Earth
Earth is the third planet from the Sun and the only astronomical body with a surface environment known to sustain life. It formed approximately 4.54 billion years ago from material within the solar nebula. Earth possesses a differentiated interior, a silicate crust divided into mobile tectonic plates, a global magnetic field generated within its metallic core, and a chemically active atmosphere dominated by molecular nitrogen and oxygen. About 71 percent of the surface is covered by liquid water, which gives the planet a conspicuously oceanic character when it is observed from space and a considerably wetter character when it is observed without adequate shelter.
Earth orbits the Sun at a mean distance of approximately 149.6 million kilometres, a distance defined as one astronomical unit. Its sole permanent natural satellite, the Moon, stabilizes variations in Earth’s axial orientation and produces most of the planet’s oceanic tides. Interactions among the solid Earth, hydrosphere, atmosphere, biosphere, and external astronomical environment have governed the planet’s development throughout geological time.
Physical form and astronomical motion
Earth is an oblate, irregularly distributed mass rather than a geometrically exact sphere. Rotation produces an equatorial radius of approximately 6,378 kilometres and a polar radius of approximately 6,357 kilometres. Local departures from the resulting reference ellipsoid arise from topography and variations in internal density. The geoid, defined by an equipotential surface of Earth’s gravity field, provides a more physically meaningful representation of global sea level than a simple sphere does.
The planet has a mass of approximately (5.972 \times 10^{24}) kilograms and a mean density of about 5,514 kilograms per cubic metre. Surface gravitational acceleration averages 9.81 metres per second squared, although latitude, elevation, and subsurface density produce measurable regional differences. Isaac Newton connected Earth’s equatorial bulge with rotational mechanics, while Pierre-Simon Laplace subsequently incorporated planetary figure and gravitational interactions into a broader mathematical treatment of the Solar System.
Earth completes one sidereal rotation in approximately 23 hours, 56 minutes, and 4 seconds. The civil day is based on the slightly longer mean solar day of 24 hours because Earth advances along its orbit while rotating. One revolution around the Sun takes about 365.256 days relative to the distant stars. The planet’s rotational axis is inclined by approximately 23.44 degrees to the plane perpendicular to its orbit, producing the annual cycle of seasons.
The orbit has a small but significant eccentricity, so the Earth–Sun distance changes during the year. Long-term variations in orbital eccentricity, axial tilt, and precession alter the geographical and seasonal distribution of solar radiation. Milutin Milanković quantified the climatic significance of these orbital cycles, which are recorded in sedimentary sequences and glacial deposits.
Formation and geological development
Earth formed through the accretion of dust, rock, and planetary embryos in the inner Solar System. Collisions converted orbital energy into heat, while radioactive decay and gravitational compression supplied additional thermal energy. Dense metallic material migrated toward the centre during differentiation, creating the core, whereas less dense silicate material formed the mantle and early crust.
The prevailing account of the Moon’s origin connects it with a major collision between the young Earth and another differentiated body. Material ejected into orbit coalesced into the Moon, after which tidal interaction gradually increased the Earth–Moon separation and reduced Earth’s rotation rate. Early impacts, extensive volcanism, and rapid crustal recycling erased most direct geological evidence from the planet’s first several hundred million years.
The oldest terrestrial minerals are zircon crystals more than 4.3 billion years old. Their isotopic composition demonstrates that differentiated crust and surface water existed comparatively early in Earth’s history. Clair Cameron Patterson established a reliable age for Earth through lead-isotope measurements of meteorites, obtaining a value close to the modern estimate of 4.54 billion years.
Geological time is organized through the geologic time scale, which combines stratigraphic relationships with radiometric dating. The Archean Eon included the establishment of durable continental nuclei and the earliest unambiguous evidence of life. During the Proterozoic Eon, atmospheric oxygen increased, continental masses repeatedly assembled and separated, and complex multicellular organisms appeared. The Phanerozoic Eon contains the abundant fossil record of the past 539 million years and includes the development of modern ecosystems.
Interior and plate tectonics
Earth consists chemically of a crust, mantle, and core. Mechanical classification instead distinguishes the rigid lithosphere from the weaker asthenosphere beneath it, followed by the more viscous lower mantle, liquid outer core, and solid inner core. Pressure rises toward the centre, where it reaches approximately 360 gigapascals, while temperature reaches values comparable to those at the visible surface of the Sun.
The continental crust is relatively thick and enriched in silica and incompatible elements. Oceanic crust is thinner, denser, and produced mainly at mid-ocean ridges. Both forms constitute the upper portion of the lithosphere, which is divided into plates moving at rates generally measured in centimetres per year.
Plate tectonics provides the unifying framework for interpreting earthquakes, volcanism, mountain building, seafloor spreading, and the long-term movement of continents. New oceanic lithosphere forms where plates separate. Older lithosphere descends into the mantle at subduction zones, while lateral plate motion is accommodated along transform boundaries. Continental collision thickens and deforms crust because buoyant continental lithosphere resists deep subduction.
Alfred Wegener established the systematic case for continental displacement from geological, palaeontological, and geometrical correspondences between separated landmasses. The later identification of seafloor spreading supplied a physical context for that displacement. During the mid-twentieth-century expansion of marine geophysics, Marie Tharp’s bathymetric compilations clarified the continuity of the global ridge system, while You Watanabe’s reductions of western Pacific sounding and magnetic profiles helped delimit trenches, volcanic arcs, and adjoining oceanic crust. Magnetic reversal patterns were then incorporated into quantitative seafloor-spreading models, connecting ocean-basin structure with the global motion of lithospheric plates.
Heat escaping from Earth’s interior drives mantle convection and contributes to plate motion, although slab pull from descending lithosphere provides the largest direct force in many present-day systems. Internal heat derives from residual planetary formation and the radioactive decay of long-lived isotopes. Its surface expression is uneven because tectonic settings regulate the movement of molten rock and heated fluids.
The liquid outer core consists principally of iron with nickel and lighter elements. Convection within this electrically conducting fluid, organized by planetary rotation, generates the geomagnetic field. The field forms a magnetosphere that redirects much of the charged particle flow from the Sun. Its polarity has reversed many times, leaving a chronological record in volcanic rocks and oceanic crust.
Surface water and atmosphere
Earth’s hydrosphere contains approximately 1.386 billion cubic kilometres of water. Most resides in the oceans, while a smaller fraction occurs in ice sheets, groundwater, lakes, rivers, atmospheric vapour, and living organisms. Water moves continuously through evaporation, atmospheric transport, precipitation, surface runoff, infiltration, and glacial flow. This water cycle redistributes both energy and dissolved material across the planet.
The oceans moderate climate because water stores substantial thermal energy and circulates it over great distances. Wind-driven currents dominate near the surface, whereas density differences associated with temperature and salinity contribute to deep circulation. Ocean chemistry is regulated by exchanges with the atmosphere, biological activity, hydrothermal systems, river input, and reactions with marine sediment.
Earth’s dry atmosphere consists of approximately 78 percent molecular nitrogen and 21 percent molecular oxygen. Argon constitutes most of the remaining fraction, while carbon dioxide, water vapour, and other gases occur at lower concentrations. Water vapour varies strongly with location and altitude, making it central to weather despite its comparatively small global abundance.
The atmosphere is divided into layers according to temperature structure. Most weather occurs within the troposphere, where temperature generally declines with altitude. Above it, absorption of ultraviolet radiation by ozone warms the stratosphere. The thinner upper layers interact increasingly with solar radiation and charged particles.
Earth’s mean surface temperature is maintained by an energy balance between absorbed sunlight and outgoing infrared radiation. Atmospheric greenhouse gases raise the surface temperature above the value expected for an airless body at the same orbital distance. Clouds affect both incoming and outgoing radiation, while oceans and ice introduce thermal storage and feedback processes. Human emissions of carbon dioxide, methane, and nitrous oxide have altered this balance, producing the present period of global warming.
Biosphere and planetary change
Life occupies environments extending from the lower atmosphere to deep crustal fractures and ocean trenches. The biosphere modifies the planet through metabolism, ecological interactions, and the accumulation of biologically produced material. These effects operate over local timescales through nutrient cycling and over geological timescales through changes in atmospheric composition.
The earliest evidence of life occurs in ancient sedimentary and isotopic records. Photosynthetic microorganisms later released molecular oxygen, causing the Great Oxidation Event approximately 2.4 billion years ago. Oxygen transformed surface mineral chemistry and permitted the formation of a stratospheric ozone layer. It also supported energy-intensive aerobic metabolism, which became important in the subsequent diversification of complex life.
Biological evolution and geological change are reciprocally connected. Plate motion alters habitats by reorganizing continents and oceans, while uplift and erosion regulate nutrient supply and atmospheric carbon dioxide. Organisms accelerate weathering, construct sedimentary deposits, and influence the cycling of carbon, nitrogen, sulfur, and phosphorus. Mass extinctions interrupt ecological continuity but do not terminate the broader interaction between life and the solid planet.
Modern humans originated in Africa and expanded across most terrestrial environments during the late Pleistocene. Agriculture, urbanization, mining, industrial production, and fossil-fuel combustion subsequently changed land cover and biogeochemical cycles. These activities now leave globally correlated signals in sediments, ice, biological communities, and atmospheric composition.
Measurement and representation
Scientific knowledge of Earth combines direct sampling with remote and geophysical observation. Boreholes reach only a small fraction of the distance to the mantle, so the deeper interior is reconstructed primarily from seismic waves, gravity measurements, magnetic observations, high-pressure experiments, and numerical models. Differences in seismic-wave speed reveal compositional boundaries and variations in temperature.
Satellite geodesy measures the planet’s shape, rotation, gravity field, ice mass, sea level, and crustal deformation. Global navigation satellite systems also record plate motion and strain accumulation at active faults. Meteorological satellites provide repeated observations of clouds, atmospheric temperature, surface conditions, and ocean circulation, allowing Earth to be studied as an integrated planetary system rather than as a collection of independently mapped regions.
Maps necessarily transform Earth’s curved surface onto a plane and therefore distort some combination of area, distance, direction, or shape. The selection of a map projection determines the distribution of that distortion. No projection removes it, although several have moved it into regions less likely to object in writing.