Earth (often referred to as "the Earth", or "the earth") whose Latin name is Tellus (often incorrectly referred to as Terra, meaning soil) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. It is also the largest of its planetary system's terrestrial planets, making it the largest solid body in the solar system, and it is the only place in the universe known to support life. The Earth was formed around 4.57 billion years ago[1] and its largest natural satellite, the Moon, was orbiting it shortly thereafter, around 4.533 billion years ago.

Since it formed, the Earth has changed through geological and biological processes that have hidden traces of the original conditions. The outer surface is divided into several tectonic plates that gradually migrate across the surface over geologic time spans. The interior of the planet remains active, with a thick layer of convecting yet solid Earth mantle and an iron core that generates a magnetic field. The atmospheric conditions have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 71% of the surface is covered in salt water oceans, and the remainder consists of continents and islands.

There is significant interaction between the Earth and its space environment. The relatively large moon provides ocean tides and has gradually modified the length of the planet's rotation period. A cometary bombardment during the early history of the planet is believed to have played a role in the formation of the oceans. Later, asteroid impacts are understood to have caused significant changes to the surface environment. Changes in the orbit of the planet may also be responsible for the ice ages that have covered significant portions of the surface in glacial sheets.

The Earth's only natural orbiting body is the Moon, although the asteroid Cruithne has been erroneously described as such. Cruithne was discovered in 1986 and follows an elliptical orbit around the Sun at about the same average orbital radius as the Earth. However, from the point of view of the moving Earth, Cruithne follows a horseshoe orbit around the Sun that avoids close proximity with the Earth.

Orbital characteristics (Epoch J2000)
Aphelion 152,097,701 km
(1.016 710 333 5 AU)
Perihelion 147,098,074 km
(0.983 289 891 2 AU)
Semi-major axis 149,597,887.5 km
(1.000 000 112 4 AU)
Semi-minor axis 149,576,999.826 km
(0.999 860 486 9 AU)
Orbital circumference 924,375,700 km
( 6.179 069 900 7 AU)
Orbital eccentricity 0.016 710 219
Sidereal orbit period 365.256 366 d
(1.000 017 5 a)
Synodic period n/a
Max. orbital speed 30.287 km/s
(109,033 km/h)
Average orbital speed 29.783 km/s
(107,218 km/h)
Min. orbital speed 29.291 km/s
(105,448 km/h)
Orbital inclination to ecliptic 0
(7.25° to Sun's equator)
Longitude of the ascending node 348.739 36°
Argument of the perihelion 114.207 83°
Satellites 1 (the Moon)
(see also 3753 Cruithne)
Physical characteristics
Aspect Ratio 0.996 647 1
Ellipticity 0.003 352 9
Equatorial radius 6,378.137 km
Polar radius 6,356.752 km
Mean radius 6,372.797 km
Equatorial circumference 40,075.02 km
Meridional circumference 40,007.86 km
Mean circumference 40,041.47 km
Surface area 510,065,600 km²
Land area 148,939,100 km² (29.2 %)
Water area 361,126,400 km² (70.8 %)
Volume 1.083 207 3×1012 km³
Mass 5.9742×1024 kg
Density 5,515.3 kg/m³
Equatorial surface gravity 9.7801 m/s²
(0.997 32 g)
Escape velocity 11.186 km/s
Sidereal rotation period 0.997 258 d (23.934 h)
Rotational velocity at equator 465.11 m/s
Axial tilt 23.439 281°
Right ascension of North pole 0° (0 h 0 min 0 s)
Declination +90°
Albedo 0.367
Surface temperature 185 K (-88 °C) min
287 K (14 °C) mean
331 K (58 °C) max
Surface pressure 101.3 kPa (MSL)
Adjective Terrestrial, Terran, Tellurian, Earthling (lifeforms)
Atmospheric constituents
Nitrogen 78.08 %
Oxygen 20.94 %
Argon 0.93 %
Carbon dioxide 0.038%
Water vapor Trace (varies with climate)
Lexicography:-
In English usage, the name can be capitalized or spelled in lowercase interchangeably, both when used absolutely or prefixed with "the" (i.e. Earth, the Earth, earth or the earth). Many deliberately spell the name of the planet with a capital, both as "Earth" or "the Earth", so as to distinguish it as a proper noun, distinct from the senses of the term as a count noun or verb (e.g. referring to soil, the ground, earthing in the electrical sense, etc.). Oxford Spelling recognizes the lowercase form as the most common, with the capitalized form as a variant of it. Another convention that is very common is to spell the name with a capital when occurring absolutely (e.g. Earth's atmosphere) and lowercase when preceded by "the" (e.g. the atmosphere of the earth). The term almost exclusively exists in lowercase when appearing in common phrases, even without "the" preceding it (e.g. it doesn't cost the earth; what on earth are you doing?).

Terms that refer to the Earth can use the Latin root terr-, as in terraform and terrestrial. An alternative Latin root is tellur-, which is used in words such as telluric, tellurian, tellurion and tellurium. Both terms derive from terra and tellus respectively, which are Latin words meaning "earth", Terra(soil) and Tellus(the planet). Scientific terms such as geography, geocentric and geothermal use the Greek prefix geo- (γαιο-, gaio-), from gē (again meaning "earth").[4] In many science fictions books and video games, Earth is referred to as Terra or Gaia. Astronauts refer to the Earth as "Terra Firma".

The English word "earth" has cognates in many modern and ancient languages. Examples in modern tongues include aarde in Dutch and Erde in German. The root has cognates in extinct languages such as ertha in Old Saxon and ert (meaning "ground") in Middle Irish, derived from the Old English eorðe. All of these words derive from the Proto-Indo-European base *er-.

Several Semitic languages have words for "earth" similar to those in Indo-European languages. Arabic has aard; Akkadian, irtsitu; Aramaic, araa; Phoenician, erets (which appears in the Mesha Stele); and Hebrew, ארץ (arets, or erets when followed by a noun modifier). The etymological connection between the words in Indo-European and Semitic languages are uncertain, though, and may simply be coincidence.

Words for Earth in other languages include: पृथ्वी pr̥thvī (Sanskrit), maa (Finnish), föld (Hungarian), zemlja (Russian), diqiu (Mandarin), deiqao (Cantonese), jigu (Korean), Bumi (Malay), chikyuu (Japanese), and dunia (Swahili)

Symbol
The astrological symbol for Earth consists of a circled cross, the arms of the cross representing a meridian and the equator. Astronomical symbol has the cross atop the circle (♁).