The earth moves round the sun in an oval track that has an average radius of 93 million miles, at a speed of 18.5 miles a second.
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the world or the Blue Planet.
Earth formed approximately 4.54 billion years ago, and life appeared on its surface within its first billion years. Earth's biosphere then significantly altered the atmospheric and other basic physical conditions, which enabled the proliferation of organisms as well as the formation of the ozone layer, which together with Earth's magnetic field blocked harmful solar radiation, and permitted formerly ocean-confined life to move safely to land. The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist. Estimates on how much longer the planet will be able to continue to support life range from , to as long as .
Earth's lithosphere is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice that is the solid ice of the Antarctic ice sheet and the sea ice that is the polar ice packs. The planet's interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solid mantle.
Earth gravitationally interacts with other objects in space, especially the Sun and the Moon. During one orbit around the Sun, the Earth rotates about its own axis 366.26 times, creating 365.26 solar days, or one sidereal year. The Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface 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 . The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation.
The planet is home to millions of species of life, including humans. Both the mineral resources of the planet and the products of the biosphere contribute resources that are used to support a global human population. These inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade, and military action. Human cultures have developed many views of the planet, including its personification as a planetary deity, its shape as flat, its position as the center of the universe, and in the modern Gaia Principle, as a single, self-regulating organism in its own right.
The modern English noun earth developed from Middle English erthe (recorded in 1137), itself from Old English eorthe (dating from before 725), deriving from Proto-Germanic *erthō. Earth has cognates in all other Germanic languages, including Dutch aarde, German Erde, and Swedish, Norwegian, and Danish jord. The Earth is personified as a goddess in Germanic paganism (appearing as Jörð in Norse mythology, mother of the god Thor).
In general English usage, the name earth can be capitalized or spelled in lowercase interchangeably, either 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". This is to distinguish it as a proper noun, distinct from the senses of the term as a mass 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 common convention 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 does not cost the earth.", "What on earth are you doing?").
The earliest material found in the Solar System is dated to ; therefore, it is inferred that the Earth must have been formed by accretion around this time. By the primordial Earth had formed. The formation and evolution of the Solar System bodies occurred in tandem with the Sun. In theory a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that in tandem with the star. A nebula contains gas, ice grains and dust (including primordial nuclides). In nebular theory planetesimals commence forming as particulate accrues by cohesive clumping and then by gravity. The assembly of the primordial Earth proceeded for 10–. The Moon formed shortly thereafter, about .
The Moon's formation remains debated. The working hypothesis is that it formed by accretion from material loosed from the Earth after a Mars-sized object dubbed Theia impacted with Earth. The model, however, is not self-consistent. In this scenario, the mass of Theia is 10% of the Earth's mass, it impacts with the Earth in a glancing blow, and some of its mass merges with the Earth. Between approximately 3.8 and , numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon, and by inference, to the Earth.
Earth's atmosphere and oceans formed by volcanic activity and outgassing that included water vapor. The origin of the world's oceans was condensation augmented by water and ice delivered by asteroids, proto-planets, and comets. In this model, atmospheric "greenhouse gases" kept the oceans from freezing while the newly forming Sun was only at 70% luminosity. By , the Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.
A crust formed when the molten outer layer of the planet Earth cooled to form a solid as the accumulated water vapor began to act in the atmosphere. The two models that explain land mass propose either a steady growth to the present-day forms or, more likely, a rapid growth early in Earth history followed by a long-term steady continental area. Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from the earth's interior. On time scales lasting hundreds of millions of years, the supercontinents have formed and broken up three times. Roughly (million years ago), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–, then finally Pangaea, which also broke apart .
Highly energetic chemistry is thought to have produced a self-replicating molecule around and half a billion years later the last common ancestor of all life existed. The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed a layer of ozone (a form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes. True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.
Since the 1960s, it has been hypothesized that severe glacial action between 750 and , during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.
Following the Cambrian explosion, about , there have been five major mass extinctions. The most recent such event was , when an asteroid impact triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared some small animals such as mammals, which then resembled shrews. Over the past , mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright. This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of the human race. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had, affecting both the nature and quantity of other life forms.
The present pattern of ice ages began about and then intensified during the Pleistocene about . High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–. The last continental glaciation ended 10,000 years ago.
The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next and by 40% over the next . Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the loss of the planet's oceans.
The Earth's increasing surface temperature will accelerate the inorganic cycle2CO, reducing its concentration to levels lethally low for plants ( for C4 photosynthesis) in approximately 500-. The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years. After another billion years all surface water will have disappeared and the mean global temperature will reach (). The Earth is expected to be effectively habitable for about another from that point, although this may be extended up to if the nitrogen is removed from the atmosphere. Even if the Sun were eternal and stable, 27% of the water in the modern oceans will descend to the mantle in one billion years, due to reduced steam venting from mid-ocean ridges.
The Sun, as part of its evolution, will become a red giant in about . Models predict that the Sun will expand out to about 250 times its present radius, roughly 1 AU (150,000,000 km). Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 km) from the Sun, when the star reaches its maximum radius. The planet was, therefore, initially expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life would have been destroyed by the Sun's increased luminosity (peaking at about 5,000 times its present level). A 2008 simulation indicates that the Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be vaporized. After that, the Sun's core will collapse into a white dwarf, as its outer layers are ejected into space as a planetary nebula. The matter that once made up the Earth will be released into interstellar space, where it may one day become incorporated into a new generation of planets and other celestial bodies.
Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four terrestrial planets in size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation, and is probably the only one with active plate tectonics.
The shape of the Earth approximates an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator. This bulge results from the rotation of the Earth, and causes the diameter at the equator to be (kilometer) larger than the pole-to-pole diameter. For this reason the furthest point on the surface from the Earth's center of mass is the Chimborazo volcano in Ecuador. The average diameter of the reference spheroid is about , which is approximately 40,000 km/π, as the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.
Local topography deviates from this idealized spheroid, although on a global scale, these deviations are small: Earth has a tolerance of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in billiard balls. The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m above local sea level) and the Mariana Trench ( below local sea level). Due to the equatorial bulge, the surface locations farthest from the center of the Earth are the summits of Mount Chimborazo in Ecuador and Huascarán in Peru.
The mass of the Earth is approximately . It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.
The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right), with the other constituents occurring in minute quantities.
The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties, but unlike the other terrestrial planets, it has a distinct outer and inner core. The outer layer of the Earth is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging (kilometers) under the oceans and 30- on the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are comprised. Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core. The inner core may rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.
Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%). The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. At the center of the planet, the temperature may be up to , and the pressure could reach 360 GPa. Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. This extra heat production, twice present-day at approximately , would have increased temperature gradients within the Earth, increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatiites that are not formed today.
The mean heat loss from the Earth is , for a global heat loss of . A portion of the core's thermal energy is transported toward the crust by mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts. More of the heat in the Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs in the oceans because the crust there is much thinner than that of the continents.
The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: Convergent boundaries, at which two plates come together, Divergent boundaries, at which two plates are pulled apart, and Transform boundaries, in which two plates slide past one another laterally. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries. The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates, and their motion is strongly coupled with convection patterns inside the Earth's mantle.
As the tectonic plates migrate across the planet, the ocean floor is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes continually recycles the oceanic crust back into the mantle. Due to this recycling, most of the ocean floor is less than old in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about . By comparison, the oldest dated continental crust is .
The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American. Other notable plates include the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between 50 and . The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/year and the Pacific Plate moving 52–69 mm/year. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/year.
The Earth's terrain varies greatly from place to place. About 70.8% of the surface is covered by water, with much of the continental shelf below sea level. This equates to (139.43 million sq mi). The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes, oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% (, or 57.51 million sq mi) not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.
The planetary surface undergoes reshaping over geological time periods due to tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts also act to reshape the landscape.
The continental crust consists of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors. Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust. The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine. Common carbonate minerals include (found in limestone) and dolomite.
The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops. Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3107 km2 of cropland and 3.4107 km2 of pastureland.
The elevation of the land surface of the Earth varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 840 m.
Besides being divided logically into Northern and Southern Hemispheres centered on the earths poles, the earth has been divided arbitrarily into Eastern and Western Hemispheres.
The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the Solar System. The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911.4 m.
The mass of the oceans is approximately 1.351018 metric tons, or about 1/4400 of the total mass of the Earth. The oceans cover an area of with a mean depth of , resulting in an estimated volume of . If all the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km. About 97.5% of the water is saline, while the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is currently ice.
The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (35 ‰ salt). Most of this salt was released from volcanic activity or extracted from cool, igneous rocks. The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms. Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir. Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.
The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km. It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation resulting from weather and seasonal factors.
Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved , forming the primarily nitrogen–oxygen atmosphere of today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature. This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Water vapor, carbon dioxide, methane and ozone are the primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the average surface would be −18 °C, in contrast to the current +15 °C, and life would likely not exist.
The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower-density air then rises, and is replaced by cooler, higher-density air. The result is atmospheric circulation that drives the weather and climate through redistribution of thermal energy.
The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°. Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.
Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation. Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.
The amount of solar energy reaching the Earth's decreases with increasing latitude. At higher latitudes the sunlight reaches the surface at lower angles and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4 °C per degree of latitude away from the equator. The Earth can be subdivided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates. Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly used Köppen climate classification system (as modified by Wladimir Köppen's student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.
Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere. Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere, where the Earth's magnetic fields interact with the solar wind. Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. The Kármán line, defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.
Thermal energy causes some of the molecules at the outer edge of the Earth's atmosphere to increase their velocity to the point where they can escape from the planet's gravity. This causes a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate than other gasses. The leakage of hydrogen into space contributes to the pushing of the Earth from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is believed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere. Hence the ability of hydrogen to escape from the Earth's atmosphere may have influenced the nature of life that developed on the planet. In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.
The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. At the equator of the magnetic field, the magnetic field strength at the planet's surface is , with global magnetic dipole moment of . According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.
The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora.
Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds). As the Earth's solar day is now slightly longer than it was during the 19th century due to tidal acceleration, each day varies between 0 and 2 SI ms longer.
Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is of mean solar time (UT1), or Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is of mean solar time (UT1) as of 1982[update]. Thus the sidereal day is shorter than the stellar day by about 8.4 ms. The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005 and 1962–2005.
Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in the 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 Moon every two minutes; from the planet's surface, the apparent sizes of the Sun and the Moon are approximately the same.
Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one sidereal year. From Earth, 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 the Earth averages about 29.8 km/s (107,000 km/h), which is fast enough to travel a distance equal to the planet's diameter, about 12,742 km, in seven minutes, and the distance to the Moon, 384,000 km, in about 3.5 hours.
The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common revolution 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 north poles of both the Sun and the Earth, the Earth revolves in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.4 degrees from the 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 gravitational sphere of influence, of the Earth is about 1.5 Gm or 1,500,000 km in radius. This is the maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the 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 galaxy and orbits about 28,000 light years from the center of the galaxy. It is currently about 20 light years above the galactic plane in the Orion spiral arm.
Due to the axial tilt of the Earth, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes seasonal change in climate, with summer in the northern hemisphere occurring when the North Pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year—a polar night. In the southern hemisphere the situation is exactly reversed, with the South Pole oriented opposite the direction of the North Pole.
By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere, Winter Solstice occurs on about December 21, Summer Solstice is near June 21, Spring Equinox is around March 20 and Autumnal Equinox is about September 23. In the Southern hemisphere, the situation is reversed, with the Summer and Winter Solstices exchanged and the Spring and Autumnal Equinox dates switched.
The angle of the Earth's tilt is relatively stable over long periods of time. The tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years. The orientation (rather than the angle) of the Earth's axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth's equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the surface. This polar motion has multiple, cyclical components, which collectively 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 of the Earth also varies in a phenomenon known as length of day variation.
In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth–Sun distance causes an increase of about 6.9% in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. This effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.
The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the Earth's. It is the largest moon in the Solar System relative to the size of its planet, although Charon is larger relative to the dwarf planet Pluto. The natural satellites orbiting other planets are called "moons" after Earth's Moon.
The gravitational attraction between the Earth and Moon causes tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases; the dark part of the face is separated from the light part by the solar terminator.
Due to their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm a year. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs a year—add up to significant changes. During the Devonian period, for example, (approximately ) there were 400 days in a year, with each day lasting 21.8 hours.
The Moon may have dramatically affected the development of life by moderating the planet's climate. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon. Some theorists believe that without this stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of years, as appears to be the case for Mars.
Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant. This allows total and annular solar eclipses to occur on Earth.
The most widely accepted theory of the Moon's origin, the giant impact theory, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.
Earth has at least five co-orbital asteroids, including 3753 Cruithne and .
On July 27, 2011, astronomers reported a trojan asteroid companion, , librating around the leading Lagrange triangular point, L4, of Earth in Earth's orbit around the Sun.
As of 2011, there are 931 operational, man-made satellites orbiting the Earth. There are also inoperative satellites and over 300,000 pieces of space debris. Earth's largest artificial satellite is the International Space Station.
A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism. The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the current climatic conditions at the surface.
A planet's life forms are sometimes said to form a "biosphere". The Earth's biosphere is generally believed to have begun evolving about . The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals. On land, biomes are separated primarily by differences in latitude, height above sea level and humidity. Terrestrial biomes lying within the Arctic or Antarctic Circles, at high altitudes or in extremely arid areas are relatively barren of plant and animal life; species diversity reaches a peak in humid lowlands at equatorial latitudes.
The Earth provides resources that are exploitable by humans for useful purposes. Some of these are non-renewable resources, such as mineral fuels, that are difficult to replenish on a short time scale.
Large deposits of fossil fuels are obtained from the Earth's crust, consisting of coal, petroleum, natural gas and methane clathrate. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed in Earth's crust through a process of Ore genesis, resulting from actions of erosion and plate tectonics. These bodies form concentrated sources for many metals and other useful elements.
The Earth's biosphere produces many useful biological products for humans, including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land. In 1980, 5,053 Mha of the Earth's land surface consisted of forest and woodlands, 6,788 Mha were grasslands and pasture, and 1,501 Mha was cultivated as croplands. The estimated amount of irrigated land in 1993 was 2,481,250 square kilometres (958,020 sq mi). Humans also live on the land by using building materials to construct shelters.
Large areas of the Earth's surface are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. From 1980 to 2000, these events caused an average of 11,800 deaths per year. Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, wildfires, and other calamities and disasters.
Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of invasive species.
According to the United Nations, a scientific consensus exists linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather and a global rise in average sea levels.
Cartography, the study and practice of map making, and vicariously geography, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, and to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.
Earth has reached approximately seven billion human inhabitants as of October 31, 2011. Projections indicate that the world's human population will reach 9.2 billion in 2050. Most of the growth is expected to take place in developing nations. Human population density varies widely around the world, but a majority live in Asia. By 2020, 60% of the world's population is expected to be living in urban, rather than rural, areas.
It is estimated that only one-eighth of the surface of the Earth is suitable for humans to live on—three-quarters is covered by oceans, and half of the land area is either desert (14%), high mountains (27%), or other less suitable terrain. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada. (82°28′N) The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)
Independent sovereign nations claim the planet's entire land surface, except for some parts of Antarctica and the odd unclaimed area of Bir Tawil between Egypt and Sudan. As of 2013, there are 206 sovereign states, including the 193 United Nations member states. In addition, there are 59 dependent territories, and a number of autonomous areas, territories under dispute and other entities. Historically, Earth has never had a sovereign government with authority over the entire globe, although a number of nation-states have striven for world domination and failed.
The United Nations is a worldwide intergovernmental organization that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict. The U.N. serves primarily as a forum for international diplomacy and international law. When the consensus of the membership permits, it provides a mechanism for armed intervention.
The first human to orbit the Earth was Yuri Gagarin on April 12, 1961. In total, about 487 people have visited outer space and reached Earth orbit as of July 30, 2010, and, of these, twelve have walked on the Moon. Normally the only humans in space are those on the International Space Station. The station's crew, currently six people, is usually replaced every six months. The furthest humans have travelled from Earth is 400,171 km, achieved during the 1970 Apollo 13 mission.
The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle.
Unlike the rest of the planets in the Solar System, humankind did not begin to view the Earth as a moving object in orbit around the Sun until the 16th century. Earth has often been personified as a deity, in particular a goddess. In many cultures a mother goddess is also portrayed as a fertility deity. Creation myths in many religions recall a story involving the creation of the Earth by a supernatural deity or deities. A variety of religious groups, often associated with fundamentalist branches of Protestantism or Islam, assert that their interpretations of these creation myths in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life. Such assertions are opposed by the scientific community and by other religious groups. A prominent example is the creation–evolution controversy.
In the past, there were varying levels of belief in a flat Earth, but this was displaced by spherical Earth, a concept that has been credited to Pythagoras (6th century BC).
Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Virgo Supercluster → Pisces–Cetus Supercluster Complex → Observable universe → Universe
Earth → Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Virgo Supercluster → Pisces–Cetus Supercluster Complex → Observable universe → Universe
In classical geometry, the radius
of a circle or sphere is the length of a line segment from its center to its perimeter. The name comes from Latin radius
, meaning "ray" but also the spoke of a chariot wheel. The plural of radius
can be either radii
(from the Latin plural) or the conventional English plural radiuses
. The typical abbreviation and mathematic variable name for "radius" is r
. By extension, the diameter d
is defined as twice the radius:
If the object does not have an obvious center, the term may refer to its circumradius, the radius of its circumscribed circle or circumscribed sphere. In either case, the radius may be more than half the diameter, which is usually defined as the maximum distance between any two points of the figure. The inradius of a geometric figure is usually the radius of the largest circle or sphere contained in it. The inner radius of a ring, tube or other hollow object is the radius of its cavity.
For regular polygons, the radius is the same as its circumradius. The inradius of a regular polygon is also called apothem. In graph theory, the radius of a graph is the minimum over all vertices u
of the maximum distance from u
to any other vertex of the graph.
The radius of the circle with perimeter (circumference) C
The radius of a circle with area A
The radius is half the diameter.
To compute the radius of a circle going through three points P1
, the following formula can be used:
is the angle
This formula uses the Sine Rule.
If the three points are given by their coordinates
, one can also use the following formula :
These formulas assume a regular polygon with n
The radius can be computed from the side s
The radius of a d
-dimensional hypercube with side s
Oval track racing, also known as oval racing, is a form of closed-circuit automobile racing that is contested on an oval-shaped track. An oval track differs from a road course in that the layout resembles an oval with turns in only one direction, almost universally left (counter-clockwise orientation). Oval tracks are dedicated motorsport circuits, used predominantly in North America. They often have banked turns and most, despite the name, are not precisely oval, and can have unique variances in shape.
Oval racing involves racing cars wheel-to-wheel around the track. Major forms of oval racing include stock car racing, open-wheel racing, sprint car racing, modified car racing and midget car racing.
Among the most famous oval tracks in North America are the Indianapolis Motor Speedway and Daytona International Speedway. Notable ovals in other countries include Rafaela in Argentina, Motegi in Japan, Lausitzring in Germany, Brooklands and Rockingham in the United Kingdom, Monza in Italy, and Montlhéry in France.
Oval tracks are classified based upon their size, surface, and shape. Their size can range from only a few hundred feet to over two and a half miles. Track surfaces can be dirt, concrete, asphalt, or a combination of concrete and asphalt. Some ovals in the early twentieth century had wood surfaces.
The definitions used to differentiate track sizes have changed over the years. It should be noted that while some tracks use terms such as "speedway" or "superspeedway" in their name, they may not meet the specific definitions used in this article. At times, the specific terms used to describe the tracks can be ambiguous or loosely defined.
The typical oval track consists of two parallel straights, connected by two 180° turns. Although most ovals generally have only two radii curves, they are usually advertised and labeled as four 90° turns.
A short track is an oval equal to or less than 1.0 miles (1.6 km) long. Short tracks are often used as a starting point for drivers seeking careers in oval track racing. Professional-level examples include Bristol, Martinsville, Richmond and Stafford.
Professional-level NASCAR races on short tracks usually utilize a 500-lap or 400-lap distance - with "laps" as the advertised measurement rather than miles. Short tracks in many cases, both professional-level and tracks at the local level, have lights installed and routinely host night races.
These are sometimes classified as a subcategory of short tracks. Synonymous with the name, a 1-mile (1.61 km) oval is a popular and common length for oval track racing. The exact measurements, however, can vary by as much as a tenth of a mile and still fall into that category. Most mile ovals are relatively flat-banked, with Dover being a notable exception. Other examples include Phoenix, Loudon, Milwaukee and Nazareth. Due to their short length and fast speeds, these tracks are often nicknamed "bullrings."
Also referred to with the general term of "speedways," these courses are 1 to 2 miles (1.6 to 3.2 km) in length, but the term is particularly reserved for 1.5-mile (2.4 km) tracks. Since their size allows them to compromise high speeds with sightlines, they have become commonplace in major racing series that utilize oval tracks. During the race track construction boom of the 1990s, these tracks began to be labeled with the rather derogatory term "cookie cutter" tracks, as their differences were perceived to be minimal. Examples include Las Vegas, Atlanta, Kansas, Texas, Kentucky, Chicagoland and Charlotte.
In 1992, Charlotte became the first intermediate track to install lights and allow for night racing. It is now commonplace for these types of tracks to host night races.
Superspeedways are at least 2.0 miles (3.2 km) in length. There are seven superspeedways in the United States, the most famous being Indianapolis Motor Speedway and Daytona International Speedway, both 2.5 miles (4.0 km) long. These tracks were built in 1909 and 1959 respectively. Indianapolis Motor Speedway was built as a facility for the automotive industry to conduct research and development. Daytona International Speedway was built as a replacement for the Daytona Beach Road Course, which combined the town's main street and its famous beach. Daytona hosts the Daytona 500, NASCAR's most prestigious race, while the Indianapolis Motor Speedway is home to the Indianapolis 500 and the Brickyard 400.
The longest superspeedway is the Talladega Superspeedway in Talladega, Alabama. Built in 1969, it is 2.66 miles (4.28 kilometers) long, and holds the current record for fastest speed in a stock car, lapping at an average of 216.309 mph (348.116 km/h) and reaching 228 miles per hour (367 km/h) at the end of the backstretch. Daytona and Talladega are the pinnacle of stock car superspeedway racing, where restrictor plates are mandated by the sport's ruling body to reduce the high speeds created by their steep banking.
Other superspeedways used by NASCAR include the Michigan International Speedway (2.0 miles), Fontana (2.0 miles), and Pocono Raceway (2.5 miles). Fontana and Michigan are often considered intermediate tracks due to their similarities with 1.5-mile tracks, while Pocono and Indianapolis are sometimes classified separately, as they are the only long, flat tracks on the schedule. Fontana, which joins Indianapolis and Pocono as the only three superspeedways currently in use in the IZOD IndyCar Series, was the site of Gil de Ferran's qualifying lap of 241.426 mph (388.537 km/h) in the CART FedEx Championship Series in 2000, presently the fastest lap recorded at an official race meeting.
The seventh Superspeedway is Texas World Speedway, which is the original "sister track" to Michigan. The 2.0-mile oval, with its 22-degree banking, was the site of Mario Andretti's closed-course record of 214.158 mph which stood for 12 years. No major professional series have raced at TWS since the 1990s, however, the track's 1.8, 2.9 and 3.1-mile road courses are very popular with amateur racing clubs.
While many oval tracks conform to the traditional symmetrical design, asymmetrical tracks are not uncommon. Pocono Raceway, for example, has the shape of a scalene triangle with rounded corners. Darlington Raceway was built with an egg-shape to accommodate a nearby minnow pond. Tracks that are built in the traditional "oval" shape include Homestead and Dover. Prior to its reconfiguration in 1997, Atlanta also conformed to this design.
One sub-classification of the traditional oval shape is the "paper clip" oval. The layout consists of two long straights, connected by two sharp, tight-radius turns, giving the track a shape resembling a paper clip. The courses are usually very challenging, and usually offer little banking, making the turns very slow and tight to maneuver. Notable "paper clip" ovals include Loudon and Martinsville.
Tri-ovals have become preferable to track builders as they offer superior sightlines. A tri-oval resembles an isosceles triangle with rounded-off corners. The circuits typically have a straight backstretch, while the main straightaway where the pit area and most grandstands are located, has a "tri-oval" curve (sometimes characterized as a fifth curve) that makes the mainstretch skewed.
The 1990s race track construction boom of 1 ½ mile tri-oval shaped tracks has given these tracks a "cookie-cutter" label. Chicagoland is unique among tri-ovals, in that it has a slightly curved backstretch, and an overall more rounded shape.
Some oval tracks have minor variations, such as kinks or doglegs. A "dogleg" is a defined as a soft curve down one of the straights, either inward or outward, which skews the oval into a non-sysmetric or non-traditional shape. Phoenix is one example of a dogleg shape. While the extra curve would seemingly give the oval 5 turns, the dogleg is normally omitted from identification, and the ovals are still labeled with 4 turns. Trenton Speedway had a unique right-hand dogleg, giving the track what was called a "kidney bean" shape.
A tri-oval with a "double dogleg" is often called a "quad-oval". The first such was Charlotte built in 1960. The shape has become a signature for SMI, which owns the other two major quad-ovals, Atlanta and Texas.
The quad-oval is very similar to the tri-oval in sightlines and layout. One specific feature is that the start/finish line segment actually falls on a straight section, rather than along a curve.
Another common shape variation is the "D-Shaped oval." Similar to a tri-oval, the D-Shaped oval has a straight backstrech, but a long, sweeping frontstrech, giving the circuit a layout resembling the letter "D." The shape originated with a pair of "sister tracks" built in the 1960s: Michigan and Texas World. Auto Club Speedway, built in 1997, was built to the same configuration. Two short tracks, Richmond and PPIR were also designed to this standard.
A "Roval," sometimes referred to as a "Combined road course", is an oval track racing facility that features a road course in the infield (or outfield), that may or may not be directly linked to the oval circuit. The name is a portmanteau of road course and oval. A roval allows the facility to be used for road racing (Formula One, sports cars, motorcycles, etc.) Some classes of racing do not allow a full oval, and therefore would not have the ability to otherwise compete at some of the most famous speedways in the world.
Rovals typically consist of the oval portion of the track, utilizing the same start/finish line, and same pit area, but a mid-course diversion to a winding road circuit in the infield. At some point, the circuit leads back to the main oval, and completes the rest of the lap. On some of the faster ovals, a chicane is present on long back-straights, to keep speeds down, and create additional braking/passing zones.
Rovals combine the high speed characteristics of ovals and technical precision of road courses, and allow road racing the unique experience of being held in the "stadium style" atmosphere of an oval superspeedway. Since 1962, the Rolex 24 Hours of Daytona has been one of the most notable roval races. However, due to the limitations of infield dimensions, and the common lack of topography, rovals often compromise the road racing experience. While the oval's sightlines are popular, some fans may feel that the racing itself is lackluster. In many cases, the grandstand seating around the oval offers the least popular view of the road racing, as fans are drawn to observe from the more challenging infield portion.
Notable rovals (U.S.)
Notable rovals (international)
The term rovals can also be used to loosely describe an oval track with an asymmetrical, unusual, or oblong shape. While these speedways are still technically ovals, their unique shape often produces driving characteristics similar to those of a road course.
Notable asymmetrical rovals
In 1991, Charlotte Motor Speedway created the first notable "Legends" oval course. The existing quad oval start/finish straight was connected to the pit lane by two 180 degree turns, resulting in a 1/4-mile short track oval. A special exhibition race featuring former NASCAR legends headlined one time on the course.
A year later, the same 1/4-mile layout became a popular venue for Legends car racing. The name "Legends oval" was derived from this use. They have also seen use with go-karts, short track stock cars, and other disciplines.
The Legends oval concept allows minor league levels of racing to compete in the stadium-style atmosphere of large speedways, when they would normally be confined to small, stand-alone 1/4-mile venues. It also allows them to serve as support races at tracks where they would not normally be able to compete (due to the track lengths and speeds) without track or car modification.
Tracks with Legends ovals
Oval tracks usually have slope in both straight and in curves, but the slope on the straights is usually smaller, circuits without any slope are rare to find, low-slope are usually old or small tracks, high gradient are more common in new circuits.
Circuits like Milwaukee Mile and Indianapolis Motor Speedway are approximately 9° tilt in curves are considered low slope, superspeedways like Talladega has up to 33° tilt in curves, Daytona has up to 32°, both are considered high inclination. Charlotte and Dover are the intermediate with the highest bank, 24° tilt. Bristol is the short oval with up to 30°.
Pack racing is a phenomenon found on fast, high-banked superspeedways. It occurs when the vehicles racing are cornering at their limit of aerodynamic drag, but within their limit of traction. This allows drivers to race around the track constantly at wide open throttle. Since the vehicles are within their limit of traction, drafting through corners will not hinder a vehicle's performance. As cars running together are faster than cars running individually, all cars in the field will draft each other simultaneously in one large pack. In stock car racing this is often referred to as "restrictor plate racing" because NASCAR mandates that each car on its two longest high-banked ovals, Talladega and Daytona, use an air restrictor to reduce horsepower.
The results of pack racing may vary. As drivers are forced to race in a confined space, overtaking is very common as vehicles may travel two and three abreast. This forces drivers to use strong mental discipline in negotiating traffic. There are drawbacks, however. Should an accident occur at the front of the pack, the results could block the track in a short amount of time. This leaves drivers at the back of the pack with little time to react and little room to maneuver. The results are often catastrophic as several cars may be destroyed in a single accident. This type of accident is often called "The Big One".
Oval track racing requires different tactics than road racing. While the driver doesn't have to shift gears nearly as frequently, brake as heavily or as often, or deal with turns of various radii in both directions as in road racing, drivers are still challenged by negotiating the track. Both types of racing place physical demands on the driver. A driver in an IndyCar race at Richmond International Raceway may be subject to as many lateral g-forces (albeit in only one direction) as a Formula One driver at Istanbul Park.
Weather also plays a different role in each discipline. Road racing offers a variety of fast and slow corners that allow the use of rain tires. Paved ovals cannot support rain tires because the turns are all very fast and the soft rubber compound used in the tread would not survive long against the forces inflicted upon it. Dirt ovals will sometimes support a light rain. Some tracks (e.g., Evergreen Speedway in Monroe, WA) have "rain or shine" rules requiring races to be run in rain.
Safety has also been a point of difference between the two. While a road course usually has abundant run-off areas, gravel traps, and tire barriers, ovals usually have a concrete retaining wall separating the track from the fans. Innovations have been made to change this, however. The SAFER barrier was created to provide a less dangerous alternative to a traditional concrete wall. The barrier can be retrofit onto an existing wall or may take the place of a concrete wall completely.
In astronomy, a lunar distance (LD) is a measurement of the distance from the Earth to the Moon. The average distance from Earth to the Moon is 384,400 km (238,900 mi). The actual distance varies over the course of the orbit of the moon, from 356,700 km (221,600 mi) at the perigee and 406,300 km (252,500 mi) at apogee.
High-precision measurements of the lunar distance are made by measuring the time taken for light to travel between LIDAR stations on Earth and retroreflectors placed on the Moon.
The Moon is spiraling away from Earth at an average rate of 3.8 cm (1.5 in) per year, as detected by the Lunar Laser Ranging Experiment. The recession rate is considered anomalously high. By coincidence, the diameter of corner cubes in retroreflectors on the Moon is also 3.8 cm (1.5 in).
The tidal dissipation rate varied in the Earth geological history.
The first person to measure the distance to the Moon was the 2nd-century-BC astronomer and geographer Hipparchus, who exploited the lunar parallax using simple trigonometry. He was approximately 26,000 km (16,000 mi) off the actual distance, an error of about 6.8%.
The NASA Near Earth Object Catalog includes the distances of asteroids and comets measured in Lunar Distances.
Earth's rotation is the rotation of the solid Earth around its own axis. The Earth rotates from the west towards the east. As viewed from the North Star or polestar Polaris, the Earth turns counter-clockwise.
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface. This point is distinct from the Earth's North Magnetic Pole. The South Pole is the other point where the Earth's axis of rotation intersects its surface, in Antarctica.
The Earth rotates once in about 24 hours from the point of view of the sun and once every 23 hours 56 minutes and 4 seconds from the point of view of the stars (see below). Earth's rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the tidal effects the Moon has on Earth's rotation. Atomic clocks show that a modern day is longer by about 1.7 milliseconds than a century ago, slowly increasing the rate at which UTC is adjusted by leap seconds.
Earth's rotation period relative to the Sun (true noon to true noon) is its true solar day or apparent solar day. It depends on the Earth's orbital motion and is thus affected by changes in the eccentricity and inclination of Earth's orbit. Both vary over thousands of years so the annual variation of the true solar day also varies. Generally, it is longer than the mean solar day during two periods of the year and shorter during another two. The true solar day tends to be longer near perihelion when the Sun apparently moves along the ecliptic through a greater angle than usual, taking about longer to do so. Conversely, it is about shorter near aphelion. It is about longer near a solstice when the projection of the Sun's apparent movement along the ecliptic onto the celestial equator causes the Sun to move through a greater angle than usual. Conversely, near an equinox the projection onto the equator is shorter by about . Currently, the perihelion and solstice effects combine to lengthen the true solar day near by solar seconds, but the solstice effect is partially cancelled by the aphelion effect near when it is only longer. The effects of the equinoxes shorten it near and by and , respectively.
The average of the true solar day during the course of an entire year is the mean solar day, which contains solar seconds. Currently, each of these seconds is slightly longer than an SI second because Earth's mean solar day is now slightly longer than it was during the 19th century due to tidal friction. The average length of the mean solar day since the introduction of the leap second in 1972 has been about 0 to 2 ms longer than 86,400 SI seconds. Random fluctuations due to core-mantle coupling have an amplitude of about 5 ms. The mean solar second between 1750 and 1892 was chosen in 1895 by Simon Newcomb as the independent unit of time in his Tables of the Sun. These tables were used to calculate the world's ephemerides between 1900 and 1983, so this second became known as the ephemeris second. In 1967 the SI second was made equal to the ephemeris second.
The apparent solar time is a measure of the Earth's rotation and the difference between it and the mean solar time is known as the equation of time.
Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is seconds of mean solar time (UT1) , mean solar days). Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is seconds of mean solar time (UT1) , mean solar days). Thus the sidereal day is shorter than the stellar day by about .
Both the stellar day and the sidereal day are shorter than the mean solar day by about . The mean solar day in SI seconds is available from the IERS for the periods and .
Recently (1999–2010) the average annual length of the mean solar day in excess of 86,400 SI seconds has varied between and , which must be added to both the stellar and sidereal days given in mean solar time above to obtain their lengths in SI seconds (see Fluctuations in the length of day).
The angular speed of Earth's rotation in inertial space is radians per SI second (mean solar second). Multiplying by (180°/π radians)×(86,400 seconds/mean solar day) yields 360.9856°/mean solar day, indicating that Earth rotates more than 360° relative to the fixed stars in one solar day. Earth's movement along its nearly circular orbit while it is rotating once around its axis requires that Earth rotate slightly more than once relative to the fixed stars before the mean Sun can pass overhead again, even though it rotates only once (360°) relative to the mean Sun. Multiplying the value in rad/s by Earth's equatorial radius of (WGS84 ellipsoid) (factors of 2π radians needed by both cancel) yields an equatorial speed of , or . Some sources state that Earth's equatorial speed is slightly less, or . This is obtained by dividing Earth's equatorial circumference by . However, the use of only one circumference unwittingly implies only one rotation in inertial space, so the corresponding time unit must be a sidereal hour. This is confirmed by multiplying by the number of sidereal days in one mean solar day, , which yields the equatorial speed in mean solar hours given above of .
The tangential speed of Earth's rotation at a point on Earth can be approximated by multiplying the speed at the equator by the cosine of the latitude. For example, the Kennedy Space Center is located at 28.59° North latitude, which yields a speed of: 1,674.4 kilometres per hour (1,040.4 mph) × cos (28.59) = 1,470.23 kilometres per hour (913.56 mph)
In the Earth's rotating frame of reference, a freely moving body follows an apparent path that deviates from the one it would follow in a fixed frame of reference. Because of this Coriolis effect, falling bodies veer eastward from the vertical plumb line below their point of release, and projectiles veer right in the northern hemisphere (and left in the southern) from the direction in which they are shot. The Coriolis effect has many other manifestations, especially in meteorology, where it is responsible for the differing rotation direction of cyclones in the northern and southern hemispheres. Hooke, following a 1679 suggestion from Newton, tried unsuccessfully to verify the predicted eastward deviation of a body dropped from a height of , but definitive results were only obtained later, in the late 18th and early 19th century, by Giovanni Battista Guglielmini in Bologna, Johann Friedrich Benzenberg in Hamburg and Ferdinand Reich in Freiberg, using taller towers and carefully released weights.
The most celebrated test of Earth's rotation is the Foucault pendulum first built by physicist Léon Foucault in 1851, which consisted of a lead-filled brass sphere suspended from the top of the Panthéon in Paris. Because of the Earth's rotation under the swinging pendulum the pendulum's plane of oscillation appears to rotate at a rate depending on latitude. At the latitude of Paris the predicted and observed shift was about clockwise per hour. Foucault pendulums now swing in museums around the world.
The permanent monitoring of the Earth's rotation requires the use of Very Long Baseline Interferometry coordinated with the Global Positioning System, Satellite laser ranging, and other satellite techniques. This provides the absolute reference for the determination of universal time, precession, and nutation.
The Earth's rotation axis moves with respect to the fixed stars (inertial space); the components of this motion are precession and nutation. The Earth's rotation axis also moves with respect to the Earth's crust; this is called polar motion.
Precession is a rotation of the Earth's rotation axis, caused primarily by external torques from the gravity of the Sun, Moon and other bodies. The polar motion is primarily due to free core nutation and the Chandler wobble.
Over millions of years, the rotation is significantly slowed by gravitational interactions with the Moon; both rotational energy and angular momentum are being slowly transferred to the Moon: see tidal acceleration. However some large scale events, such as the 2004 Indian Ocean earthquake, have caused the rotation to speed up by around 3 microseconds by affecting the Earth's moment of inertia. Post-glacial rebound, ongoing since the last Ice age, is also changing the distribution of the Earth's mass thus affecting the moment of inertia of the Earth and, by the conservation of angular momentum, the Earth's rotation period.
The Earth formed as part of the birth of the Solar System: what eventually became the solar system initially existed as a large, rotating cloud of dust, rocks, and gas. It was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements ejected by supernovas. As this interstellar dust is inhomogeneous, any asymmetry during gravitational accretion results in the angular momentum of the eventual planet. The current rotation period of the Earth is the result of this initial rotation and other factors, including tidal friction and the hypothetical impact of Theia.
is the distance from Earth's center to its surface, about 6,371 kilometers (3,959 mi). This length is also used as a unit of distance, especially in astronomy and geology, where it is usually denoted by
This article deals primarily with spherical and ellipsoidal models of the Earth. See Figure of the Earth for a more complete discussion of models. The Earth is only approximately spherical, so no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km (3,947–3,968 mi). Several different ways of modeling the Earth as a sphere each yield a mean radius of 6,371 kilometers (3,959 mi).
While "radius" normally is a characteristic of perfect spheres, the term as used in this article more generally means the distance from some "center" of the Earth to a point on the surface or on an idealized surface that models the Earth. It can also mean some kind of average of such distances, or of the radius of a sphere whose curvature matches the curvature of the ellipsoidal model of the Earth at a given point.
The first scientific estimation of the radius of the earth was given by Eratosthenes about 240 BC. Estimates of the accuracy of Eratosthenes’s measurement range from within 2% to within 15%.
Earth's rotation, internal density variations, and external tidal forces cause it to deviate systematically from a perfect sphere. Local topography increases the variance, resulting in a surface of unlimited complexity. Our descriptions of the Earth's surface must be simpler than reality in order to be tractable. Hence we create models to approximate the Earth's surface, generally relying on the simplest model that suits the need.
Each of the models in common use come with some notion of "radius". Strictly speaking, spheres are the only solids to have radii, but looser uses of the term "radius" are common in many fields, including those dealing with models of the Earth. Viewing models of the Earth from less to more approximate:
In the case of the geoid and ellipsoids, the fixed distance from any point on the model to the specified center is called "a radius of the Earth"
or "the radius of the Earth at that point"
. It is also common to refer to any mean radius
of a spherical model as "the radius of the earth"
. On the Earth's real surface, on other hand, it is uncommon to refer to a "radius", since there is no practical need. Rather, elevation above or below sea level is useful.
Regardless of model, any radius falls between the polar minimum of about 6,357 km and the equatorial maximum of about 6,378 km (≈3,950 – 3,963 mi). Hence the Earth deviates from a perfect sphere by only a third of a percent, sufficiently close to treat it as a sphere in many contexts and justifying the term "the radius of the Earth". While specific values differ, the concepts in this article generalize to any major planet.
Rotation of a planet causes it to approximate an oblate ellipsoid/spheroid
with a bulge at the equator and flattening at the North and South Poles, so that the equatorial radius
is larger than the polar radius
where the oblateness constant
is the angular frequency,
is the gravitational constant, and
is the mass of the planet. For the Earth , which is close to the measured inverse flattening . Additionally, the bulge at the equator shows slow variations. The bulge had been declining, but since 1998 the bulge has increased, possibly due to redistribution of ocean mass via currents.
The variation in density and crustal thickness causes gravity to vary on the surface, so that the mean sea level will differ from the ellipsoid. This difference is the geoid height
, positive above or outside the ellipsoid, negative below or inside. The geoid height variation is under 110 m on Earth. The geoid height can change abruptly due to earthquakes (such as the Sumatra-Andaman earthquake) or reduction in ice masses (such as Greenland).
Not all deformations originate within the Earth. The gravity of the Moon and Sun cause the Earth's surface at a given point to undulate by tenths of meters over a nearly 12 hour period (see Earth tide).
Given local and transient influences on surface height, the values defined below are based on a "general purpose" model, refined as globally precisely as possible within 5 m of reference ellipsoid height, and to within 100 m of mean sea level (neglecting geoid height).
Additionally, the radius can be estimated from the curvature of the Earth at a point. Like a torus the curvature at a point will be largest (tightest) in one direction (North-South on Earth) and smallest (flattest) perpendicularly (East-West). The corresponding radius of curvature depends on location and direction of measurement from that point. A consequence is that a distance to the true horizon at the equator is slightly shorter in the north/south direction than in the east-west direction.
In summary, local variations in terrain prevent the definition of a single absolutely "precise" radius. One can only adopt an idealized model. Since the estimate by Eratosthenes, many models have been created. Historically these models were based on regional topography, giving the best reference ellipsoid for the area under survey. As satellite remote sensing and especially the Global Positioning System rose in importance, true global models were developed which, while not as accurate for regional work, best approximate the earth as a whole.
The following radii are fixed and do not include a variable location dependence. They are derived from the WGS-84 ellipsoid.
The value for the equatorial radius is defined to the nearest 0.1 meter in WGS-84. The value for the polar radius in this section has been rounded to the nearest 0.1 meter, which is expected to be adequate for most uses. Please refer to the WGS-84 ellipsoid if a more precise value for its polar radius is needed.
The radii in this section are for an idealized surface. Even the idealized radii have an uncertainty of ± 2 meters. The discrepancy between the ellipsoid radius and the radius to a physical location may be significant. When identifying the position of an observable location, the use of more precise values for WGS-84 radii may not yield a corresponding improvement in accuracy.
The symbol given for the named radius is used in the formulae found in this article.
The Earth's equatorial radius
, or semi-major axis, is the distance from its center to the equator and equals 6,378.1370 kilometers (3,963.1906 mi). The equatorial radius is often used to compare Earth with other planets.
The Earth's polar radius
, or semi-minor axis, is the distance from its center to the North and South Poles, and equals 6,356.7523 kilometers (3,949.9028 mi).
The distance from the Earth's center to a point on the spheroid surface at geodetic latitude
are the equatorial radius and the polar radius, respectively.
These are based on an oblate ellipsoid.
Eratosthenes used two points, one almost exactly north of the other. The points are separated by distance
, and the vertical directions at the two points are known to differ by angle of
, in radians. A formula used in Eratosthenes' method is
which gives an estimate of radius based on the north-south curvature of the Earth.
Note that N=R
at the equator:
At geodetic latitude 48.46791 degrees (e.g., Lèves, Alsace, France), the radius R
is 20000/π ≈ 6,366.197, namely the radius of a perfect sphere for which the meridian arc length from the equator to the North Pole is exactly 10000 km, the originally proposed definition of the meter.
The Earth's mean radius of curvature (averaging over all directions) at latitude
The Earth's radius of curvature along a course at geodetic bearing (measured clockwise from north)
is derived from Euler's curvature formula as follows:
The Earth's meridional radius of curvature at the equator equals the meridian's semi-latus rectum:
The Earth's polar radius of curvature is:
The Earth can be modeled as a sphere in many ways. This section describes the common ways. The various radii derived here use the notation and dimensions noted above for the Earth as derived from the WGS-84 ellipsoid; namely,
A sphere being a gross approximation of the spheroid, which itself is an approximation of the geoid, units are given here in kilometers rather than the millimeter resolution appropriate for geodesy.
The International Union of Geodesy and Geophysics (IUGG) defines the mean radius (denoted
) to be
For Earth, the mean radius is 6,371.009 kilometers (3,958.761 mi).
Earth's authalic ("equal area") radius is the radius of a hypothetical perfect sphere which has the same surface area as the reference ellipsoid. The IUGG denotes the authalic radius as
A closed-form solution exists for a spheroid:
is the surface area of the spheroid.
For Earth, the authalic radius is 6,371.0072 kilometers (3,958.7603 mi).
Another spherical model is defined by the volumetric radius, which is the radius of a sphere of volume equal to the ellipsoid. The IUGG denotes the volumetric radius as
For Earth, the volumetric radius equals 6,371.0008 kilometers (3,958.7564 mi).
Another mean radius is the rectifying radius
, giving a sphere with circumference equal to the perimeter of the ellipse described by any polar cross section of the ellipsoid. This requires an elliptic integral to find, given the polar and equatorial radii:
The rectifying radius is equivalent to the meridional mean, which is defined as the average value of M
For integration limits of [0…π/2], the integrals for rectifying radius and mean radius evaluate to the same result, which, for Earth, amounts to 6,367.4491 kilometers (3,956.5494 mi).
The meridional mean is well approximated by the semicubic mean of the two axes:
yielding, again, 6,367.4491 km; or less accurately by the quadratic mean of the two axes:
about 6,367.454 km; or even just the mean of the two axes:
about 6,367.445 kilometers (3,956.547 mi).
The orbital speed
of a body, generally a planet, a natural satellite, an artificial satellite, or a multiple star, is the speed at which it orbits around the barycenter of a system, usually around a more massive body. It can be used to refer to either the mean orbital speed, i.e., the average speed as it completes an orbit, or the speed at a particular point in its orbit.]
The orbital speed at any position in the orbit can be computed from the distance to the central body at that position, and the specific orbital energy, which is independent of position: the kinetic energy is the total energy minus the potential energy.
In the case of radial motion:]
The transverse orbital speed is inversely proportional to the distance to the central body because of the law of conservation of angular momentum, or equivalently, Kepler's second law. This states that as a body moves around its orbit during a fixed amount of time, the line from the barycenter to the body sweeps a constant area of the orbital plane, regardless of which part of its orbit the body traces during that period of time. This law is usually stated as "equal areas in equal time."]
This law implies that the body moves faster near its periapsis than near its apoapsis, because at the smaller distance it needs to trace a greater arc to cover the same area.
For orbits with small eccentricity
, the length of the orbit is close to that of a circular one, and the mean orbital speed can be approximated either from observations of the orbital period and the semimajor axis of its orbit, or from knowledge of the masses of the two bodies and the semimajor axis.]
is the orbital velocity,
is the length of the semimajor axis,
is the orbital period, and
is the standard gravitational parameter. Note that this is only an approximation that holds true when the orbiting body is of considerably lesser mass than the central one, and eccentricity is close to zero.
Taking into account the mass of the orbiting body
is now the mass of the body under consideration,
is the mass of the body being orbited,
is specifically the distance between the two bodies (which is the sum of the distances from each to the center of mass), and
is the gravitational constant. This is still a simplified version; it doesn't allow for elliptical orbits, but it does at least allow for bodies of similar masses.
When one of the masses is almost negligible compared to the other mass as the case for Earth and Sun, one can approximate the previous formula to get:
is the (greater) mass around which this negligible mass or body is orbiting, and ve
is the escape velocity.
For an object in an eccentric orbit
orbiting a much larger body, the length of the orbit decreases with eccentricity
, and is given at ellipse. This can be used to obtain a more accurate estimate of the average orbital speed:
The mean orbital speed decreases with eccentricity.