The third planet from the Sun and the largest of the four inner, or terrestrial, planets. The Sun is an average-sized, middle-aged star situated toward the outer edge of one of the spiral arms of the Milky Way Galaxy. So far as is known, Earth is unique in the solar system in having life. Whether life exists in the universe beyond the solar system is unknown.

Earth has one natural satellite, the Moon. Otherwise, Earth's  nearest neighbors in space are Venus, which is about 108 × 10⁶ km (67 × 10⁶ mi) from the Sun, and Mars, about 228 × 10⁶ km (141 × 10⁶ mi) from the Sun. Earth is about 150 × 10⁶ km (93 × 10⁶ mi) from the Sun.

Earth completes an orbit around the Sun in 365 days, 5 h, 48 min, 46 s; the orbit defines the length of the year. The length of the day is determined by the period of Earth's  rotation about its axis. The fact that the year is not a whole number of days has affected the development of the calendar.

Earth rotates on its axis once each day. The axis of rotation is perpendicular to the Equator, and the Equator is inclined at about 23.5° to the plane of Earth's  orbit around the Sun. As Earth moves in its orbit, the north spin axis, or north geographic pole, points in the direction of the star Polaris, making it the North Star or polestar. One result of the tilt of the Equator relative to the orbital plane is that different parts of Earth receive differing amounts of sunlight through the year; this is the primary cause of seasons.Equator

Earth is an oblate spheroid. The mean equatorial radius is 6378.139 km (3963.37 mi), and the polar radius is 6356.779 km (3950.10 mi), the difference being 21.360 km (13.27 mi).

Earth's  mass is 5.976 × 10²⁷ g (0.2108 × 10²⁷ oz), being the sum of 5.974 × 10²⁷ g (0.2107 × 10²⁷ oz) for solid Earth, 1.4 × 10²⁴ g (0.049 × 10²⁴ oz) for the ocean, and 5.1 × 10²¹ g (0.18 × 10²¹ oz) for the atmosphere. Earth's  average density is 5.518 g/cm³, which is just about double the density of the common rocks that form at Earth's  surface, indicating that Earth's  interior is more dense than the surface. Seismic studies have confirmed that Earth is layered both compositionally and mechanically . Atmosphere Oceanography

The deepest compositional layer is the core, which is divided into a solid inner core and a liquid outer core. Both the inner and outer core have the same composition, believed to be nickel-iron plus a small amount of lighter elements such as sulfur and silicon. Electric currents moving in the molten metal outer core are believed to be the origin of Earth's  magnetic field. Above the core is the mantle which, on the basis of density of rare rock samples brought up from deep in the mantle in kimberlite pipes, and other evidence, is believed to be composed of silicate minerals, and in particular olivine and pyroxene. A rock composed largely of olivine and pyroxene is called a peridotite.

Above the mantle is Earth's  crust, and between the crust and the mantle there is a pronounced seismic discontinuity known as the Mohoroviić discontinuity, or Moho. The crust is of two kinds, both of which are less dense and compositionally different from the peridotitic mantle below. Beneath the ocean the crust is basaltic in composition and about 8 km (5 mi) thick. The crust beneath the continents is granitic in composition and averages 35 km (21.7 mi) in thickness but ranges up to 80 km (49.7 mi), as beneath Tibet. The oceanic crust is geologically young because it is continually created and destroyed through the process of plate tectonics. No part of the oceanic crust that is older than about 180 × 10⁶ years has yet been discovered. The continental crust is much older than the oceanic crust. Continental rocks as old as 4 × 10⁹ years have been discovered in Canada, and the fact that they are highly deformed indicates a long and eventful history.Earth crust

The surface of solid Earth has a bimodal distribution of elevations. If the water from the ocean could be removed, it would be apparent that continents stand high (average elevation is 840 m or 2755 ft above sea level), while the ocean floor sits low (average elevation is 3800 m or 12,464 ft below sea level). This difference in elevation arises because rigid lithosphere floats on the weak asthenosphere, and because the density of oceanic lithosphere (that is, lithosphere capped by oceanic crust) is greater than the density of continental lithosphere.

On the continents, mountain belts are the most dramatic features. They range in elevation from Mount Everest, 8848 m (29,030 ft), in the Himalaya Mountains to older, deeply eroded ranges that are now barely above sea level. Granitic and metamorphic rocks are generally exposed in the cores of mountain ranges. The overlying rocks that cover most of the Earth's  surface are sedimentary, mainly of shallow marine origin, that may or may not have been deformed. The deformation is the result of compression and tension that causes folding and faulting, and may be accompanied by intrusion and metamorphism. Movements and collisions of tectonic plates are the principal cause of mountain building. Mountains generally are formed over several tens of millions of years. The rocks deformed in the process are generally marine sedimentary rocks formed along the margins of continents.Deep-marine sediments Marine sediments Sedimentary rocks

The topographic features underlying the oceans are similarly diverse and reveal more evidence of a dynamic Earth. The continental shelf, an area covered by shallow water, generally less than 150 m (500 ft) deep, surrounds the continents at most places. Such areas are generally underlain by continental, granitic rocks, and are submerged parts of the continents. Continental slopes are the transition between the continental shelf and the ocean floors. Their tops are generally less than 150 m below sea level, and they slope down to about 4400 m (14,000 ft). They are narrow, steep features, with slopes generally between 2 and 6°, but some are up to 45°. They are generally underlain by thick accumulations of sedimentary rocks.Sea-floor imaging

Submarine trenches and their associated volcanic island arcs are formed as a result of a tectonic plate of lithosphere sinking into the mantle beneath the edge of an overriding plate. The deepest place on Earth is in the Mariana Trench, 11,022 m (36,152 ft) below sea level.

The ocean floor is the most widespread surface feature of Earth. Beneath an average of 4.4 km (2.75 mi) of seawater are about 2.3 km (1.4 mi) of sedimentary rocks with some intercalated basalt, and below that is the oceanic crust, consisting of 4–6 km (2.5–3.7 mi) of basaltic rocks. Interrupting the ocean floor at many places are submarine mountains formed by basaltic volcanoes. Some of these volcanoes are very large and form oceanic islands such as the Hawaiian Islands.

New oceanic lithosphere capped by basaltic crust is created at the mid-ocean ridges, and this newly formed plate moves away from the ridges. The tectonic plates formed in this way may carry continents on them, and are the mechanism of continental drift. Paleomagnetic data from the continents indicate that the continents have moved relative to each other. The tectonic plates capped by basaltic crust plates are consumed at the trench-volcanic island arc areas.

As well as the ridge and the trench, a third type of plate boundary occurs where two plates slide past each other at a transform fault. Such collisions account for the deformed rocks found in the crust.

The evidence for continental drift in the geological past includes matching of rock types, ages, fossils, climates, and structures (mountain ranges), as well as the paleomagnetic data. Evidence showing or suggesting present movements consists of shallow earthquakes along mid-ocean ridges and transform faults that offset them; deep earthquakes associated with deep-sea trench-volcanic island arc areas; direct measurement of movement; volcanic activity at mid-ocean ridges; and volcanic activity at trench-island arc areas.

Earth's  temperature and gravitation are such that an atmosphere is present. The major constituents are nitrogen and oxygen. A thin ozone layer in the atmosphere shields the Earth from lethal ultraviolet radiation from the Sun. The atmosphere, especially oxygen, and the presence of water, both at the surface and in the atmosphere, make life possible. Precipitation, mainly rain, results in running water such as streams and rivers on the continents. Running water is the main cause of erosion of the continents, and most of the landscapes have been eroded by water, although some are eroded by wind or ice (glaciers).Erosion Glaciology

Earth, along with the rest of the solar system, is believed to have formed about 4.55 × 10⁹ years ago. This age is determined by dating radioactive isotopes in meteorities. Meteorites are believed to be fragments produced by collisions among small bodies formed by the same process that created the solar system. Theoretical studies of the Sun and other studies of radioactive isotopes also suggest a similar age.