Earth is the third largest planet in order of distance from the Sun and the fifth largest in the Solar System in both mass and diameter. Moreover, it is the only celestial object known to harbor life. It orbits the Sun in 365.256 solar days — a sidereal year — and rotates relative to the Sun in one sidereal day (about 23:56:04), slightly less than its 24-hour solar day because of this movement around the Sun. The Earth’s axis of rotation has an inclination of 23°, which causes the onset of seasons.
|Semi-major axis||149,597,887.5 km
(1,000,000,112 4 au)
(0.983,289,891 2 au)
|Orbital circumference||939,885,629.3 km
|Eccentricity||0,016 710 22|
|Period of revolution||365,256 363 days|
|Average orbital speed||29.783 km/s|
|Maximum orbital speed||30.287 km/s|
|Minimum orbital speed||29.291 km/s|
|Tilt on the ecliptic||(by definition) 0°|
|Known satellites||1, the Moon|
|Equatorial radius||6,378.137 km|
|Polar radius||6,356.752 km|
|Flattening||0.003,353 ≈ 1⁄300 (1⁄(298.25±1))|
|Equatorial perimeter||40,075.017 km|
|Southern perimeter||40,007.864 km|
|Volume||1,083 21 × 1012 km3|
|Mass||5.973 6 × 1024 kg|
|Global density||5.515 × 103 kg/m3|
|Surface gravity||9.806 65 m/s2
|Release speed||11.186 km/s|
period (sidereal day)
|0.997 269 49 days
(23 h 56 min 4.084 s)
speed (at the equator)
|Axis tilt||23,436 690 775 2°|
|Declination of the North Pole||90°|
|Visual geometric albedo||0,367|
|Solar irradiance||1,367.6 W/m2
|254.3 K (−18.7 °C)|
|•Maximum temperature||56.7 °C|
|•Average temperature||15 °C|
|•Minimum temperature||−93.2 °C|
|Characteristics of the atmosphere|
|Atmospheric pressure||101,325 Pa|
|Density on the ground||1,217 kg/m3|
|Total mass||5.148 × 1018 kg|
|Ladder height||8.5 km|
|Average molar mass||28.97 g/mol|
|Nitrogen N2||78.084% dry volume|
|Oxygen O2||20.946% dry volume|
|Argon Ar||0.9340% dry volume|
|Carbon dioxide CO2||413 ppm dry volume|
|Neon Ne||18.18 ppm dry volume|
|Helium He||5.24 ppm dry volume|
|Methane CH4||1.79 ppm dry volume|
|Krypton Kr||1.14 ppm dry volume|
|Hydrogen H2||550 ppb dry volume|
|Nitrous oxide N2O||300 ppb dry volume|
|Carbon monoxide CO||100 ppb dry volume|
|Xenon Xe||90 ppb dry volume|
|Ozone O3||0 to 70 ppb dry volume|
|Nitrogen dioxide NO2||20 ppb dry volume|
|Iodine I||10 ppb dry volume|
|Water vapor H2O||~ 0.4% overall
volume ~ 1 to 4% surface area (typical values)
|Discovered by||• Planetary nature sensed by
the Pythagorean school (Philolaos of Crotone).
• Attested in
the Hellenistic period (Aristarchus of Samos, then Eratosthenes).
|Discovered on||• Vth century BC
• Third century BC
According to radiometric dating, Earth was formed 4.54 billion years ago. It has a single natural satellite, the Moon, which formed soon after. The gravitational interaction with its satellite creates the tides, stabilizes its axis of rotation and gradually reduces its rotational speed. Life is thought to have appeared in the oceans at least 3.5 billion years ago, which affected the atmosphere and the Earth’s surface through the proliferation of organisms first anaerobic and, following the Cambrian explosion, aerobic.
A combination of factors such as Earth’s distance from the Sun (about 150 million kilometers — an astronomical unit), its atmosphere, ozone layer, magnetic field, and geological evolution have allowed life to evolve and develop. During the evolutionary history of life, biodiversity has experienced long periods of expansion occasionally punctuated by mass extinctions; about 99% of the species that once lived on Earth are now extinct. In 2022, more than 7.9 billion human beings live on Earth and depend on its biosphere and natural resources for their survival.
Earth is the densest planet in the Solar System as well as the largest and most massive of the four terrestrial planets. Its rigid envelope — called the lithosphere — is divided into different tectonic plates that migrate a few centimeters per year. About 71% of the planet’s surface is covered by water — including oceans, but also lakes and rivers, which make up the hydrosphere — and the remaining 29% are continents and islands. Most of the polar regions are covered by ice, including the Antarctic ice sheet and the Arctic Ocean sea ice. The internal structure of Earth is geologically active, the solid inner core and the liquid outer core (both composed mainly of iron) allowing, in particular, to generate the Earth’s magnetic field by dynamo effect and the convection of the Earth’s mantle (composed of silicate rocks) being the cause of plate tectonics.
The age of the planet is now estimated at 4.54 billion years. The history of Earth is divided into four major time intervals, called eons.
The Hadean begins 4.54 billion years ago (Ga), when Earth formed at the same time as the other planets from a solar nebula — a disk-shaped mass of dust and gas, detached from the forming Sun.
The formation of Earth by accretion ends in less than 20 million years. Initially molten, the Earth’s outer layer cools to form a solid crust as water begins to accumulate in the atmosphere, culminating in the first rains and oceans. The Moon formed shortly thereafter, 4.53 billion years ago. The consensus regarding the formation of the Moon is the giant impact hypothesis, according to which an impactor commonly called Theia, the size of Mars and with a mass approximately one-tenth of the Earth’s mass, collided with Earth. In this model, part of this object would have agglomerated with Earth while another part, mixed with about 10% of the total mass of Earth, would have been ejected into space and then would have agglomerated to form the Moon.
The volcanic activity that follows the impact, associated with the very high temperatures (up to 10,000 ° C), produces a primitive atmosphere by degassing. Condensed water vapor with several possible origins, mixed with ice brought by comets, produces the oceans when temperatures drop. The greenhouse gases in this atmosphere maintain a temperature compatible with the presence of liquid water on the Earth’s surface and prevent the oceans from freezing while the planet received only about 70% of the current solar luminosity.
Two main models are proposed to explain the speed of continental growth: constant growth up to the present day and rapid growth at the beginning of Earth’s history. The consensus is that the second hypothesis is the most likely with the rapid formation of the continental crust followed by small variations in the global surface of the continents. On a time scale of several hundred million years, continents or supercontinents are thus formed and then divided.
Together with the Archean and Proterozoic (the next two aeons), they form a supereon called the Precambrian.
The Archean begins about 4 billion years ago and is the aeon marked by the first traces of life. Indeed, it is assumed that intense chemical activity in a highly energetic medium then made it possible to produce a molecule capable of reproducing. Life itself would have appeared between 200 and 500 million years later, before about −3.5 Ga, the starting point of the evolution of the biosphere. In addition, the date of appearance of the last universal common ancestor is estimated to be between −3.5 and −3.8 Ga.
Early signs of life include biomolecules in 3.7 Ga granite in Greenland or traces of potentially biogenic carbon in a 4.1 Ga zircon in Australia. However, the oldest fossilized evidence of microorganisms dates from 3.5 Ga ago and has also been found in Australia.
Moreover, around −3.5 billion years ago, the Earth’s magnetic field is formed and prevents the atmosphere from being swept away by the solar wind.
The Proterozoic begins 2.5 Ga ago and marks the appearance of photosynthesis in cyanobacteria, producing free oxygen O 2 and forming stromatolites. This led to a major ecological upheaval towards −2.4 Ga, called the Great Oxidation, by forming the ozone layer and gradually changing the then methane-rich atmosphere into the current one, composed mainly of nitrogen and oxygen. It is still photosynthesis that maintains oxygen levels in the Earth’s atmosphere and is at the origin of organic matter — essential for life on Earth.
Due to the increase in the concentration of oxygen in the atmosphere, multicellular organisms called eukaryotes (although some of them are unicellular), more complex, are emerging by a mechanism thought to be endosymbiosis. The oldest found date from −2.1 Ga and were called Gabonionta, because they were discovered in Gabon. Eukaryotes subsequently form colonies and, protected from ultraviolet rays by the ozone layer, these life forms could have colonized the Earth’s surface.
From −750 to −580 million years ago, during the Neoproterozoic, Earth would have experienced one or more series of global glaciations that would have covered the planet with a layer of ice. This hypothesis is called snowball Earth, and is of particular interest because it directly precedes the Cambrian explosion and may have triggered the evolution of multicellular life.
Moreover, the oldest of the known supercontinents, Rodinia, began to break up about 750 million years ago. The continents between which it split later recombined to form Pannotia, 650 to 540 million years ago.
The Phanerozoic is marked by the appearance of the first shelled animals. It begins 541 ± 0.1 million years ago and extends to the present day. Its beginning coincides with the Cambrian explosion, the rapid appearance of most of the current large phyla of metazoans (multicellular animals).
The last supercontinent, Pangea, formed approximately 335 million years ago and began to break up 175 million years ago.
During this eon, the biosphere experienced five mass extinctions. The last of these occurred 66 million years ago, its generally accepted cause being a meteorite colliding with Earth that would have created the Chicxulub impact. The consequence is the extermination of dinosaurs (except avians) and other large reptiles, affecting without extinguishing smaller animals such as mammals, birds, or lizards.
Over the next 66 Ma, mammals diversified and, about 6 Ma ago, hominian such as Orrorin Tugenensis developed the ability to stand. This resulted in a simultaneous development of tool use and brain development throughout the evolutionary history of the human lineage. The development of agriculture and civilizations allowed humans to influence Earth, nature and other forms of life.
The current pattern of ice ages is established during the Pleistocene about 2.6 Ma ago. Since then, high-latitude regions have experienced glaciation cycles of about 80,000 years, the last of which ended about 10,000 years ago.
The future of Earth is closely linked to that of the Sun. Due to the accumulation of helium in the core of the star, its solar luminosity slowly increases on the scale of geological time. Thus, the luminosity will increase by 10% over the next 1.1 billion years and by 40% over the next 3.5 billion years. Climate models indicate that the increase in radiation reaching Earth will probably have dramatic consequences on the sustainability of its “terrestrial” climate, including the disappearance of the oceans.
However, Earth is expected to remain habitable for more than 500 million years, this duration could increase to 2.3 billion years if atmospheric pressure decreases by removing some of the nitrogen from the atmosphere. The increase in Earth’s temperature will accelerate the cycle of inorganic carbon, reducing its concentration to levels that could become too low for plants (10 ppm for C4 photosynthesis) in about 500 to 900 million years.
The reduction of vegetation will lead to a decrease in the amount of oxygen in the atmosphere, which will cause the gradual disappearance of most animal life forms. Then, the average temperature of Earth will rise faster due to the runaway greenhouse effect by water vapor. In 1 to 1.7 Ga, the temperature will be so high that the oceans will evaporate, precipitating the Earth’s climate into the Venusian-type climate, and wiping out all simple life on the Earth’s surface.
Even if the Sun were eternal and stable, the internal cooling of Earth would cause the level of CO2 to drop due to a reduction in volcanism and 35% of the water in the oceans would descend into the mantle due to the decrease in exchanges at the oceanic ridges.
As part of its evolution, the Sun will become a red giant in more than 5 billion years. Models predict that it will swell to about 250 times its current radius.
The fate of Earth is less clear. As a red giant, the Sun is expected to lose about 30% of its mass. Thus, without taking tidal effects into account, Earth would move into an orbit 1.7 AU (about 250 million km) from the Sun when it reaches its maximum radius of 1.2 AU (about 180 million km).
In this model, the planet should not be engulfed by the outer layers of the Sun even if the remaining atmosphere will eventually be “blown” into space, and the Earth’s crust will eventually melt into an ocean of lava, when the solar luminosity reaches about 5,000 times its current level. However, a 2008 simulation indicates that the Earth’s orbit will change due to tidal effects and will actually push Earth into the Sun’s atmosphere where it will be absorbed and vaporized — just like Mercury and Venus but not Mars.
Shape and size of Earth
The shape of Earth is approached by an ellipsoid of revolution, a sphere flattened at the poles. More precisely, it is called oblate — or flattened — because its secondary axis is also its axis of rotation. Indeed, the rotation of Earth causes a flattening at the poles due to centrifugal force, so that the Earth’s radius at the equator is about 21 km larger than that at the North and South Poles, a variation of less than 1% of the radius.
The average diameter of the reference spheroid — called geoid, the equipotential surface of the Earth’s gravity field, that is, the shape that the Earth’s oceans would take in the absence of continents and disturbances such as wind — is about 12,742 km, which is approximately 40,008 km/π because the meter was originally defined as 1/10,000,000e (ten-millionth) of the distance from the equator to the North Pole via Paris (thus half a terrestrial meridian).
The greatest variations in the Earth’s rocky surface are Everest (8,849 m altitude, a 0.14% change in radius) and the Mariana Trench (10,984 ± 25 m below sea level, a change of 0.17%). Because of the flattening at the poles and the larger diameter at the equator, the places furthest from the center of Earth are the summits of Chimborazo in Ecuador, 6,384.4 km from the center of Earth — even though it rises to 6,263 m above sea level — followed by Huascarán in Peru. and not Everest as is sometimes thought. For the same reason, the mouth of the Mississippi is farther from the center of Earth than its source.
Moreover, because of its shape, the circumference of Earth is 40,075.017 km at the equator and 40,007.863 km for a meridian.
The equatorial radius of Earth is 6,378.137 km while the polar radius is 6,356.752 km (ellipsoid model of the flattened sphere at the poles). In addition, the distance between its center and the surface also varies according to geographical characteristics from 6,352.8 km at the bottom of the Arctic Ocean to 6,384.4 km at the summit of Chimborazo. Because of these variations, the mean radius of a planet according to the model of an ellipsoid is defined by convention by the International Union of Geodesy and Geophysics as being equal to:
, where has the equatorial radius and b the polar radius.
For Earth, this gives
6,371.008 8 km.
The mass of Earth is determined by dividing the standard gravitational parameter
= GM — also called, in the case of Earth, geocentric gravitational constant — by the gravitational constant G. In fact, the accuracy of its measurement is therefore limited by that of G, the GM product can be deduced for a body with satellites with high precision thanks to measurements of gravitational acceleration GM/d2 (where d is the planet-satellite distance). Famous experiments for measuring this mass include the Cavendish experiment — using a torsion pendulum to determine G — and methods related to calculating the density of Earth.
The IAU gives an estimate
|Planet||Equatorial radius||Mass||Gravity||Axis tilt|
|3,301 × 1023 kg
|4,867 5 × 1024 kg
|Earth||6,378.137 km||5,972 4 × 1024 kg||9.780 m/s2
(0.997 32 g)
|6,441 71 × 1023 kg
Composition and Structure of Earth
Earth is a terrestrial planet, that is to say, an essentially rocky planet with a metallic core, unlike gas giants such as Jupiter, essentially made up of light gases (hydrogen and helium). It is the largest of the four terrestrial planets in the Solar System, either in size or mass. Of these four planets, Earth also has the highest global density, the highest surface gravity, the strongest global magnetic field, the highest rotational speed and is probably the only one with active plate tectonics.
The Earth’s outer surface is divided into several rigid segments — called tectonic plates — that migrate a few centimeters per year and thus experience major shifts on the planet’s surface on a geological scale. About 71% of the surface is covered by saltwater oceans, with the remaining 29% being continents and islands. Liquid water, necessary for life as we know it, is very abundant on Earth, and no other planet has yet been discovered with such bodies of liquid water (lakes, seas, oceans) on its surface.
Chemical composition of Earth
|Compound||turn of phrase||composition|
|Silica||SiO2||60,2 %||48,6 %|
|Aluminum oxide||Al2O3||15,2 %||16,5 %|
|Calcium oxide||Cad||5,5 %||12,3 %|
|Magnesium oxide||MgO||3,1 %||6,8 %|
|Iron(II) oxide||FeO||3,8 %||6,2 %|
|Sodium oxide||Na2O||3,0 %||2,6 %|
|Potassium oxide||K2O||2,8 %||0,4 %|
|Iron(III) oxide||Fe2O3||2,5 %||2,3 %|
|Water||H2O||1,4 %||1,1 %|
|Carbon dioxide||CO2||1,2 %||1,4 %|
|Titanium dioxide||TiO2||0,7 %||1,4 %|
|Phosphorus pentoxide||P2O5||0,2 %||0,3 %|
|Total||99,6 %||99,9 %|
Earth is mainly composed of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%) and aluminum (1.4%), the rest (1.2%) consisting of traces of other elements. Since the densest elements tend to concentrate at the center of Earth (a phenomenon of planetary differentiation), it is estimated that the Earth’s core is composed mainly of iron (88.8%), with a smaller amount of nickel (5.8%), sulfur (4.5%) and less than 1% of other elements.
Geochemist F. W. Clarke calculated that 47% (by weight, or 94% by volume) of the Earth’s crust was composed of oxygen, mainly in the form of oxides, the main ones being silicon oxides (in the form of silicates), aluminum (aluminosilicates), iron, calcium, magnesium, potassium and sodium. Silica is the major constituent of the crust, in the form of pyroxenoids, the most common minerals in magmatic and metamorphic rocks. After a synthesis based on the analysis of many types of rocks, Clarke obtained the percentages presented in the table opposite.
The interior of Earth, like that of other terrestrial planets, is stratified, that is to say, organized in superimposed concentric layers, having increasing densities with depth. These various layers are distinguished by their petrological nature (chemical and mineralogical contrasts) and their physical properties (changes of physical state, rheological properties).
The outer layer of the solid Earth, thin to very thin relative to the Earth’s radius, is called the crust; it is solid, and chemically distinct from the mantle, solid, on which it rests; Under the combined effect of pressure and temperature, with depth, the mantle passes from a brittle solid state (brittle, seismogenic, “lithospheric”) to a ductile solid state (plastic, “asthenospheric”, and therefore characterized by a lower viscosity, although still extremely high). The contact surface between the crust and the mantle is called the Moho; It is visualized very well by seismic methods because of the high-speed contrast of seismic waves, between the two sides. The thickness of the crust varies from 6 kilometers under the oceans to more than 50 kilometers on average under the continents.
The crust and the cold, rigid upper part of the upper mantle are called lithosphere; Their horizontally rigid behavior on a scale of one million to ten million years is at the origin of plate tectonics. The asthenosphere lies beneath the lithosphere and is a convective, relatively less viscous layer over which the lithosphere moves in “thin plates”. Essential changes in the crystallographic structure of the various minerals of the mantle, which are phase changes in the thermodynamic sense, towards the depths of 410 km and 670 km below the surface respectively, frame a so-called transition zone, initially defined on the basis of the first seismological images.
The upper mantle is the layer that goes from the Moho to the phase transition to 670 kilometers deep, the transition to 410 kilometers deep being recognized not to have a major importance on the mantle convection process, unlike the other. Therefore, the area between this phase transition at a depth of 670 km and the core-mantle boundary is called the lower mantle.
Beneath the lower mantle, the Earth’s core, composed of about 88% iron, is a chemically original entity of all that is above, namely the silicate Earth. This core is itself laminated into a liquid outer core and very little viscous (viscosity of the order of that of an engine oil at
20°C), which surrounds a solid inner core, also called seed. This seed results from the crystallization of the core due to the secular cooling of Earth. This crystallization, by the latent heat it releases, is a source of convection of the outer core, which is the source of the Earth’s magnetic field.
The absence of such a magnetic field on other terrestrial planets suggests that their metallic nuclei, whose presence is necessary to explain astronomical density and moment of inertia data, are totally crystallized. According to a still debated interpretation of seismological data, the Earth’s inner core seems to rotate at an angular velocity slightly higher than that of the rest of the planet, advancing relatively 0.1 to 0.5° per year.
|100–670||Asthenosphere||570||1 100–2 000|
|670–2 890||Lower mantle||4,4–5,6||2 220||2 000–4 000|
|2 890–5 100||External core||9,9–12,2||2 210||4 000–6 000|
|5 100–6 378||Inner core||12,8–13,1||1 278||6 000|
The Earth’s internal heat comes from a combination of residual energy from planetary accretion (about 20%) and heat produced by radioactive elements (80%). The Earth’s main heat-producing isotopes are potassium-40, uranium-238, uranium-235 and thorium-232. At the center of the planet, the temperature could reach 6,726.85 °C and the pressure would be 360 GPa.
Since most of the heat comes from the decay of radioactive elements, scientists believe that early in Earth’s history, before short-lived isotopes decayed, Earth’s heat production would have been much greater. This additional production, twice as important three billion years ago as today, would have increased the temperature gradients in Earth and thus the rate of mantle convection and plate tectonics. This would have allowed the formation of igneous rocks such as komatiites, which are no longer formed today.
release W/kg isotope
|Average concentration in mantle kg isotope/kg mantle||Heat
release W/kg mantle
|238 U||9.46 × 10−5||4.47 × 109||1.09||30.8 × 10−9||2.91 × 10−12|
|235 U||5.69 × 10−4||7.04 × 108||6.45||0.22 × 10−9||1.25 × 10−13|
|232 Th||2.64 × 10−5||1.40 × 1010||0.32||124 × 10–9||3.27 × 10−12|
|40 K||2.92 × 10−5||1.25 × 109||3.63||36.9 × 10−9||1.08 × 10−12|
The average heat loss by Earth is 87 mW/m 2 for an overall loss of 4.42 × 1013W(44.2 TW). A portion of the core’s thermal energy is transported to the crust by plumes, a form of convection where semi-molten rocks rise to the crust. These plumes can produce hot spots and traps. Most of the Earth’s heat is lost through plate tectonics at oceanic ridges. The last major source of heat loss is conduction through the lithosphere, most of which takes place in the oceans, as the crust is thinner than that of the continents, especially at the ridges.
|North American Plaque||55,4|
|South American Plate||41,8|
Tectonic plates are rigid segments of lithosphere that move relative to each other. The kinematic relationships that exist at plate boundaries can be grouped into three domains: domains of convergence where two plates meet, divergence domains where two plates separate, and domains of transcurrence where plates move laterally relative to each other. Earthquakes, volcanic activity, mountain formation, and ocean trenches are more frequent along these boundaries. The movement of tectonic plates is related to convection movements taking place in the Earth’s mantle.
When the density of the lithosphere exceeds that of the underlying asthenosphere, the former plunges into the mantle, forming a subduction zone. At the same time, the adiabatic rise of the asthenospheric mantle leads to the partial fusion of peridotites, which forms magma at divergent boundaries and creates ridges. The combination of these processes allows a continuous recycling of the oceanic lithosphere which returns to the mantle. As a result, most of the ocean floor is less than 100 million years old. The oldest oceanic crust is located in the western Pacific and is estimated to be 200 million years old. By comparison, the oldest elements of the continental crust are 4,030 million years old.
There are seven major plates, Pacific, North American, Eurasian, African, Antarctic, Australian and South American. Other important plates include the Arabian, Caribbean, Nazca plates west of the west coast of South America and the Scotia plate in the southern Atlantic Ocean. The Indian plate sank 50 to 70 million years ago under the Eurasian plate by subduction, creating the Tibetan plateau and the Himalayas. The oceanic plates are the fastest: the Cocos plate is advancing at a rate of 75 mm/year and the Pacific plate at 52–69 mm/year. At the other extreme, the slowest is the Eurasian plate progressing at a speed of 21 mm/year.
The relief of Earth differs enormously depending on the location. About 70.8% of the world’s surface is covered by water and much of the continental shelf lies below sea level. The submerged areas have relief as varied as the others including an oceanic ridge circling Earth as well as underwater volcanoes, ocean trenches, submarine canyons, plateaus and abyssal plains. The 29.2% not covered by water is composed of mountains, deserts, grasslands, plateaus and other geomorphologies.
The planetary surface undergoes many changes due to plate tectonics and erosion. Surface elements constructed or deformed by tectonics are subject to constant weathering due to precipitation, thermal cycling, and chemical effects. Glaciations, coastal erosion, coral reef construction, and meteorite impacts also contribute to landscape changes.
The continental lithosphere is composed of low-density materials such as igneous rocks: granite and andesite. Basalt is less common and this dense volcanic rock is the main constituent of the ocean floor. Sedimentary rocks are formed by the accumulation of sediments that compact. About 75% of continental surfaces are covered by sedimentary rocks even though they represent only 5% of the crust. The third type of rock encountered on Earth is a metamorphic rock, created by the transformation of other types of rock in the presence of high pressures, high temperatures, or both.
Among the most abundant silicates on the Earth’s surface are quartz, feldspar, amphibole, mica, pyroxene and olivine. Common carbonates are calcite (a component of limestone) and dolomite. The pedosphere is the outermost layer of Earth. It is composed of soil and is subject to the process of soil formation. It is at the meeting of the lithosphere, the atmosphere, the hydrosphere and the biosphere.
The elevation of the Earth’s land surface varies from −418 meters at the shores of the Dead Sea to 8,849 meters at the summit of Mount Everest. The average elevation of the land area is 840 meters.
The abundance of water on the Earth’s surface is a unique feature that distinguishes the “blue planet” from other planets in the Solar System. The Earth’s hydrosphere is mainly composed of oceans, but technically it also includes seas, lakes, rivers and water supports. The Challenger Deep of the Mariana Trench in the Pacific Ocean is the deepest submerged place with a depth of 10,911 meters.
The mass of the oceans is about 1.37 × 1018t or about 1/4,400th of the total mass of Earth. The oceans cover an area of 3,618 × 108 km2 with an average depth of 3,682 meters or an estimated volume of 1,332 × 109km3. About 97.5% of the Earth’s water is salt. The remaining 2.5% is freshwater, but about 68.7% of it is immobilized as ice.
The average salinity of the oceans is about 35 grams of salt per kilogram of seawater (35‰). Most of this salt was released by volcanic activity or by the erosion of igneous rocks. The oceans are also an important reservoir of dissolved atmospheric gases that are essential for the survival of many aquatic life.
Seawater has a great influence on the global climate because of the enormous heat reservoir that the oceans represent. In addition, changes in ocean temperatures can lead to very significant weather events such as El Niño.
Atmosphere on Earth
Earth is surrounded by a gaseous envelope that it retains by gravitational attraction: the atmosphere. The Earth’s atmosphere is intermediate between the very thick atmosphere of Venus and the very thin atmosphere of Mars. The atmospheric pressure at sea level averages 101,325 Pa, or 1atm by definition. The atmosphere consists (by volume) of 78.08% nitrogen, 20.95% oxygen, 0.9340% argon and 0.0415% or 415 ppmv (ppm by volume) or 0.0630% or 630 ppmm (ppm by mass) (December 27, 2020) of carbon dioxide, as well as various other gases including water vapor. The height of the troposphere varies with latitude between 8 kilometers at the poles and 17 kilometers at the equator, with some variations resulting from meteorological and seasonal factors.
The Earth’s biosphere has greatly altered its atmosphere. Oxygen-based photosynthesis that appeared more than 2.5 billion years ago helped form the current atmosphere, mainly composed of nitrogen and oxygen, during the Great Oxidation. This change allowed the proliferation of aerobic organisms as well as the formation of the ozone layer blocking ultraviolet rays emitted by the Sun. The atmosphere also supports life by transporting water vapor, providing useful gases, burning small meteorites before they hit the surface, and moderating temperatures.
This last phenomenon is known as the greenhouse effect: molecules present in small quantities in the atmosphere block the loss of heat in space and thus increase the global temperature. Water vapor, carbon dioxide, methane and ozone are the main greenhouse gases in the Earth’s atmosphere. Without this heat conservation, the average temperature on Earth would be −18°C compared to the current 15°C.
Meteorology and Climate
The Earth’s atmosphere has no clearly defined limit, it is slowly disappearing into space. Three-quarters of the mass of air surrounding Earth is concentrated in the first 11 kilometers of the atmosphere. This lowest layer is called the troposphere. The Sun’s energy heats this layer and the surface below, which causes an expansion of the atmospheric volume by the expansion of the air, which has the effect of reducing its density and causing it to rise and be replaced by denser, colder air. The resulting atmospheric circulation is a determining factor in climate and meteorology because of the redistribution of heat between the different layers of air that it involves.
The main bands of circulation are the trade winds in the equatorial region at less than 30° and the westerly winds in the intermediate latitudes between 30° and 60°. Ocean currents are also important in determining climate, especially the thermohaline circulation that distributes thermal energy from the equatorial regions to the polar regions.
Water vapor generated by surface evaporation is transported by atmospheric movements. When atmospheric conditions allow warm, moist air to rise, this water condenses and falls back to the surface as precipitation. Most of the water is then transported to lower elevations by river systems and returns to oceans or lakes. This water cycle is a vital mechanism to support life on Earth and plays a key role in the erosion of Earth’s landforms. The distribution of precipitation varies greatly depending on the region considered, from several meters to less than one millimeter per year. Atmospheric circulation, topological features and temperature gradients determine the average precipitation over a given region.
The amount of solar energy reaching Earth decreases with increasing latitude. At higher latitudes, solar rays reach the surface at a lower angle and must pass through a larger column of the atmosphere. As a result, the average sea temperature decreases by about 0.4°C at each degree of latitude away from the equator. Earth can be divided into similar climate latitudinary belts according to climate classification. Starting from the equator, these are tropical (or equatorial), subtropical, temperate and polar zones. Climate can also be based on temperature and precipitation. The Köppen classification (modified by Rudolph Geiger, a student of Wladimir Peter Köppen) is the most widely used and defines five major groups (humid tropical, arid, temperate, continental and polar) that can be divided into more precise subgroups.
Above the troposphere, the atmosphere is usually divided into three layers, the stratosphere, the mesosphere and the thermosphere. Each layer has a different adiabatic thermal gradient defining the evolution of temperature with altitude. Beyond that, the exosphere transforms into a magnetosphere, where the Earth’s magnetic field interacts with the solar wind. The ozone layer is in the stratosphere and blocks some of the ultraviolet rays, which is essential for life on Earth. The Kármán line, defined as being 100 kilometers above the Earth’s surface, is the usual boundary between the atmosphere and space.
Thermal energy can increase the speed of certain particles in the upper part of the atmosphere, which can escape Earth’s gravity. This results in a slow but constant “leakage” of the atmosphere into space called atmospheric escape. Because unbound hydrogen has a low molecular weight, it can reach escape velocity more easily and disappears into space at a higher rate than other gases. The leakage of hydrogen into space moves Earth from an initially reducing state to an oxidizing state. Photosynthesis provides an unbound source of oxygen, but the loss of reducing agents such as hydrogen is considered a necessary condition for the massive accumulation of oxygen in the atmosphere. Thus, the ability of hydrogen to leave the Earth’s atmosphere could have influenced the nature of life that developed on the planet.
Currently, most hydrogen is converted into water before it escapes due to the oxygen-rich atmosphere. Thus, the hydrogen that manages to escape comes mainly from the destruction of methane molecules in the upper atmosphere.
Earth magnetic field
The Earth’s magnetic field is essentially shaped like a magnetic dipole with its poles currently located near the planet’s geographic poles, with the axis of the magnetic dipole making an 11° angle to the Earth’s axis of rotation. Its intensity at the Earth’s surface varies from 0.24 to 0.66 Gauss (0.24 × 10−5 T to 0.66 × 10−5 T), with maximum values at low latitudes. Its global magnetic moment is 7.94 × 10 15T m3.
According to the theory of the dynamo effect, the magnetic field is generated by convection movements of conductive materials within the molten outer core. Although most often more or less aligned with the axis of rotation of Earth, the magnetic poles move and change alignment irregularly due to disturbances in the stability of the core. This results in reversals of the Earth’s magnetic field—the magnetic North Pole moves to the geographic South Pole, and vice versa—at very irregular intervals, approximately several times per million years for the current period, the Cenozoic. The last reversal occurred about 780,000 years ago.
The magnetic field forms the magnetosphere that deflects particles from the solar wind and six to ten times the Earth’s radius in the direction of the Sun and up to sixty times the Earth’s radius in the opposite direction. The collision between the magnetic field and the solar wind forms the Van Allen belts, a pair of toroidal regions containing a large number of ionized energetic particles. When, on the occasion of solar plasma arrivals more intense than the average solar wind, for example during coronal mass ejection events towards Earth, the deformation of the geometry of the magnetosphere under the impact of this solar flux allows the process of magnetic reconnection. Part of the electrons of this solar plasma enters the Earth’s atmosphere in a belt around the magnetic poles: it then forms aurora borealis.
Orbit and rotation of Earth
The period of rotation of Earth relative to the Sun — called the solar day — is about 86,400 seconds or 24 hours. The rotation period of Earth relative to fixed stars — called stellar day — is 86,164,098,903,691 seconds of mean solar time (UT1), or 23 h 56 min 4.098903691 s, according to the International Earth Rotation and Reference Systems Service. Because of the precession of the equinoxes, the period of rotation of Earth relative to the Sun — called the sidereal day — is 23 h 56 min 4.09053083288 s. Thus the sidereal day is shorter than the stellar day by about 8.4 ms. Moreover, the average solar day is not constant over time and has varied by about ten milliseconds since the beginning of the seventeenth century due to fluctuations in the speed of rotation of the planet.
Apart from meteorites in the atmosphere and satellites in low orbit, the main apparent movement of celestial bodies in the Earth’s sky is westward at a rate of 15°/hour or 15°/minute. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Moon or Sun every two minutes.
Earth orbits the Sun at an average distance of about 150 million kilometers — thus defining the astronomical unit — with a period of revolution of 365.256 solar days — called a sidereal year. From Earth, this gives an apparent eastward motion of the Sun relative to the stars at a rate of about 1°/day, which corresponds to a solar or lunar diameter every 12 hours. Because of this movement and displacement of 1 °/day, it takes on average 24 hours — a solar day — for Earth to achieve a complete rotation around its axis and for the Sun to return to the meridian plane, about 4 minutes longer than its sidereal day. The orbital speed of Earth is about 29.8 km/s (107,000 km/h).
The Moon and Earth revolve around their common barycenter in 27.32 days relative to fixed stars. By associating this movement with that of the Earth-Moon couple around the Sun, we obtain that the period of the synodic month — from a new moon to the next new moon — is 29.53 days. Seen from the north celestial pole, the motions of Earth, the Moon and their axial rotations are all in the direct direction — the same as that of the Sun’s rotation and all planets except Venus and Uranus. The orbital and axial planes are not precisely aligned, the Earth’s axis is inclined 23.44° to the perpendicular to the Earth-Sun orbital plane, and the Earth-Moon orbital plane is inclined 5° to the Earth-Sun orbital plane. Without this tilt, there would be an eclipse every two weeks or so, alternating between lunar and solar eclipses.
The Hill sphere, Earth’s gravitational sphere of influence, has a radius of about 1,500,000 kilometers or 0.01 AU. This is the maximum distance up to which the gravitational influence of Earth is greater than that of the Sun and other planets. As a result, objects orbiting Earth must remain in this sphere so as not to be out of their orbit due to disturbances due to the gravitational pull of the Sun. However, this is only an approximation and numerical simulations have shown that satellite orbits must be less than about half or even a third of the Hill sphere to remain stable. For Earth, this would correspond to 500,000 kilometers (for comparison, the semi-major Earth-Moon axis is about 380,000 kilometers).
Earth, within the Solar System, is located in the Milky Way and is 28,000 light-years from the galactic center. Specifically, it is currently in the arm of Orion, about 20 light-years from the equatorial plane of the galaxy.
Axis tilt and seasons on Earth
The axial inclination of Earth with respect to the ecliptic is exactly 23.439281° — or 23°26’21.4119″ — by convention. Due to the axial tilt of Earth, the amount of solar radiation reaching any point on the surface varies throughout the year. This results in seasonal changes in climate with a summer in the northern hemisphere when the North Pole points towards the Sun and winter when the same pole points in the other direction. During the summer, the days last longer and the sun rises higher in the sky. In winter, the climate usually becomes colder and the days get shorter. The periodicity of the seasons is given by a tropical year worth 365,242 solar days.
Beyond the Arctic Circle, the sun no longer rises during part of the year — called the polar night — and, conversely, does not set during another time of the year — called polar day. This phenomenon also appears beyond the Antarctic Circle in a reciprocal way.
By astronomical convention, the four seasons are determined by the solstices — moments when the apparent position of the Sun as seen from Earth reaches its southern or northern extremity relative to the plane of the celestial equator, resulting in a minimum or maximum daylight duration respectively — and the equinoxes — the moment when the apparent position of the Sun is located on the celestial equator. resulting in a day and night of equal duration. In the Northern Hemisphere, the winter solstice occurs around December 21 and the summer one around June 21, the spring equinox takes place around March 21 and the autumnal equinox around September 21. In the southern hemisphere, the dates of the winter and summer solstices and those of the spring and autumn equinoxes are reversed.
The angle of inclination of Earth is relatively stable over time. Thus, in modern times, the perihelion of Earth takes place in early January and the aphelion in early July. However, these dates change over time due to precession and other orbital factors that follow a cyclical pattern known as Milanković parameters. Thus, tilting causes nutation, a periodic sway with a period of 18.6 years, and the orientation—not the angle—of the Earth’s axis evolves and completes a nutation cycle in about 25,800 years.
This precession of the equinoxes is the cause of the difference in duration between a sidereal year and a tropical year. These two movements are caused by the torque exerted by the tidal forces of the Moon and the Sun on the equatorial bulge of Earth. In addition, the poles periodically move relative to the Earth’s surface in a motion that lasts about 14 months and is known as the Chandler oscillation.
Before the formation of the Moon, the axis of rotation of Earth oscillated chaotically, which made it difficult for life to appear on its surface because of the climatic disturbances caused. Following the collision of the Theia impactor with the proto-Earth that allowed the formation of the Moon, the axis of rotation of Earth was stabilized due to the gravitational locking by tidal effect between Earth and its natural satellite.
Procession of Earth
|Mass||7.349 × 1022 kg|
|Semi-major axis||384,400 km|
27 d 7 h 43.7 min
Earth has a single known permanent natural satellite, the Moon, located about 380,000 km from Earth. Relatively large, its diameter is about a quarter of that of Earth. Within the Solar System, it is one of the largest natural satellites (after Ganymede, Titan, Callisto and Io) and the largest of a non-gaseous planet. In addition, it is the largest moon in the Solar System relative to the size of its planet (note that Charon is relatively larger compared to the dwarf planet Pluto). It is relatively close to the size of the planet Mercury (about three-quarters of its diameter). Natural satellites orbiting other planets are commonly referred to as ” moons” in reference to Earth’s moon.
The gravitational pull between Earth and the Moon causes the tides on Earth. The same effect takes place on the Moon so that its rotation period is identical to the time it takes to orbit Earth, which implies that it always has the same face towards Earth: we speak of gravitational lock. While orbiting Earth, different parts of the visible side of the Moon are illuminated by the Sun, causing the lunar phases.
Because of the tidal torque, the Moon moves away from Earth at a rate of about 38 millimeters per year, also producing the Earth’s day lengthening by 23 microseconds per year. Over several million years, the cumulative effect of these small changes produces significant changes. Thus, during the Devonian period, approximately 410 million years ago, there were 400 days in a year, each day for 21.8 hours.
The Moon may have had an influence on the development of life by regulating the Earth’s climate. Paleontological observations and computer simulations in planetary mechanics show that the tilt of the Earth’s axis is stabilized by tidal effects with the Moon. Without this stabilization against the torques applied by the Sun and the planets on the equatorial bulge, it is assumed that the axis of rotation could have been very unstable. This would then have caused chaotic changes in its inclination over geological time and for scales of duration typically greater than a few tens of millions of years, as seems to have been the case for Mars.
The Moon is now at such a distance from Earth that, when viewed from Earth, our satellite is about the same apparent size (angular size) as the Sun. The angular diameter (or solid angle) of the two bodies is almost identical because even if the diameter of the Sun is 400 times larger than that of the Moon, it is 400 times closer to Earth than our star. This is what makes it possible to see on Earth and in our geological epoch total or annular solar eclipses (depending on the small variations in Earth-Moon distance, related to the very slight ellipticity of the Selene orbit).
The current consensus on the origins of the Moon is in favor of the hypothesis of the giant impact between a planetoid the size of Mars, called Theia, and the newly formed proto-Earth. This hypothesis explains, among other things, the fact that there is little iron on the Moon and that the chemical composition of the lunar crust (especially for trace elements as well as in isotopy for oxygen) is very similar to that of the Earth’s crust.
A second natural satellite of Earth?
Computer models by astrophysicists Mikael Granvik, Jérémie Vaubaillon and Robert Jedicke suggest that “temporary satellites” should be quite common and that “at any given time there should be at least one natural satellite, with a diameter of 1 meter, orbiting Earth”. These objects would remain in orbit for an average of ten months before returning to a solar orbit.
One of the first mentions in the scientific literature of a temporary satellite is that of Clarence Chant during the great meteoric procession of 1913:
“It would seem that bodies that had traveled through space, probably in an orbit around the Sun and passing close to Earth, could have been captured by the Sun and moved around it like a satellite.”
Examples of such objects are known. For example, between 2006 and 2007, 2006 RH120 is effectively temporarily orbiting Earth rather than the Sun.
In April 2020, there are 2,666 artificial satellites orbiting Earth, compared to 1,167 in 2014 and 931 in 2011. Some are no longer in operation like Vanguard 1, the oldest of them still in orbit. These satellites can serve different purposes such as scientific research (e.g. the Hubble Space Telescope), telecommunications or observation (e.g. Meteosat).
In addition, these artificial satellites generate space debris: in 2020 there are more than 23,000 more than 10 cm in diameter in orbit and about half a million between 1 and 10 cm in diameter.
Since 1998, the largest artificial satellite around Earth is the International Space Station, measuring 110 m in length, 74 m in width and 30 m in height and orbiting at about 400 km altitude.
Other objects of the procession
Earth has multiple quasi-satellites and co-orbiters. Among them are 3753 Cruithne, a horseshoe-orbiting near-Earth asteroid sometimes erroneously nicknamed “Earth’s second moon” and (469219) Kamoʻoalewa, the most stable quasi-satellite known to which space exploration projects have been announced.
In the Sun-Earth system, Earth has a single Trojan asteroid: 2010 TK7. It oscillates around the Lagrangian point L4 of the Earth-Sun couple, 60° ahead of Earth in its orbit around the Sun.
In September 2018, the existence of Kordylewski clouds at points L4 and L5 of the Earth-Moon system is confirmed. These large concentrations of dust were detected only late because of their low luminosity.
A planet that can harbor life is said to be habitable even if life is not present there, or does not originate there. Earth provides liquid water, environments where complex organic molecules can assemble and interact, and enough so-called “soft” energy to maintain, for a sufficiently long time, the metabolism of living beings. The distance separating Earth from the Sun placing it in a habitable zone, as well as its orbital eccentricity, its speed of rotation, the inclination of its axis, its geological history, its atmosphere remained non-aggressive for organic molecules despite a very large evolution of chemical composition, and its protective magnetic field are all favorable parameters for the appearance of terrestrial life and the conditions of habitability on its surface.
Of the 4,500 exoplanets discovered so far, a number have been deemed habitable, although this term is somewhat ambiguous. This does not designate a planet where humans could land and begin to settle, but a rocky world in the right orbital region around his star, where the temperature is moderate enough for liquid water to exist on its surface without freezing or boiling. If Earth obviously meets these conditions, it is also the case of Mars, which is nevertheless far from being as hospitable as the latter. Among these discovered planets, 24 could be more conducive to life than Earth, therefore super-habitable. Earth could therefore be in 25th place in the ranking of the most habitable planets known.
The latter corresponds to all living organisms and their living environments and can therefore be broken down into three zones where life is present on Earth: the lithosphere, the hydrosphere and the atmosphere, these also interacting with each other. The appearance of life on Earth is estimated at least 3.5 billion years ago, the starting point of the evolution of the biosphere. In addition, the date of appearance of the last universal common ancestor is estimated at between 3.5 and 3.8 billion years ago. Also, about 99% of the species that once lived on Earth are now extinct.
The biosphere is divided into about fifteen biomes, inhabited by similar groups of plants and animals. These are a set of ecosystems characteristic of a biogeographical area and named from the vegetation and animal species that predominate and are adapted to it. They are mainly separated by differences in latitude, altitude or humidity. Some terrestrial biomes beyond the Arctic and Antarctic circles (such as tundra), at high altitudes or in very arid areas are relatively devoid of animal and plant life while biodiversity is highest in tropical rainforests.
Earth natural resources
Earth provides natural resources that are exploitable and exploited by humans for various uses. These may be, for example, mineral raw materials (freshwater, ore, etc), products of wild origin (wood, game, etc). or fossil organic matter (oil, coal, etc).
They are distinguished between renewable resources — which can be replenished over a short period of time on a human scale — and non-renewable resources — where, on the contrary, the speed of consumption greatly exceeds their speed of creation. The latter include fossil fuels, which take millions of years to build. Large quantities of these fossil fuels can be obtained from the Earth’s crust, such as coal, oil, natural gas or methane hydrates. These deposits are used for energy production and as raw materials for the chemical industry. These energy sources are then opposed to renewable energy sources — such as solar and wind energy — which are not exhaustible. Ores, too, are formed in the earth’s crust and consist of various chemical elements useful for human production such as metals.
The terrestrial biosphere produces many essential resources for humans such as food, fuel, medicines, oxygen and also ensures the recycling of many organic wastes. Terrestrial ecosystems depend on arable land and freshwater, while marine ecosystems are based on nutrients dissolved in water.
In 2019, land use—representing 29% of the planet’s surface, or 149 million km²—is roughly distributed as follows:
|Land use||Non-fertile land (including deserts)||Glaciers||Permanent pasture||Permanent crops||Forests||Fruticae||Freshwater||Urban areas|
|Surface area (million km²)||28||15||40||11||39||12||1.5||1.5|
|percentage||18,8 %||10,1 %||26,7 %||7,4 %||26,2 %||8,1 %||1 %||1 %|
In 2019, a UN report estimated that the use of natural resources is expected to increase by 110% between 2015 and 2060, resulting in a reduction of more than 10% of forests and about 20% for other habitats such as grasslands.
Large areas of the Earth’s surface are subject to extreme weather events such as extratropical cyclones (Cape Hatteras storms, European storms, etc) or tropical (called hurricanes, typhoons or cyclones depending on the region).
Between 1998 and 2017, nearly half a million people died during an extreme weather event. In addition, other areas are exposed to earthquakes, landslides, volcanic eruptions, tsunamis, tornadoes, sinkholes, blizzards, floods, droughts, or forest fires.
Human activities induce air and water pollution and also create events in some places such as acid rain, loss of vegetation (overgrazing, deforestation, desertification), loss of biodiversity, soil degradation, erosion and introduction of invasive species. In addition, air pollution is responsible for a quarter of premature deaths and diseases worldwide.
According to the United Nations, there is a scientific consensus linking human activities to global warming due to industrial emissions of carbon dioxide, and more generally greenhouse gases. This change in climate is likely to cause melting glaciers and ice sheets, extreme temperature ranges, significant changes in meteorology, and rising sea levels.
Human Geography on Earth
In 2022, Earth has approximately 7.97 billion inhabitants. Projections indicate that the world’s population will reach 9.7 billion in 2050, with growth expected to take place in particular in developing countries. For example, the sub-Saharan Africa region has the highest birth rate in the world. Human population density varies considerably around the world: about 60% of the world’s population lives in Asia, particularly in China and India—which alone account for 35% of the world’s population—compared to less than 1% in Oceania. “In addition, approximately 56% of the world’s population lives in urban rather than rural areas. In 2018, according to the UN, the three largest cities in the world (with megacity status) are Tokyo (37 million inhabitants), Delhi (29 million) and Shanghai (26 million).
About one-fifth of Earth is in favor of human exploitation. Indeed, the oceans represent 71% of the Earth’s surface and, of the remaining 29%, 10% are covered by glaciers (especially in Antarctica) and 19% by deserts or high mountains. 68% of the land surface is in the northern hemisphere and 90% of humans live there. The northernmost permanent human settlement is at Alert on Ellesmere Island in Canada (82°28′N) while the southernmost is Amundsen-Scott Antarctic Base in Antarctica (89°59’S).
All land surfaces, with the exception of Marie Byrd Land in Antarctica and Bir Tawil in Africa, which are terra nullius, are claimed by independent nations. As of 2020, the United Nations recognizes 197 states including 193 member states. The World Factbook, meanwhile, counts 195 countries and 72 territories with limited sovereignty or autonomous entities. Historically, Earth has never known sovereignty extending over the entire planet — even though many nations have attempted to achieve world domination and failed.
The United Nations (UN) is an international organization that was created for the purpose of peacefully settling conflicts between nations. The United Nations serves primarily as a forum for diplomacy and public international law. When consensus is reached among the various members, an armed operation may be envisaged.
The first human astronaut to orbit Earth was Yuri Gagarin on April 12, 1961. Since then, about 550 people have been in space and twelve of them walked on the Moon (between Apollo 11 in 1969 and Apollo 17 in 1972). Normally, at the beginning of the twenty-first century, the only humans in space are those on the International Space Station, which is permanently inhabited. The astronauts of the Apollo 13 mission are the humans who have farthest from Earth at 400,171 kilometers in 1970.
Philosophical and cultural point of view
The belief in a flat Earth has been refuted by experience since antiquity and then by practice through circumnavigations in the early Renaissance. The model of a spherical Earth has therefore historically always prevailed.
In the fifth century BC, Pythagoras and Parmenides begin to picture Earth in the form of a sphere. This is a logical inference from observing the curvature of the horizon on board a ship. Because of this work, Earth is already considered spherical by Plato (fifth century BC), by Aristotle (IVth century BC) and generally by all Greek scholars. The origin of a belief of its rotation on itself is attributed to Hicetas by Cicero. According to Strabo, the Crates of Mallos built in the second century BC. a sphere to represent Earth according to the theory known as the “five climatic zones”.
Eratosthenes deduced the circumference of Earth (length of the meridian) geometrically around 230 BC; it would have obtained a value of about 40,000 km, which is a measurement very close to reality (40,075 km at the equator and 40,008 km on a meridian passing through the poles). The astronomer is also at the origin of the first evaluations of the inclination of the axis. In his Geography, Ptolemy (second century) takes up the calculations of Eratosthenes and clearly states that Earth is round.
The idea that in the Middle Ages theologies imagined Earth as flat would be a myth invented in the nineteenth century to blacken the image of this period and it is commonly accepted that no medieval scholar supported the idea of a flat Earth. Thus, medieval texts generally refer to Earth as “the globe” or “the sphere”—referring in particular to the writings of Ptolemy, one of the most widely read and taught authors at the time.
Unlike the other planets of the Solar System, humanity did not consider Earth as a moving object rotating around the Sun until the beginning of the seventeenth century, it being commonly thought of as the center of the universe before the development of heliocentric models.
Because of Christian influences, and the work of theologians like James Ussher based solely on the analysis of genealogies in the Bible to date the age of Earth, most Western scientists still thought in the nineteenth century that Earth was a few thousand years old at most. It was only from the development of geology that the age of Earth was reassessed. In the 1860s, Lord Kelvin, with the help of thermodynamic studies, first estimated the age of Earth to be of the order of 100 million years, launching a great debate. The discovery of radioactivity by Henri Becquerel at the end of the nineteenth century provides a reliable means of dating and proves that the age of Earth is actually counted in billions of years.
Earth has often been personified as a deity, especially in the form of a goddess as with Gaia in Greek mythology. As such, Earth is then represented by the mother goddess, the goddess of fertility. In addition, the goddess gave her name to Gaia theories, twentieth-century environmentalist hypotheses comparing terrestrial environments and life to a single self-regulating organism toward stabilizing habitability conditions.
Its equivalent in Roman mythology is Tellus (or Terra mater), goddess of fertility. The name of the planet in English derives indirectly from the name of this goddess, derived from the Latin terra meaning the terrestrial globe.
Also, the creation myths of many religions, for example, the first account of the creation of Genesis in the Bible, relate the creation of Earth by one or more deities.
Some religious groups, often affiliated with the fundamentalist branches of Protestantism and Islam, argue that their interpretation of creation myths in sacred texts is truth and that it should be considered equal to conventional scientific assumptions about the formation of Earth and the development of life. or should replace them. Such claims are rejected by the scientific community. and other religious groups.
Different astronomical symbols are and have been used to represent Earth. The most common in a contemporary way is (Unicode U+1F728), representing a globe cut by the equator and a meridian and, consequently, the “four corners of the world” or the cardinal points. There is also a crucigerous orb, ♁ (U+2641). More anciently, there is also a globe cut only by the equator, (U+1F714).
Nevertheless, their use is discouraged by the International Astronomical Union, which favors abbreviations. Only the first is common, found for example in M🜨 for the unity of an Earth mass.
The human vision concerning Earth evolves thanks in particular to the beginnings of astronautics and the biosphere is then seen from a global perspective. This is reflected in the development of ecology which is concerned about humanity’s impact on the planet.
As early as 1931, Paul Valéry, in his book Regards sur le Monde Actuel, estimated that “the time of the finite world begins”. By “world” he does not mean the world-universe of the Ancients, but our present world, that is to say, Earth and all its inhabitants. In continuity, Bertrand de Jouvenel evokes the finitude of Earth as early as 1968.
The philosopher Dominique Bourg, a specialist in the ethics of sustainable development, evokes in 1993 the discovery of the ecological finitude of Earth in Nature in politics or the philosophical issue of ecology. Believing that this finitude is sufficiently known and proven that it is useless to illustrate it, he stresses that it has led to a radical change in the relationship between the universal and the singular in our representations.
While the classical modern paradigm postulated that the universal commands the singular, and the general the particular, the relationship between the planetary and the local cannot be reduced to it. In the systemic universe of ecology, the biosphere (the planetary) and the biotopes (the local) are interdependent. This interdependence of the local and the planetary shatters the driving principle of modernity, which tended to abolish all local particularity in favor of general principles, in which the modern project is utopian according to him.
Experimental evidence of the symbolic connection of ecology to culture is provided by the reactions of the first astronauts who, in the 1960s, were able to observe the planet in orbit or from the Moon — and bring back iconic photographs such as The Blue Marble or Earthrise. These returns describing an Earth “beautiful, precious and fragile” – which man, therefore, has the duty to protect – had an influence on the worldview of the population in general.
The ecological finitude of Earth is a question that has become so significant that some philosophers (Heidegger, Grondin, Schürch) have been able to speak of an ethic of finitude. In addition, the concepts of ecological footprint and biocapacity make it possible to understand the problems related to this finitude of Earth.