Image the Earth. Credit: NASA

Earth is an ocean planet. Our house world’s diversity of water — and also life — renders it unique in our solar system. Other planets, add to a couple of moons, have ice, atmospheres, seasons and even weather, however only on earth does the whole facility mix come with each other in a means that encourages life — and lots of it.

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

Earth formed approximately 4.54 billion years ago, and life showed up on its surface within its an initial billion years. Earth’s biosphere climate significantly altered the atmospheric and also other basic physical conditions, which allowed the proliferation that organisms and also the formation of the ozone layer, which along with Earth’s magnetic ar blocked harmful solar radiation, and also permitted formerly ocean-confined life to move safely come land. The physical properties of the Earth, and its geological history and orbit, have allowed life to persist. Estimates on how much longer the world will have the ability to continue to assistance life range from 500 million year (myr), come as long as 2.3 billion year (byr).

Earth’s lithosphere is separated into several rigid segments, or tectonic plates, the migrate throughout the surface over durations of numerous millions that years. Around 71% that the surface is extended by salt water oceans, through the remainder consisting of continents and islands which together have plenty of lakes and also other resources of water that contribute to the hydrosphere. Earth’s poles are mostly covered v ice the is the solid ice of the Antarctic ice sheet and also the sea ice that is the polar ice packs. The planet’s inner remains active, through a solid stole inner core, a liquid external core the generates the magnetic field, and also a special layer of fairly solid mantle.

Earth gravitationally interacts with various other objects in space, especially the Sun and also the Moon. During one orbit roughly the Sun, the earth rotates about its own axis 366.26 times, developing 365.26 solar days, or one sidereal year. The Earth’s axis of rotation is tilted 23.4° away from the perpendicular the its orbit plane, creating seasonal sport on the planet’s surface ar with a period of one tropical year (365.24 solar days).

The Moon is Earth’s only natural satellite. It began orbiting the Earth about 4.53 exchange rate years earlier (bya). The Moon’s gravitational communication with earth stimulates s tides, stabilizes the axial tilt, and gradually slowly the planet’s rotation. The planet is house to millions of species of life, consisting of humans. Both the mineral sources of the planet and also the assets of the biosphere contribute resources the are offered to support a an international human population. These citizens are group into around 200 independent sovereign states, which interact through diplomacy, travel, trade, and military action. Human societies have emerged many views of the planet, including its personification together a planetary deity, its shape as flat, its place as the center of the universe, and in the contemporary Gaia Principle, as a single, self-regulating organism in its very own right.

Formation that Earth

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

The Moon’s formation remains debated. The functioning hypothesis is that it formed by accretion from product loosed from the earth after a Mars-sized object referred to as Theia affected with Earth. The model, however, is no self-consistent. In this scenario, the mass of Theia is 10% the the Earth’s mass, it impacts with the planet in a glancing blow, and also some the its fixed merges v the Earth. Between roughly 3.8 and 4.1 bya, plenty of asteroid impacts throughout the Late heavy Bombardment caused significant changes come the higher surface environment of the Moon, and also by inference, come the Earth. Earth’s atmosphere and also oceans created by volcanic activity and outgassing that included water vapor. The beginning of the world’s seas was condensation augmented by water and ice yielded by asteroids, proto-planets, and also comets. In this model, atmospheric “greenhouse gases” retained the seas from freeze while the newly creating Sun was only 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 created when the molten outer layer that the planet earth cooled to kind a solid as the collected water vapor started to plot in the atmosphere. The 2 models that describe land mass suggest either a steady growth to the present-day forms or, more likely, a fast growth at an early stage in Earth history followed by a long-term steady continent area. Continents created by key tectonics, a procedure ultimately pushed by the consistent loss of warm from the earth’s interior. On time scales lasting thousands of millions of years, the supercontinents have formed and also broken up three times. Roughly 750 mya (million years ago), among the earliest well-known supercontinents, Rodinia, began to break apart. The continents later on recombined to kind Pannotia, 600–540 mya, then lastly Pangaea, which additionally broke personal 180 mya.


The shape of the earth approximates an oblate spheroid, a round flattened along the axis indigenous pole to pole such the there is a bulge about the equator. This bulge results 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. Thus the furthest allude on the surface ar from the Earth’s facility of fixed is the Chimborazo volcano in Ecuador. The average diameter that the recommendation spheroid is about 12,742 km, i m sorry is around 40,000 km/π, together the meter to be originally defined as 1/10,000,000 of the street from the equator come the north Pole through Paris, France.

Local topography deviates from this idealized spheroid, although on a global scale, this deviations room small: earth has a tolerance of around one part in around 584, or 0.17%, from the recommendation spheroid, i beg your pardon is less than the 0.22% tolerance enabled in billiard balls. The largest local deviations in the rocky surface of the earth are mountain Everest (8,848 m above local sea level) and also the Mariana Trench (10,911 m listed below local sea level). Due to the equatorial bulge, the surface places farthest indigenous the center of the planet are the summits of mount Chimborazo in Ecuador and Huascarán in Peru.

Chemical Composition

The fixed of the earth is about 5.98×1024 kg. It is composed mainly of stole (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and also aluminium (1.4%); through the staying 1.2% consists of trace amounts of other elements. Due to mass segregation, the core region is thought to be mainly composed of iron (88.8%), v smaller amounts of nickel (5.8%), sulfur (4.5%), and also less 보다 1% trace elements.

The geochemist F. W. Clarke calculated that a little much more than 47% the the Earth’s crust is composed of oxygen. The much more common absent constituents that the Earth’s late are practically all oxides; chlorine, sulfur and fluorine space the only vital exceptions to this and their total amount in any type of rock is typically much much less than 1%. The major oxides room silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica attributes principally together an acid, creating silicates, and all the commonest minerals of igneous rocks room of this nature. Native a computation based on 1,672 analyses of all kinds that rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table in ~ right), v the other constituents occurring in minute quantities.

Internal Structure

The internal of the Earth, choose that of the other terrestrial planets, is separated into class by their chemical or physics (rheological) properties, yet unlike the other terrestrial planets, it has actually a unique outer and also inner core. The outer layer the the planet is a chemically distinctive silicate solid crust, which is underlain by a highly viscous heavy mantle. The late is separated indigenous the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 kilometres (kilometers) under the oceans and 30-50 kilometres on the continents. The crust and the cold, rigid, peak of the upper mantle are jointly known as the lithosphere, and it is that the lithosphere that the tectonic plates are comprised. Beneath the lithosphere is the asthenosphere, a reasonably low-viscosity great on i m sorry the lithosphere rides. Important alters in crystal framework within the mantle take place at 410 and also 660 km listed below the surface, extending a transition zone the separates the upper and also lower mantle. Beneath the mantle, an extremely low viscosity liquid external core lies above a heavy inner core. The inside core might rotate at a slightly greater angular velocity 보다 the remainder of the planet, proceeding 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 come the Sun—its average solar day—is 86,400 seconds of average solar time (86,400.0025 SI seconds). As the Earth’s solar day is currently slightly much longer than it was throughout the 19th century because of tidal acceleration, each day varies between 0 and also 2 SI multiple sclerosis longer.

Apart native meteors within the atmosphere and low-orbiting satellites, the main noticeable motion of celestial body in the Earth’s skies is come the west in ~ a price of 15°/h = 15’/min. For bodies near the celestial equator, this is indistinguishable to an noticeable diameter the the sun or Moon every two minutes; native the planet’s surface, the apparent sizes the the Sun and the Moon are roughly the same.


Earth orbits the sun at an median distance of around 150 million kilometers every 365.2564 mean solar days, or one sidereal year. Native Earth, this provides an noticeable movement of the sun eastward with respect to the stars at a price of about 1°/day, i beg your pardon is one obvious Sun or Moon diameter every 12 hours. Due to this motion, on average it bring away 24 hours—a solar day—for planet to finish a full rotation around its axis so that the sun returns come the meridian. The orbital speed of the planet averages about 29.8 km/s (107,000 km/h), which is fast enough to take trip a distance equal to the planet’s diameter, about 12,742 km, in 7 minutes, and also the street to the Moon, 384,000 km, in about 3.5 hours.

The Moon revolves with the Earth about a usual barycenter every 27.32 days family member to the lift stars. When combined with the Earth–Moon system’s common change around the Sun, the period of the synodic month, from brand-new moon to brand-new moon, is 29.53 days. Regarded 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 that both the Sun and also the Earth, the earth revolves in a counterclockwise direction about the Sun. The orbital and also axial planes space not precisely aligned: earth axis is tilted part 23.4 levels from the perpendicular come the Earth–Sun plane (the ecliptic), and the Earth–Moon airplane is tilted as much as ±5.1 degrees versus the Earth–Sun plane. There is no this tilt, there would be an eclipse every two weeks, alternate between lunar eclipses and solar eclipses.

The Hill sphere, or gravitational round of influence, that the earth is around 1.5 Gm or 1,500,000 kilometres in radius. This is the maximum distance at i beg your pardon the Earth’s gravitational influence is more powerful than the more distant Sun and also planets. Objects must orbit the planet within this radius, or they can end up being unbound through the gravitational perturbation of the Sun.

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

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

Axis Tilt and also Seasons

Due come the axial tilt the the Earth, the lot of sunshine reaching any given suggest on the surface ar varies end the food of the year. This causes seasonal change in climate, v summer in the northern hemisphere developing when the phibìc Pole is pointing toward the Sun, and winter following when the pole is pointed away. During the summer, the work lasts longer and the sun climbs greater in the sky. In winter, the climate becomes generally cooler and the days shorter. Over the Arctic Circle, an extreme case is reached where over there is no daylight at all for component of the year—a polar night. In the southerly hemisphere the instance is exactly reversed, through the south Pole oriented opposite the direction of the north Pole.

By expensive convention, the 4 seasons are figured out by the solstices—the point in the orbit of maximum axial tilt towards or away from the Sun—and the equinoxes, once the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere, Winter Solstice wake up on about December 21, Summer Solstice is near June 21, spring Equinox is roughly March 20 and also Autumnal Equinox is about September 23. In the southern hemisphere, the instance is reversed, through the Summer and Winter Solstices exchanged and the Spring and also Autumnal Equinox days switched.

The angle of the Earth’s tilt is relatively stable over lengthy periods that time. The tilt does undergo nutation; a slight, irregular movement with a main period of 18.6 years. The orientation (rather 보다 the angle) the the Earth’s axis also changes end time, precessing about in a finish circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and also a tropic year. Both of these movements are brought about by the differing attraction of the Sun and Moon top top the Earth’s equatorial bulge. Native the perspective of the Earth, the poles additionally migrate a few meters throughout the surface. This polar motion has multiple, cyclical components, which jointly 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 that the Earth also varies in a phenomenon well-known as length of work variation.

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In modern times, Earth’s perihelion occurs approximately January 3, and the aphelion roughly July 4. These dates adjust over time because of precession and also other orbit factors, which monitor cyclical patterns recognized as Milankovitch cycles. The an altering Earth–Sun street causes rise of around 6.9% in solar power reaching the planet at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the sun at about the exact same time that the earth reaches the closest approach to the Sun, the southerly hemisphere obtain slightly much more energy from the sun than go the north over the course of a year. This result is lot less significant than the complete energy adjust due come the axial tilt, and also most of the excess energy is took in by the higher proportion of water in the southern hemisphere.