Image of Earth. Credit: NASA

Earth is an ocean 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 encourages life — and also lots the it.

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Earth is the 3rd planet from the Sun, and also the densest and fifth-largest the the eight planets in the Solar System. It is likewise the biggest of the Solar System’s 4 terrestrial planets. It is occasionally referred to as the civilization or the Blue Planet.

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

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

Earth gravitationally interacts with various other objects in space, specifically the Sun and 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 that rotation is tilted 23.4° away from the perpendicular that its orbit plane, producing seasonal variations on the planet’s surface ar with a duration of one tropic year (365.24 solar days).

The Moon is Earth’s only natural satellite. It began orbiting the Earth about 4.53 billion years ago (bya). The Moon’s gravitational communication with planet stimulates ocean tides, stabilizes the axial tilt, and also gradually slowly the planet’s rotation. The planet is house to millions of varieties of life, consisting of humans. Both the mineral sources of the planet and also the assets of the biosphere contribute resources that are supplied to support a worldwide human population. These residents are grouped into around 200 independent sovereign states, which connect through diplomacy, travel, trade, and also military action. Human societies have emerged many see of the planet, including its personification as a planetary deity, its shape as flat, its place as the facility of the universe, and in the modern-day Gaia Principle, as a single, self-regulating biology in its own right.

Formation that Earth

The faster material discovered in the Solar system is date to 4.5672±0.0006 bya; therefore, the is inferred that the earth must have been formed by accretion around this time. By 4.54±0.04 bya the primordial planet had formed. The formation and also evolution that the Solar mechanism bodies arisen in tandem v the Sun. In theory a solar nebula partitions a volume the end of a molecular cloud by gravitational collapse, which starts to spin and also flatten right into a circumstellar disk, and also then the planets prosper out of the in tandem v the star. A nebula has gas, ice cream grains and also dust (including primordial nuclides). In nebular concept planetesimals commence forming as particulate accrues by cohesive clumping and then by gravity. The assembly of the primordial earth proceeded because that 10–20 myr. The Moon developed shortly thereafter, about 4.53 bya.

The Moon’s development remains debated. The functioning hypothesis is the it created by accretion from material loosed native the earth after a Mars-sized object dubbed Theia impacted with Earth. The model, however, is no self-consistent. In this scenario, the mass of Theia is 10% of the Earth’s mass, it impacts with the planet in a glancing blow, and some that its mass merges v the Earth. Between about 3.8 and 4.1 bya, plenty of asteroid impacts throughout the Late heavy Bombardment caused significant changes to the better surface environment of the Moon, and by inference, to the Earth. Earth’s atmosphere and oceans developed by volcanic task and outgassing that contained water vapor. The beginning of the world’s seas was condensation augmented through water and also ice ceded by asteroids, proto-planets, and also comets. In this model, atmospheric “greenhouse gases” retained the oceans from freezing while the newly developing Sun was only at 70% luminosity. By 3.5 bya, the Earth’s magnetic ar was established, which aided prevent the environment from gift stripped away by the solar wind.

A crust created when the molten outer layer that the planet planet cooled to kind a solid together the built up water vapor began to act in the atmosphere. The two models that explain land mass suggest either a steady expansion to the present-day creates or, more likely, a fast growth early in Earth history followed by a permanent steady continental area. Continents developed by bowl tectonics, a process ultimately pushed by the constant loss of warmth from the earth’s interior. Top top time scales lasting thousands of millions of years, the supercontinents have formed and also broken up three times. About 750 mya (million year ago), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to type Pannotia, 600–540 mya, then lastly Pangaea, which likewise broke apart 180 mya.


The form of the earth approximates one oblate spheroid, a round flattened follow me the axis from pole to pole such that there is a bulge around the equator. This bulge outcomes from the rotation that the Earth, and also causes the diameter at the equator to it is in 43 km (kilometer) bigger than the pole-to-pole diameter. Hence the furthest allude on the surface from the Earth’s center of mass is the Chimborazo volcano in Ecuador. The mean diameter the the recommendation spheroid is around 12,742 km, i m sorry is around 40,000 km/π, together the meter was originally identified as 1/10,000,000 the the street from the equator to the phibìc Pole with Paris, France.

Local topography deviates indigenous this idealized spheroid, return on a global scale, these deviations space small: planet has a tolerance of around one part in about 584, or 0.17%, indigenous the recommendation spheroid, i m sorry is much less than the 0.22% tolerance permitted in billiard balls. The biggest local deviations in the rocky surface ar of the planet are mountain 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 areas farthest from the facility of the planet are the summits of mount Chimborazo in Ecuador and also Huascarán in Peru.

Chemical Composition

The massive of the earth is roughly 5.98×1024 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 continuing to be 1.2% consists of trace quantities of various other elements. As result of mass segregation, the core an ar is thought to be mostly composed of iron (88.8%), through smaller amounts of nickel (5.8%), sulfur (4.5%), and also less than 1% map elements.

The geochemist F. W. Clarke calculated the a little an ext than 47% the the Earth’s crust consists of oxygen. The much more common rock constituents that the Earth’s tardy are virtually all oxides; chlorine, sulfur and also fluorine are the only vital exceptions come this and also their full amount in any kind of rock is commonly much much less than 1%. The primary oxides space silica, alumina, stole oxides, lime, magnesia, potash and also soda. The silica features principally together an acid, creating silicates, and all the commonest mineral of igneous rocks room of this nature. From a computation based upon 1,672 analyses of all kinds of rocks, Clarke deduced the 99.22% were composed of 11 oxides (see the table at right), v the various other constituents emerging in minute quantities.

Internal Structure

The interior of the Earth, like that of the various other terrestrial planets, is separated into class by your chemical or physics (rheological) properties, however unlike the other terrestrial planets, it has a distinct outer and inner core. The outer layer that the planet is a chemically distinct silicate hard crust, i m sorry is underlain through a extremely viscous hard mantle. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the tardy varies: averaging 6 kilometres (kilometers) under the oceans and 30-50 kilometres on the continents. The crust and also the cold, rigid, height of the upper mantle are jointly 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 great on which the lithosphere rides. Important transforms in crystal structure within the mantle take place at 410 and also 660 km below the surface, extending a shift zone that separates the upper and also lower mantle. In ~ the mantle, really low viscosity liquid external core lies over a hard inner core. The inside core might rotate in ~ a slightly greater angular velocity than the remainder that the planet, advancing by 0.1–0.5° per year.

Earth cutaway from core to exosphere. Not to scale. Credit: Wikipedia

Orbit and Rotation

Earth’s rotation duration relative come the Sun—its median solar day—is 86,400 secs of median solar time (86,400.0025 SI seconds). Together the Earth’s solar day is now slightly longer than it was during the 19th century because of tidal acceleration, every day varies in between 0 and 2 SI multiple sclerosis longer.

Apart indigenous meteors within the atmosphere and also low-orbiting satellites, the main obvious motion of celestial body in the Earth’s sky is to the west in ~ a rate of 15°/h = 15’/min. Because that bodies close to the celestial equator, this is indistinguishable to an apparent diameter of the sunlight or Moon every 2 minutes; indigenous the planet’s surface, the evident sizes of the Sun and the Moon are roughly the same.


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

The Moon revolves with the Earth about a common barycenter every 27.32 days relative to the lift stars. When linked with the Earth–Moon system’s common transformation around the Sun, the duration of the synodic month, from brand-new moon to new moon, is 29.53 days. Regarded from the celestial phibìc pole, the activity of Earth, the Moon and their axial rotations space all counterclockwise. Regarded from a vantage point over the north poles that both the Sun and the Earth, the planet revolves in a counterclockwise direction around the Sun. The orbital and axial planes are not specifically aligned: earth’s axis is tilted part 23.4 degrees from the perpendicular to the Earth–Sun aircraft (the ecliptic), and also the Earth–Moon plane is tilted as much as ±5.1 degrees against the Earth–Sun plane. Without this tilt, there would be one eclipse every 2 weeks, alternative between lunar eclipses and solar eclipses.

The Hill sphere, or gravitational sphere of influence, of the planet is around 1.5 Gm or 1,500,000 kilometres in radius. This is the maximum street at which the Earth’s gravitational affect is more powerful than the an ext distant Sun and planets. Objects have to orbit the planet within this radius, or lock can end up being unbound by the gravitational perturbation the the Sun.

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

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

Axis Tilt and Seasons

Due come the axial tilt that the Earth, the quantity of sunlight reaching any type of given allude on the surface varies end the food of the year. This reasons seasonal adjust in climate, v summer in the northern hemisphere developing when the phibìc Pole is pointing toward the Sun, and also winter taking place when the pole is pointed away. During the summer, the day lasts longer and also the sun climbs greater in the sky. In winter, the climate becomes generally cooler and the work shorter. Over the Arctic Circle, severe case is got to where over there is no daylight at all for component of the year—a polar night. In the southern hemisphere the instance is specifically reversed, v the south Pole oriented opposite the direction of the phibìc Pole.

By huge convention, the four seasons are identified by the solstices—the allude in the orbit of best axial tilt towards or away from the Sun—and the equinoxes, once the direction that the tilt and also the direction to the Sun room perpendicular. In the north hemisphere, Winter Solstice occurs on about December 21, Summer Solstice is near June 21, feather Equinox is around March 20 and also Autumnal Equinox is around September 23. In the southern hemisphere, the instance is reversed, through the Summer and also Winter Solstices exchanged and also the Spring and Autumnal Equinox dates switched.

The angle of the Earth’s tilt is fairly stable over lengthy periods the time. The tilt does experience nutation; a slight, irregular activity with a main period of 18.6 years. The orientation (rather than the angle) that the Earth’s axis also changes over 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 tropic year. Both the these movements are brought about by the differing attraction the the Sun and also Moon ~ above the Earth’s equatorial bulge. Native the perspective of the Earth, the poles additionally migrate a few meters across the surface. This polar motion has actually multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component come this motion, over there is a 14-month cycle dubbed the Chandler wobble. The rotational velocity that the Earth likewise varies in a phenomenon known as size of day variation.

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In modern times, Earth’s perihelion occurs around January 3, and the aphelion around July 4. This dates readjust over time because of precession and other orbital factors, which monitor cyclical patterns well-known as Milankovitch cycles. The an altering Earth–Sun street causes an increase of about 6.9% in solar power reaching the planet at perihelion loved one to aphelion. Since the southern hemisphere is tilted towards the sun at about the exact same time the the planet reaches the closest technique to the Sun, the southerly hemisphere receive slightly an ext energy from the sun than go the north over the food of a year. This result is much less significant than the total energy adjust due come the axial tilt, and also most the the excess power is soaked up by the higher proportion that water in the southerly hemisphere.