Life on Earth

Life on Earth

Giancarlo Varnier

Copyright: 2012 Ed Alvis

Published by Editions ALVIS at Smashwords

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CONTENTS

Premise

Planet Earth

Age of Precambrian

Age of Ancient Life

Age of Reptiles

Age of Recent Life

Age of Man

Age of Future

Premise

The Earth is the third planet in order of distance from the Sun, and the largest of the terrestrial planets of the solar system, both in terms of the mass and the diameter. It is the planet they live on all living species known, the only planetary body in the solar system adequate to sustain life.

In this text, after a brief description of the general characteristics of our planet, we focus on its dynamic training through the various geologic eras to arrive, then, the emergence and evolution of life in its many forms, with particular reference to the human species. Finally may try to understand the great events that could lead to the end of life on Earth and fate that is reserved our planet in the context of cosmic events of the future.

Planet Earth

Our planet was formed about 4.57 billion years ago from the solar nebula. At the beginning, there was a disk of dust and gas in rotation, after the formation of the Sun excess material began to congregate in different areas, forming the planets known today. First, the material was united in bodies of a few kilometers, which began to collide, like billiard balls, to collide, to join and merge into other bodies and to form more and more massive protoplanets. Initially liquefied went to cool the planet, forming a crust more of granitic, similar to today's. The Moon was formed immediately after, probably due to the impact between the Earth and Mars and a planet as large as one having about 10% of the mass of the earth, known as Theia. In the collision between the two bodies, a bit 'of the mass of this small celestial body joined the Earth and a portion was ejected into space, but enough material survived to form an orbiting satellite. The age of the Earth was determined by Clair Patterson in 1953, using radiometric methods related to the decay of uranium Chemical models based on current abundance of radioactive isotopes with long decay times and compositional analysis of undifferentiated material from meteorites and dating from the Moon to the Earth's formation 4.65 billion years ago. The main difficulty in determining the age of the Earth is due to the fact that currently no rock outcropping on the planet has this age, due to the nature of fluid or plastic of all the earth's crust during the first billion years or so. In addition, the processes of magmatic differentiation separated in this first phase the various elements concentrating only some within the earth's crust. The oldest rocks are continental rocks be found on the planet, are found in cratons and have an age of 4.1 billion years. Most of the oceanic crust is younger, he has continuously recycled by mechanisms related to plate tectonics: the oldest rocks in this type of crust are Jurassic and have an age of 100 million years.

Its axis of rotation is tilted with respect to the perpendicular to the ecliptic plane: this inclination, combined with the Earth's revolution around the Sun, is due to the alternation of the seasons. The weather conditions have been altered primordial predominantly by the presence of life forms, which have created a different ecological balance, shaping the planet's surface. Approximately 71% of the surface is covered by salt water oceans, with the remaining 29% is represented by continents and islands. The outer surface is divided into several rigid segments, or tectonic plates, which move along the surface in periods of several million years. The inner part, active from geological point of view, is composed of a thick layer of relatively solid or plastic, called mantle, and by a nucleus, divided in turn into the outer core, where it generates the magnetic field, and a solid inner core consisting mainly of iron and nickel. Everything related to the composition of the inner part of the earth remains pure hypothesis and lacks direct observation and verification. Important are the influences on the Earth from outer space, because the Moon is at the origin of the phenomenon of the tides, stabilizes the axial shift and has slowly changed the length of the period of rotation of the planet (slowing it down), a bombardment of comets during primordial played a key role in the formation of the oceans and, in a subsequent period, some asteroid impacts caused significant changes in the characteristics of the surface and have altered the present life. The astronomical symbol of the Earth is a circle with a cross inside (Unicode: U +2641, Hexadecimal: & x2641, ♁): the horizontal line represents the equator, while a vertical meridian). The shape of the Earth is properly defined as geoid, but is much similar to an oblate spheroid (solid of revolution that is obtained by the rotation of an oval about its minor axis), from which it differs for a maximum of 100 meters. The average diameter of the spheroid of reference is approximately 12 742 km, however, in a more approximate can be defined as 40 009 km / π, since the meter was originally defined as 1/10 000 000 of the distance between the equator and the North Pole through Paris, France. The Earth's rotation is the cause of the equatorial bulge, which involves an equatorial diameter of 43 km greater than the polar. The largest local deviations on the surface are: Mount Everest with 8850 m (above local sea level) and the Mariana Trench, with 10 924 m (below local sea level). If one compares the earth to a perfect ellipsoid, it has a tolerance of about one part in 584, or 0.17%, which is lower than the 0.22% tolerance allowed in billiard balls. Due to the presence of the bulge, also, the place more distant from the center of the Earth is currently located on Mount Chimborazo in Ecuador.

The interior of the Earth, said even geosphere, consists of rocks of different composition and phase (solid, mainly, but sometimes also liquid). Thanks to the study of seismograms it has come to consider the interior of the earth is divided into a series of shells; fact it was noted that the seismic waves undergo phenomena of refraction in crossing the planet. The refractive index consists of a modification of the speed and trajectory of a wave when this is transmitted to a medium with a different density. So we were able to detect areas in depth that are experiencing a sharp acceleration and deflection of the waves, and based on these four areas have been identified concentric spherical: the crust, the mantle, the outer core and the inner core. The interior of the Earth, like that of the other terrestrial planets, is chemically divided into a crust formed by rocks, basic or acidic a mantle ultrabasic and the Earth's core consists mainly of iron. The planet is large enough to have a nucleus differentiated in a solid inner core and an outer core liquid, which produces a weak magnetic field due to the convection of its electrically conductive material. From the point of view of mechanical properties, the crust and the upper portion of the mantle form the lithosphere, rigid; while an intermediate portion of the mantle, which behaves in a sense as a fluid enormously viscous, constitutes the asthenosphere. Material from asthenosphere down continuously to the surface through volcanoes and oceanic ridges not yet maintains the original composition because it is subject to fractional crystallization.

It is believed that the core is composed mainly of iron (88.8%) with small amounts of nickel (5.8%) and sulfur (4.5%). The geochemical F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen. The most common constituents are represented by oxides. Chlorine, sulfur and fluorine are the only important exceptions, although their presence in the total rock is less than 1%. The principal oxides are the silicates, oxides of aluminum, iron, calcium, magnesium, potassium and sodium. Silicates are the acid component of the earth's crust, and be going on to represent all the major minerals of intrusive rocks. 1672 analyzing samples of all types of rocks, Clarke concluded that the 99.22% of them were composed of only 11 oxides, while the remaining constituents were present only in very small quantities. The temperature inside the Earth rises with a temperature gradient of about 25 ° C / km in the crust, and then decreased to 0.7 ° C - 0.8 ° C / km in other areas, is directly connected to the pressure. Reaches 5270 K (approx. 5000 ° C) and a pressure of 3600 kbar in the portion of the inner core. The internal heat has been generated in part during the formation of the planet, and since then has been continuously further heat generated by radioactive decay of isotopes of uranium, thorium and potassium. The heat transmitted from inside to outside of the planet derives from the convective motions of the mantle, even if, being the rocks bad thermal conductive, represents only a twenty-thousandth of the energy that the planet receives from the Sun The average density of the Earth is 5.515 g / cm ³, making it the densest planet in the solar system, but is not constant, but is directly proportional to the increase of depth. In the crust changes from 2.2 to 2.9 g / cm ³, to increase progressively in the mantle, with a density of 3.4 to 5.6 g / cm ³, and reach the old to between 9 and 13 g / cm ³.

According to the theory of plate tectonics, which is now accepted by almost all experts in Earth Sciences, the outer area of the Earth is divided into two parts: the lithosphere, comprising the crust and the top layer of the mantle upper, and the asthenosphere which forms the innermost part of the mantle and deep. The Earth behaves as a superheated liquid which moves the lithosphere plates, and is extremely viscous. The lithosphere essentially floats on the asthenosphere and is divided into those which commonly are called tectonic plates. These plaques are rigid segments that move relative to each other according to three types of movement: convergent, divergent and transform. A final type of movement occurs when two plates move laterally with respect to another, using a strike-slip fault. Through the movement of these plates the planet was formed, alternating moments when there was only one super-continent, in situations similar to today. There are lithospheric plates of continental and oceanic. In addition, the collision between two or more tectonic plates is the basis for the genesis of the mountain ranges on the continental lithospheric plate, while their difference may lead to the development of a mid-ocean ridge, on the plaque-type oceanic lithosphere and then, of new crust. Therefore the boundaries between tectonic plates are areas of high geological activity and intense efforts, and along them are concentrated most earthquake-prone areas, even with high intensity earthquakes, and volcanic areas.

The plates are: African Plate, Antarctic Plate, Australian Plate, Eurasian Plate, North American Plate, South American Plate, Pacific Plate. There are numerous smaller plates or smaller size, among them the most important are: the Indian Plate, the Arabian Plate, the Caribbean Plate, the Nazca plate along the western coast of South America and the Scotia Plate in the Atlantic Ocean south . The plaques at more rapid movement are located in oceanic areas, with the Cocos plate that moves with a rate of 75 mm / year and the Pacific plate that moves with a rate of 52-69 mm / year. At the extreme, the plate with the Eurasian movement slower, moving at an average rate of about 21 mm / year.

The Earth's surface can vary enormously from place to place. Approximately 70.8% of the surface is covered by water; also most of the continental shelf is located below the sea level. In the submerged part of the planet are all the characteristics of a mountainous terrain, including a system of mid-ocean ridges, volcanoes submerged, ocean trenches, submarine canyons, highlands and abyssal plains. The remaining 29.2% is found in mountains, deserts, plains, plateaus, and other areas geomorphological children. The planetary surface changes constantly second geologic time due to the movements of the various tectonic plates and erosion, in addition to its geographical characteristics, created or deformed by tectonic movements, are subject to the influence of the weather (rain, snow, ice, wind), to several thermal cycles (eg. freeze / thaw of alpine areas or high daily temperature in the case of deserts) and chemical action. Finally, the modeling of the planet, are also included major events such as glaciations and meteorite impacts. During the migration of two tectonic plates continental, the oceanic crust is subducted below the margins of the latter. At the same time, due to the rise of mantle material, new oceanic crust is created along divergent margins in the mid-ocean ridges. This cycle continuously replaces the material of oceanic crust in a process that has led it to be at less than 100 million years. The oldest oceanic plate, located in the Western Pacific, was estimated with an age of about 200 million years. For comparison, the oldest continental crust, dated thanks to the presence of fossils, has an age of about 3 billion years. The movements of the various subduction plates are regulated by density contrasts, because the continental plates are made up of less dense rocks, especially intrusive rocks, such as granite and andesite, while oceanic areas are formed by effusive rocks, mainly basalt. This difference explains why the constitutive contrast between two plates of different types is always to go to the oceanic subduction. Different development has the case in which the two plates belong to the same type, for which factors intervene more sensitive as the efforts and directions of movement. Of both types of crust can be found, in favorable cases to their emplacement, sedimentary rocks. They are formed by the accumulation of sediments in a manner so often detectable, when there is a stratification, can be traced back in time to the conditions present at the time of formation of each layer and the evolution of these conditions towards the present. In addition, the sedimentary rocks are the only ones which can be found in fossils, which are essential for accurate dating of the rock and to draw paleo-environmental information on climate, geography, flora and fauna present at the time. It should be added that in such rocks are sought and exploited almost all major oil and gas fields and coal. Approximately 75% of the entire surface of the continents is covered by sediments, although they form only about 5% of the crust. The third type of rock on the planet, after the volcanic (extrusive and intrusive) and sedimentary ones, is that of the metamorphic rocks. They are derived from the processing of pre-existing rocks of any type through the influence of high pressures, high temperatures, or both of these factors. Metamorphic process can be of varying intensity, causing both a simple recrystallization of some species ores to other more stable, and the partial melting and deformation of the rock, turning it into a completely different. Through the fusion process, also creates a circulation of hot fluids inside the rock. Within these fluids are brought into solution and concentrated, if present, otherwise rare elements dispersed in minute quantities. Metamorphic rocks or deposits arising from the dismantling, therefore, are one of the preferred research fields of raw materials, precious metals and stones.

The most abundant minerals on the earth's surface are the silicates, which mainly include: quartz, feldspar, amphibole, mica, pyroxene and olivine. Among the carbonate minerals, however, the most common are: calcite, aragonite and dolomite. The soil component and the outer part of the Earth, as well as the more subtle, and relates to the soil and the processes leading to its formation. Pedosphere arises as a contact between the lithosphere, atmosphere, hydrosphere and biosphere. It is estimated that the arable area is 13.31% of the land surface, with only 4.71% of it is used for permanent crops.

The relief of the earth's surface varies from the lowest point (-418 m from the Dead Sea) to a 2005 estimate of the maximum altitude of 8848 m of the top of Mount Everest, also the average height of land located below the sea level is of 686 m.

The Earth is the only planet so far known host, with certainty, life. Current theories place his birth in a few hundred million years after the planet was formed, between 3.5 and 4 billion years ago, but the Earth is about 4.5 billion years, and it is believed that during the first hundreds of millions of years of life has been subjected to a terrific bombardment by debris in the solar system. At intervals of a few tens of millions of years, the Earth was hit by rocks large enough to sterilize it completely, as we have said, the Moon would have originated from a huge ball of molten magma launched into space by the impact an object the size of Mars. So, every form of life on Earth today must be derived from an organism that came to light after the last case of sterility. For the development of life, therefore, not much time: at most a few hundred, according to some, only a few tens of millions of years. For some not plausible. The universe would be conducive to life, that would have made available the chemicals needed for its birth, for a period of time much longer. According to the proponents of panspermia is statically more likely that life emerged during this period much longer, lasting billions of years, and not during the time window rather limited permitted by Earth. Moreover, in recent times it has been discovered that life on Earth can develop luxuriantly in a variety of environments far more widely than was thought at one time. Until a few decades ago, for example, it was believed that life was impossible to temperatures higher than 60 ° (hence the boiling in autoclaves to sterilize surgical instruments). We now know that some species of bacteria (those not at random thermophilic) not only survive, but actually thrive at temperatures above the boiling point of water. They live around volcanic vents located on the bottom of the oceans, known as black smokers, and are completely independent from the light of the Sun, according to some microorganisms such as these were, the early pioneers of our planet. Life, moreover, can cope even in conditions of extreme cold. Below the polar ice has been found a bacterium called Colwellia who lives in temperatures of - 40 °. Microorganisms psychrophilic, which like the cold, have resorted to complicated and ingenious mechanisms to prevent water contained in their cells from freezing: some protein jelly eso-polymers calls, prevent the formation of ice crystals that break up the cells. Life can tolerate temperatures not only absurd, but also extreme pH levels, as evidenced by the discovery of microorganisms that grow easily in hot springs from which flows sulfuric acid. Deinococcus radiodurans, is able to survive extreme levels of radiation lethal to any other living being. Although there is still disagreement on the subject in the scientific community, some findings seem to show that even some tiny organisms, the hypothetical nanobi, are able to survive even between 10 and 20 km depth, temperatures and pressures extreme. Spores were then discovered in crystals that go back hundreds of millions of years ago, yet now able to develop living beings. These groups of living beings are collectively known as extremophiles. Their existence makes it difficult to decide which is the habitat to be considered normal for life on Earth. This knowledge expands, as a result, the range of possible habitats for life in the universe: if we find alien life, we no longer limit ourselves to search for planets covered with blue-green liquid water. If there are psychrophilic bacteria able to metabolize at - 40 °, entire regions of the solar system, the large satellites of the outer planets to the countless swarms of icy objects in the Kuiper Belt, where the temperatures are very low, appear suddenly very more hospitable, taking into account the fact that within these objects may be warmer due to the decay of radioactive substances. The discovery that life is more durable and adaptable than previously thought is considered by most evidence against the panspermia, since it suggests that the range of places where land can be born life could be much larger than when written on academic until a few years ago. It may be, for example, that the assumption of uninhabitabilty of the Earth during the first few hundred million years is to be reviewed, and if bacteria can survive at 20 kilometers underground, maybe life could withstand the most violent attacks by meteorites during the period Hadean. But extremophiles, as well as to suggest that perhaps early life was less difficult to gain a foothold on Earth than biologists thought, also teach that life, even if only at the level of spores, you may be strong enough to survive the rigors of the interplanetary and even interstellar, as suggested by the hypothesis of panspermia: the cold of space is not after all a hindrance if a microbe that has evolved in the mild climate of the Earth can continue to survive - 200 °! In short, the same tests used by some against panspermia, are used by others as evidence in favor of it. If you really during the Hadean all the families living were canceled by the violent impact with planetoids, the oldest fossils we have, in fact, could be those of alien life forms. In an experiment carried out in 2001, NASA has used a high velocity bullet to try to reproduce the effect of a comet rich in amino acids that falls on the Earth at a speed of thousands of kilometers per hour. It has been found that the impact, instead of breaking the amino acids, as it was assumed, pushes them instead to join forming peptide chains, polymeric compounds which have a complexity level immediately below that of proteins. In fact they found almost everywhere aromatic nitrogen compounds in space in comets, in the clouds of interstellar dust and the atmosphere of the outer planets. The carbonaceous chondrites, a particular type of meteorites, contain the carbonaceous compounds aliens more complex ones known, including amino acids and sugars. In an article published in Science in February 2004, Professor Sandra Pizzarello of Arizona State University argued that the chirality, the tendency of the molecules to be laevogyrate or right-handed, proteins and sugars in the forms of terrestrial life could be connected to the meteoritic material that hit our planet for billions of years. Sandra Pizzarello found that in experiments in which the synthesis of sugars was performed in the laboratory under conditions similar to those considered in the early Earth, a constant rain of chemicals with the right chirality (specifically laevogyrate) reverses the chirality sugars terrestrial . This does not mean that Sandra Pizzarello has shown that life came from meteorites, of course, but it is at least possible that the arrival of meteorites has influenced the evolution of life measure deep, whatever the way they began on our planet. We know that these organic molecules are present in the universe for a very long time, no doubt, before the birth of our world. One of the main candidates for the title of oldest object of the Earth is a meteorite crashed in 2000 on the frozen surface of Tagish Lake in Canada, visible in the photo at left. The analysis of the Tagish Lake meteorite performed by researchers at the University of Western Ontario and published in Science in 2006 revealed that the hollow carbon spheres with a diameter of a few thousandths of a millimeter, found in the fragments of the meteorite would be even the oldest in the Sun! In other words, it is possible that within these rock fragments there are particles as old as the universe itself, containing complex compounds, including amino acids, mixed with granules of clay minerals, silicates with a layered structure. Someone has speculated that these inorganic compounds may be possible uteri for the formation of a kind of entity prebiotic, as precursors of nucleic acids. At this point it is easy to imagine how much feeling aroused the announcement, made in March 2011 by Richard Hoover, an astrobiologist who works for the Marshall Space Flight Center of NASA, the discovery of a fossilized alien life form on Earth and come in a meteorite. His extraordinary statement is supported by ten years of study of fossil bacteria: Hoover considered extremely rare form of meteorites, called carbonaceous chondrites, found in every part of the world, from Antarctica to Siberia to Alaska. The fossils he identified as aliens appear long and filamentous, and resemble terrestrial bacteria known as Velox Titanospirillum. The fact that many of the bacteria are recognizable and can be compared with terrestrial species inextricably linked to life on our planet from space to that extent, said Hoover wave of enthusiasm. Hoover contends, however, that bacteria identified by him would possess no nitrogen. Of course most of the academic world were skeptical, waiting for new independent tests. In any case, however bizarre it may appear, today the idea that life, or at least the chemical precursors of life, can be arrived on Earth from cometary material transported, is no longer considered totally absurd: at least some of the processes that it was assumed they had made a small puddle of warm early Earth could have taken place in outer space, in a time when the universe was young.

Life forms on the planet make up the biosphere. The biosphere is divided into various biomes, inhabited by a population of flora and fauna somewhat similar. On land, biomes are separated primarily according to the latitude. Biomes north of the Arctic Circle and south of the Antarctic Circle are relatively devoid of plant and animal life, while most populated are located near the equator. The complex interaction between biosphere and individual forms of life has led some authors to the Gaia hypothesis, according to which life on earth is possible thanks to the behavior of living beings that maintain a delicate homeostasis.

The Earth has an atmosphere relatively thick, composed for 78% nitrogen, 21% oxygen and 1% argon, plus traces of other gases including carbon dioxide and water. The atmosphere separates the earth's surface from the environment inhospitable space, blocks most of the harmful solar radiation, moderates the temperatures on the surface and is the vehicle of transportation of water vapor and other gaseous substances. Its various layers, the troposphere, the stratosphere, the mesosphere, thermosphere and exosphere are different around the globe and also vary with the seasons. It is precisely the atmosphere to the greenhouse effect, consisting in the absorption and re-emission thermal infrared by certain gaseous species. The main gases responsible for this phenomenon are carbon dioxide, water vapor, methane and ozone. The greenhouse effect, as appropriate, is essential for life on the planet, because without this heat shield, the average surface temperature would be -18 ° C, which is incompatible with the maintenance of liquid water and , accordingly, with life.

Above the troposphere, the atmosphere is usually divided into: the stratosphere, mesosphere and thermosphere. Each of these zones has a typical variation of the temperature as a function of the altitude. Continuing in altitude, there is the exosphere, and then the magnetosphere (where is the interaction between the magnetic field and the solar wind). A fundamental area for life on the planet is the ozonosphere, part of the stratosphere where a high concentration of ozone shields the earth's surface from ultraviolet rays. The Kármán line, located at an altitude of 100 km, is commonly used to define the boundary between the Earth's atmosphere and space. Due to the high thermal energy, some molecules of the outer part of the atmosphere can accelerate until reaching a speed that allows them to escape from the gravity of the planet. The effect is that the atmosphere is very slow, but steady loss of matter in space. Since hydrogen has a low molecular weight, reaches its escape velocity more rapidly and more easily compared to other molecules, and abandons the atmosphere at a greater rate. For this reason, the Earth is in an oxidizing environment, rather than reducing, with important consequences on the chemical nature of life. However, the oxygen-rich atmosphere can preserve most of the hydrogen remaining tying it in the form of water molecules.

The magnetosphere is a natural phenomenon, a dipole magnetic poles do not coincide with the geographical ones - not static - and having dipole moment (axis) at an angle of 11.3 ° to the axis of rotation of the Earth. Despite the many hypotheses about the presence of this field, theories have been geared towards a model similar to that of a self-exciting dynamo to. The intensity of the earth's magnetic field is not constant over time, but there are significant variations. They have led, in the course of geological time, the drift of the magnetic poles with respect to continents and repeated phenomena of inversion of the field, with mutual exchange of the North and South magnetic poles The Earth's magnetism is of significant importance for life on Earth. In fact it extends for several tens of thousands of kilometers into space, forming an area called magnetosphere, whose presence generates a sort of shield electromagnetic which deflects and reduces the number of cosmic rays that if they arrived to the surface of the planet would lead to its sterilization. The interaction between cosmic rays (solar wind) and magnetosphere originates the wonderful phenomenon called aurora borealis.

Hydrosphere The term refers only to the oceans, but technically includes all inland seas, lakes, rivers and ground water up to 2000 m deep. The Earth is the only planet in the solar system whose surface is home to liquid water. The water covers 71% of the earth's surface and is divided into a 97% salt water and a 3% fresh water, of which approximately 68% is in the form of ice. The water divides the world into five oceans and seven continents. The deepest point in the body of water is the Mariana Trench in the Pacific Ocean with -10,911 m, while the average depth of the oceans is 3.794 m, more than five times the average height of the continents. The estimated mass of ocean water is about 1.35 x 1018 tons, comparable to 1/4400 of the entire land mass, it also occupies a volume of 1.386 x 109 km ³. The average salt in ocean water is 35 g / l, however, this value being linked to the external inputs of water and evaporation (temperature) can significantly increase or decrease in closed basins in areas with very cold water . These salts come from direct volcanic emission or dismantling chemical and physical performed over time at the expense of magmatic rocks. The masses of water are also huge tanks of gaseous substances, they possess an important function thermoregulatory and mitigating climate and are active agents from the point of view Geomorphological. They live an entire aquatic ecosystem, complete from the point of view of the food pyramid and integrated with the surface, and proved essential for human development past and present. The presence of liquid water on the earth's surface is a combination of the right orbital characteristics, the volcanism, the gravity, the greenhouse effect, the magnetic field and of the atmosphere rich in oxygen. There are several hypotheses that Europa, a satellite of Jupiter, guests of liquid water beneath the layer of ice that covers the entire surface. The Earth is actually beyond the outer edge of the orbits that would allow a planet to be warm enough for liquid water to form. Without some form of greenhouse effect, the Earth's water would freeze. The paleontological findings indicate that at some point, after the blue-green bacteria (archaea) colonized the oceans, global warming stopped working, and the Earth froze completely for a period of between 10 and 100 million years. On other planets, such as Venus, gaseous water is dissociated by ultraviolet solar, and hydrogen is ionized and blown away by the solar wind. The effect is slow, but inexorable. It is thought that this is the cause of water shortage of Venus. Private hydrogen, oxygen reacts with the surface and is incorporated in solid mineral. On Earth, a shield of ozone absorbs most of the ultraviolet energy in the upper atmosphere, reducing this effect. Finally, volcanism, helped by the tidal effects of the Moon, continuously emits water vapor from the inside. The tectonic plates of the Earth recycle carbon and water by the subduction of sediment-rich areas, and converting them into magma emitted by volcanoes such as carbon dioxide gas and steam. Ocean currents, also, is believed due to a particular oscillation of the axis of rotation of the earth, said oscillation of Chandler.

The Earth rotates from west to east once a day, understood as the sidereal day, around the axis joining the North Pole to the South Pole in 23 hours, 56 minutes and 4.091 seconds. It is for this reason that the sun and all the stars rise in the east and set in the west making a move in the sky at a speed of about 15 ° / I 15 '/ min. In addition, the Earth rotates around the Sun at an average distance of 150 million km in a sidereal year. The speed of orbit is about 30 km / s (108,000 km / h), fast enough to cover the planet's diameter (about 12,600 km) in seven minutes, and the distance to the Moon (384,000 km) in 4 hours.

The Earth's axis is tilted 23.5 degrees from the perpendicular plane of the Earth-Sun and Earth-Moon plane is tilted five degrees, which prevents the occurrence of two eclipses (a solar and a lunar) every month , and makes a rather rare event. Again because of the inclination of the Earth, the Sun's position in the sky and the effects of its radiation (seen by an observer on the surface) varies throughout the year. For example, an observer at a northern latitude, when the north pole is tilted toward the sun, notice periods of light per day longer and a more temperate climate, and have less daylight hours and a colder climate in the case opposite. Above the two polar circles to reach the extreme case of alternation of long periods of absence of light (called polar nights), with periods of no sunset the Sun This relationship between climate and the tilt of Earth's axis is defined through the 4 seasons. They, from the point of view astronomical, are determined by the solstices (the points of maximum inclination towards and against the Sun) and the equinoxes (points where the inclination is perpendicular to the direction of the Sun). The winter solstice falls on December 21, the summer June 21, while the two equinoxes fall, the spring on March 20 and Autumnal September 23. The alternation of the seasons is opposite from a hemisphere of the earth to the other, given the inclination of the axis opposite form, for example, the presence in the north of the summer and in the winter south. The angle of inclination is relatively stable when considered over long periods, however, it performs a slow and irregular motion (known as nutation), with a period of 18.6 years. The orientation of the axis varies according to a precession around a full circle in a cycle of just over 25 800 years. The presence of a precession is the cause of the phase difference between a sidereal year and a tropical year. Both changes result from the movement of the changing pull of the sun and moon on the equator of the planet. Also the speed of rotation of the planet is not constant, but varies in time according to a phenomenon known as variation of the length of the day. In modern times the perihelion falls on January 3, while aphelion around July 4 (for information about other eras, check precession and Milankovitch cycles). The difference in terms of energy received from the Sun between the position of perihelion and aphelion and that of 6.9% in favor of the first; also from the moment when the southern hemisphere is oriented towards the Sun, that in which the planet reaches the point of perihelion, the hemisphere receives slightly more energy than the northern hemisphere during the whole year. This difference, albeit present, is decidedly not significant compared to the total energy resulting from the change of orientation of the axis, and, in its greater part, is absorbed and compensated by the higher presence of masses aqueous southern hemisphere.

The Hill sphere (gravitational sphere of influence) of the Earth is about 1.5 Gm (1,496,620 km) radius. This is the maximum distance at which the gravitational influence of the planet is stronger than that of the Sun and planets. Objects in orbit must remain within this range to avoid being influenced and made unstable by gravitational perturbations outside.

Since the Earth is very large, observing the surface is not immediately obvious that he Geoidal shape, slightly flattened at the poles and with a slight bulge at the equator. For this reason the ancient civilizations, such as Mesopotamia, and the first Greek philosophers, such as Thales, who believed the Earth was flat. A first step towards the recognition of the real form was made by Anaximander, who conceived the earth as a cylinder suspended in space, then imagine that you have not only the sky above his head but also under their feet. The spherical shape was finally recognized on the basis of inferences based on observations such as the change of astronomical observations with latitude, the observation of the eclipse of the moon and the comparison with the shape of the moon and the sun. The Greeks, about 2500 years ago, they began first to argue that the earth was a sphere. The first evidence of Earth's sphericity come to us from Pythagoras (VI-V century BC) and by Parmenides (fifth century BC) and Aristotle (384 BC-322 BC) led the first demonstrations and finally Eratosthenes (274 BC-196 BC) was the first measurements. The scholars of the early Middle Ages, then, as William of Conches, John of Sacrobosco, Roger Bacon, Thomas Aquinas, Brunetto Latini, Dante Alighieri, John Buridan and others said the sphericity of the Earth with arguments, for the most part of this like this:

1.Sun at noon, indicating the south whatever our point of view: if the earth was flat, it would not be so;

2.Shadow cast by the earth on the moon during a partial eclipse, is an arc of a circle;

3.Part which first disappears of a ship on the horizon is the keel.

It is considered the unfounded belief that modern in the Middle Ages the world was widely held flat. However, there are still supporters of the flat shape of the Earth, many of which adhere to the Flat Earth Society (Flat Earth Society). The erroneous assumption of the flatness of the Earth in the most ancient civilizations, it was due to lack of knowledge of the nature center of gravity, which allows you to have the sky always high and the center of the Earth always low, and thus overcome the apparent paradox you had to walk with his head pointing down on the other side of the Earth (paradox but already Anaximander had been able to overcome).

It felt a lot longer that the Earth was the center of the universe because you have the impression that all the other heavenly bodies to revolve around it, also looking