Earth

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This article is about the planet. For its human aspects, see World. For other uses, see Earth (disambiguation) and Planet Earth (disambiguation).

Earth is the third planet from the Sun and the only astronomical object is known to harbor and support life. About 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is covered with water, mostly by oceans, seas, gulfs, and other salt-water bodies, but also by lakes, rivers, and another freshwater, which together constitute the hydrosphere. Much of Earth's polar regions are covered in ice. Earth's outer layer is divided into several rigid tectonic plates that migrate across the surface over many millions of years, while its interior remains active with a solid iron inner core, a liquid outer core that generates Earth's magnetic field, and a convective mantle that drives plate tectonics.

Earth's atmosphere consists mostly of nitrogen and oxygen. More solar energy is received by tropical regions than polar regions and is redistributed by atmospheric and ocean circulation. Greenhouse gases also play an important role in regulating surface temperature. A region's climate is not only determined by latitude, but also by elevation and proximity to moderating oceans, among other factors. Severe weather, such as tropical cyclones, thunderstorms, and heatwaves, occurs in most areas and greatly impacts life.

Earth's gravity interacts with other objects in space, especially the Moon, which is Earth's only natural satellite. Earth orbits around the Sun in about 365.25 days. Earth's axis of rotation is tilted with respect to its orbital plane, producing seasons on Earth. The gravitational interaction between Earth and the Moon causes tides, stabilizes Earth's orientation on its axis, and gradually slows its rotation. Earth is the densest planet in the Solar System and the largest and most massive of the four rocky planets.

According to radiometric dating estimation and other evidence, Earth formed over 4.5 billion years ago. Within the first billion years of Earth's history, life appeared in the oceans and began to affect Earth's atmosphere and surface, leading to the proliferation of anaerobic and, later, aerobic organisms. Some geological evidence indicates that life may have arisen as early as 4.1 billion years ago. Since then, the combination of Earth's distance from the Sun, physical properties, and geological history have allowed life to evolve and thrive. In the history of life on Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinctions. Over 99% of all species that ever lived on Earth are extinct. Almost 8 billion humans live on Earth and depend on its biosphere and natural resources for their survival. Humans increasingly impact Earth's surface, hydrology, atmospheric processes, and other life.

Etymology

The modern English word Earth developed, via Middle English, from an Old English noun most often spelled It has cognates in every Germanic language and their ancestral root has been reconstructed as  In its earliest attestation, the word eerie was already being used to translate the many senses of Latin and Greek  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/Tellus and Greek Gaia, Earth may have been a personified goddess in Germanic paganism: late Norse mythology included, a giantess often given as the mother of Thor.

Historically, earth has been written in lowercase. From early Middle English, its definite sense as "the globe" was expressed as the earth. By Early In modern English, many nouns were capitalized, 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 remain common. House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes "Earth" when appearing as a name (for example, "Earth's atmosphere") but writes it in lowercase when preceded by the (for example, "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "What on earth are you doing?"

Occasionally, the name Terra is used in scientific writing and especially in science fiction to distinguish humanity's inhabited planet from others, while in poetry Tellus 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 In romance languages the word gave rise to names with slightly altered spellings (like the Spanish Tierra and the French Terre). The Latinate form Gaea of the Greek poetic name Gaia  is rare, though the alternative spelling Gaia has become common due to the Gaia hypothesis, in which case its pronunciation is rather then the more classical English 

There are a number of adjectives for the planet Earth. From Earth, itself comes earthly. From the Latin Terra comes Terran  terrestrial and (via French) terrene and from the Latin Tellus comes tellurian and telluric.

Gravitational field

Main article: Gravity of Earth

 


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

Main article: Earth's magnetic field

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 with a magnetic dipole moment of 7.79×1022 Am2 at epoch 2000, decreasing nearly 6% per century. 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.


Magnetosphere

Main article: Magnetosphere

Schematic of Earth's magnetosphere. The solar wind flows from left to right

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 bowshock 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

Main article: Earth's rotation

Earth's rotation imaged by DSCOVR EPIC on 29 May 2016, a few weeks before a solstice.

Earth's rotation period relative to the Sun—it's mean solar day—is 86,400 seconds of mean solar time (86,400.0025 seconds). 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 then 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 means March equinox (when the Sun is at 90° on the equator), are 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 is approximately the same.

Orbit

Main article: Earth's orbit


Earth orbits the Sun at an average distance of about 150 million km (93 million mi) every 365.2564 mean solar days or one sidereal year. This gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, 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 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.