volcano in Ecuador. The mean diameter of the referral spheroid is around 12,742 km, i beg your pardon is about 40,000 km/π, as the meter to be originally identified as 1/10,000,000 the the street from the equator to the phibìc Pole through Paris, France.

Local topography deviates from this idealized spheroid, although on a an international scale, these deviations space small: planet has a tolerance of about one component in about 584, or 0.17%, indigenous the referral spheroid, i beg your pardon is less than the 0.22% tolerance enabled in billiard balls. The largest local deviations in the rocky surface ar of the earth are mount Everest (8,848 m over local sea level) and also the Mariana Trench (10,911 m below local sea level). Due to the equatorial bulge, the surface locations farthest from the center of the planet are the summits of mount Chimborazo in Ecuador and also Huascarán in Peru.

Chemical Composition

The fixed of the earth is roughly 5.98×1024 kg. The is composed greatly of stole (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 continuing to be 1.2% consist of of trace quantities of other elements. Due to mass segregation, the core an ar is thought to be primarily composed of steel (88.8%), through smaller quantities of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.

The geochemist F. W. Clarke calculated that a little much more than 47% that the Earth’s crust is composed of oxygen. The much more common rock constituents of the Earth’s late are nearly all oxides; chlorine, sulfur and fluorine room the only vital exceptions come this and also their complete amount in any rock is commonly much much less than 1%. The principal oxides are silica, alumina, stole oxides, lime, magnesia, potash and also soda. The silica attributes principally together an acid, forming silicates, and also all the commonest mineral of igneous rocks room of this nature. Native a computation based on 1,672 analyses of every kinds the rocks, Clarke deduced that 99.22% were written of 11 oxides (see the table at right), v the other constituents arising in minute quantities.

Internal Structure

The inner of the Earth, prefer that of the various other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties, yet unlike the various other terrestrial planets, it has a unique outer and also inner core. The outer layer that the earth is a chemically distinct silicate solid crust, i beg your pardon is underlain through a highly viscous hard mantle. The tardy is separated from the mantle through the Mohorovičić discontinuity, and the thickness that the crust varies: averaging 6 km (kilometers) under the oceans and 30-50 km on the continents. The crust and the cold, rigid, optimal of the upper mantle are collectively known together the lithosphere, and it is that the lithosphere that the tectonic plates are comprised. In ~ the lithosphere is the asthenosphere, a fairly low-viscosity class on i beg your pardon the lithosphere rides. Important changes in crystal framework within the mantle occur at 410 and also 660 km below the surface, extending a change zone that separates the upper and also lower mantle. Beneath the mantle, really low viscosity liquid outer core lies over a solid inner core. The within core might rotate in ~ a slightly higher angular velocity than the remainder of the planet, progressing by 0.1–0.5° per year.

Earth cutaway from main point to exosphere. Not to scale. Credit: Wikipedia

Orbit and also 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). Together the Earth’s solar work is now slightly longer than the was throughout the 19th century due to tidal acceleration, each day varies between 0 and also 2 SI multiple sclerosis longer.

Apart native meteors in ~ the atmosphere and low-orbiting satellites, the main apparent motion the celestial bodies in the Earth’s sky is to the west in ~ a rate of 15°/h = 15’/min. For bodies near the celestial equator, this is equivalent to an noticeable diameter of the sun or Moon every 2 minutes; indigenous the planet’s surface, the noticeable sizes that the Sun and also the Moon are around the same.


Earth orbits the sun at an average distance of about 150 million kilometers every 365.2564 average solar days, or one sidereal year. Native Earth, this offers an evident movement that the sun eastward with respect to the stars in ~ a price of around 1°/day, i m sorry is one noticeable Sun or Moon diameter every 12 hours. As result of this motion, on typical it take away 24 hours—a solar day—for planet to finish a complete rotation around its axis so the the sunlight returns come the meridian. The orbital speed of the planet averages about 29.8 km/s (107,000 km/h), i beg your pardon is fast enough to take trip a street equal to the planet’s diameter, about 12,742 km, in seven minutes, and also the distance to the Moon, 384,000 km, in around 3.5 hours.

The Moon revolves through the Earth approximately a usual barycenter every 27.32 days loved one to the background stars. When merged with the Earth–Moon system’s common revolution around the Sun, the duration of the synodic month, from new moon to brand-new moon, is 29.53 days. Viewed from the celestial phibìc pole, the activity of Earth, the Moon and also their axial rotations are all counterclockwise. Perceived from a vantage point over the north poles of both the Sun and the Earth, the earth revolves in a counterclockwise direction about the Sun. The orbital and axial planes are not specifically aligned: earth axis is tilted part 23.4 degrees from the perpendicular come the Earth–Sun plane (the ecliptic), and the Earth–Moon aircraft is tilted up to ±5.1 degrees versus the Earth–Sun plane. There is no this tilt, there would certainly be an eclipse every two weeks, alternating between lunar eclipses and also solar eclipses.

The Hill sphere, or gravitational sphere of influence, of the planet is about 1.5 Gm or 1,500,000 km in radius. This is the maximum distance at i beg your pardon the Earth’s gravitational affect is stronger than the more distant Sun and planets. Objects have to orbit the planet within this radius, or castle can come to be unbound by the gravitational perturbation that the Sun.

Earth, along with the Solar System, is situated in the Milky way galaxy and also orbits about 28,000 irradiate years indigenous the center of the galaxy. That is currently about 20 light years above the galactic airplane in the Orion spiral arm.

Earth’s axial tilt (or obliquity) and its relation to the rotation axis and aircraft of orbit. Credit: NASA

Axis Tilt and also Seasons

Due come the axial tilt of the Earth, the lot of sunlight reaching any given allude on the surface ar varies over the food of the year. This reasons seasonal change in climate, v summer in the north hemisphere occurring when the north Pole is pointing toward the Sun, and winter occurring when the pole is sharp away. During the summer, the work lasts longer and the sunlight climbs greater in the sky. In winter, the climate becomes generally cooler and the job shorter. Above the Arctic Circle, severe case is reached where there is no daylight at all for part of the year—a polar night. In the southerly hemisphere the instance is specifically reversed, through the southern Pole oriented opposite the direction that the north Pole.

By astronomical convention, the 4 seasons are determined by the solstices—the allude in the orbit of preferably axial tilt toward or far from the Sun—and the equinoxes, when the direction the the tilt and also the direction to the Sun room perpendicular. In the northern hemisphere, Winter Solstice occurs on about December 21, Summer Solstice is near June 21, spring Equinox is approximately March 20 and Autumnal Equinox is about September 23. In the southerly hemisphere, the situation is reversed, v the Summer and also Winter Solstices exchanged and the Spring and also Autumnal Equinox dates switched.

The edge of the Earth’s tilt is fairly stable over lengthy periods the time. The tilt does experience nutation; a slight, irregular motion with a main period of 18.6 years. The orientation (rather than the angle) that the Earth’s axis additionally changes end time, precessing around in a complete circle end each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and also a dry year. Both of these movements are resulted in by the differing attraction that the Sun and also Moon on the Earth’s equatorial bulge. From the view of the Earth, the poles also migrate a couple of meters across the surface. This polar motion has actually multiple, cyclical components, which collectively are termed quasiperiodic motion. In enhancement to an annual component come this motion, over there is a 14-month cycle called the Chandler wobble. The rotational velocity the the Earth likewise varies in a phenomenon recognized as length of day variation.

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In modern times, Earth’s perihelion occurs approximately January 3, and the aphelion approximately July 4. This dates adjust over time as result of precession and other orbit factors, which follow cyclical patterns known as Milankovitch cycles. The an altering Earth–Sun street causes boost of about 6.9% in solar power reaching the planet at perihelion loved one to aphelion. Because the southern hemisphere is tilted towards the sun at around the very same time that the earth reaches the closest technique to the Sun, the southern hemisphere receive slightly more energy indigenous the sunlight than walk the north over the food of a year. This result is lot less far-reaching than the total energy change due to the axial tilt, and also most that the excess power is soaked up by the higher proportion the water in the southern hemisphere.