My Science School

Most of the earth is made of various solid rocks. The 2000km- (1240-mile-) thick outer core is the only part of the Earth that exists in an entirely liquid form. Iron, nickel and other materials are liquefied by the extremely high temperatures. Molten rock is found in parts of the mantle, some of which comes to the surface as lava.

The Earth's interior is composed of four layers, three solid and one liquid—not magma but molten metal, nearly as hot as the surface of the sun.



The deepest layer is a solid iron ball, about 1,500 miles (2,400 kilometers) in diameter. Although this inner core is white hot, the pressure is so high the iron cannotmelt.

The iron isn't pure—scientists believe it contains sulfur and nickel, plus smaller amounts of other elements. Estimates of its temperature vary, but it is probably somewhere between 9,000 and 13,000 degrees Fahrenheit (5,000 and 7,000 degrees Celsius).



Above the inner core is the outer core, a shell of liquid iron. This layer is cooler but still very hot, perhaps 7,200 to 9,000 degrees Fahrenheit (4,000 to 5,000 degrees Celsius). It too is composed mostly of iron, plus substantial amounts of sulfur and nickel. It creates the Earth's magnetic field and is about 1,400 miles (2,300 kilometers) thick.

The next layer is the mantle. Many people think of this as lava, but it's actually rock. The rock is so hot, however, that it flows under pressure, like road tar. This creates very slow-moving currents as hot rock rises from the depths and cooler rock descends.

The mantle is about 1,800 miles (2,900 kilometers) thick and appears to be divided into two layers: the upper mantle and the lower mantle. The boundary between the two lies about 465 miles (750 kilometers) beneath the Earth's surface.

The crust is the outermost layer of the Earth. It is the familiar landscape on which we live: rocks, soil, and seabed. It ranges from about five miles (eight kilometers) thick beneath the oceans to an average of 25 miles (40 kilometers) thick beneath the continents.

Currents within the mantle have broken the crust into blocks, called plates, which slowly move around, colliding to build mountains or rifting apart to form new seafloor.

Continents are composed of relatively light blocks that float high on the mantle, like gigantic, slow-moving icebergs. Seafloor is made of a denser rock called basalt, which presses deeper into the mantle, producing basins that can fill with water.

Except in the crust, the interior of the Earth cannot be studied by drilling holes to take samples. Instead, scientists map the interior by watching how seismic waves from earthquakes are bent, reflected, sped up, or delayed by the various layers.

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     The magnetic field of the Earth at any given time is preserved in the magnetic minerals within rocks that solidified during that period. Geologists are thus able to study the magnetic field of rocks thousands of years old, such as those used to build the pyramids at Giza, Egypt.

     A magnetic field is the area around a material in which its magnetic forces can be detected. Those forces stem from the activity of tiny, negatively charged particles called electrons, which are within all atoms. A material’s magnetism is determined by the way its electrons move around the outside of its atoms’ nuclei — particularly those electrons that aren’t paired with other electrons in certain ways. If a large number of unpaired electrons rotate in the same direction (imagine a large number of tops spinning on a table or other flat surface), then an object’s magnetic field can be strong. If all of the unpaired electrons spin in random directions, the object’s magnetic field is either very weak or missing.

     Some materials, such as lodestones, create a persistent magnetic field. Others with unpaired electrons, such as iron, can become magnetized when they’re placed within a magnetic field and their atoms rotate and align.

     Scientists don’t know how some types of rocks, including lodestones, become so strongly magnetized. But new lab tests show how some other rocks can become naturally magnetized.

     Charles Aubourg is a geologist at the University of Pau and the Adour Countries in France. He and his colleagues heated samples of a type of sedimentary rock to as much as 130 degrees Celsius (about 266 degrees Fahrenheit). Sedimentary rock is made from material eroded from other rocks. The eroded materials transform into stone when exposed to high pressure deep within Earth for a lengthy period of time, sometimes millions of years.

     Aubourg’s team got its rock samples from northern France, but similar rocks can be found worldwide. Each sample contained large amounts of clay and silt (both of which are made of tiny particles eroded from other rocks). But importantly, the rocks also contained a small amount of an iron-bearing mineral called pyrite.

     First, the team used a strong magnetic field to erase any magnetism naturally trapped in the sample. Then the researchers heated the rock inside a strong magnetic field according to a specific recipe: 25 days at 50 degrees Celsius, then 25 days at 70 degrees, 25 days at 80 degrees, 10 days at 120 degrees, and a final 10 days at 130 degrees. This temperature range is the same as that of rocks located between 2 kilometers and 4 kilometers deep in Earth’s crust, explains Aubourg.

     The rocks’ magnetic field rose during each stage of heating. It increased most quickly during the earliest days of each step. The growing magnetism of the samples suggests that the heat triggered reactions that caused some of the pyrite to chemically transform into magnetic minerals.

     Analyses conducted after the heating suggest that the magnetic minerals were very tiny grains of magnetite. These grains were so small, less than 20 nanometers across, that it would take more than 1,000 of them side by side to stretch across the width of a single human hair. The researchers reported their results online August 10 in the scientific journal Geochemistry, Geophysics, Geosystems.

     Because the grains of magnetite were so small, looking for one “would be like trying to find a needle in a haystack,” says Douglas Elmore. He is a sedimentary geologist at the University of Oklahoma in Norman. Nevertheless, he notes, the evidence is convincing that the heating experiments created small grains of magnetite, not other types of magnetic minerals.

     Studies that investigated rocks in their natural environment have hinted that rocks buried in shallow layers of Earth’s crust and heated there naturally can become magnetized, says Elmore. The new lab tests provide even stronger evidence that such magnetization occurs naturally, he adds.

     Studying the magnetic field trapped in ancient rocks helps scientists better understand Earth’s history, including how the planet’s magnetic field has changed through time.

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    The earth is rather like an enormous magnet. Otherwise known as the magnetosphere, the Earth's magnetic field stretches out into space, helping to protect the Earth from the Sun's radiation. The magnetic poles are close to the geographic North and South Poles.

    A magnetic field is a vector field that describes the magnetic influence of electric charges in relative motion and magnetized materials. The effects of magnetic fields are commonly seen in permanent magnets, which pull on magnetic materials (such as iron) and attract or repel other magnets. Magnetic fields surround and are created by magnetized material and by moving electric charges (electric currents) such as those used in electromagnets. They exert forces on nearby moving electrical charges and torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. Both the strength and direction of a magnetic field vary with location. As such, it is described mathematically as a vector field.

    In electromagnetics, the term "magnetic field" is used for two distinct but closely related fields denoted by the symbols B and H. In the International System of Units, H, magnetic field strength, is measured in the SI base units of ampere per meter. B, magnetic flux density, is measured in tesla (in SI base units: kilogram per second2 per ampere), which is equivalent to newton per meter per ampere. H and B differ in how they account for magnetization. In a vacuum, B and H are the same aside from units; but in a magnetized material, B/{\displaystyle \mu _{0}}  and H differ by the magnetization M of the material at that point in the material.

    Magnetic fields are produced by moving electric charges and the intrinsic magnetic moments of elementary particles associated with a fundamental quantum property, their spin. Magnetic fields and electric fields are interrelated, and are both components of the electromagnetic force, one of the four fundamental forces of nature.

    Magnetic fields are widely used throughout modern technology, particularly in electrical engineering and electromechanics. Rotating magnetic fields are used in both electric motors and generators. The interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits. Magnetic forces give information about the charge carriers in a material through the Hall Effect. The Earth produces its own magnetic field, which shields the Earth's ozone layer from the solar wind and is important in navigation using a compass.

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The molten iron that partly makes tip the Earth's core continually flows around. As this happens, it generates powerful electric currents that create the Earth's magnetic field. This is similar to the way magnetic currents are generated by an electric motor.

Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from the Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic field is generated by electric currents due to the motion of convection currents of molten iron in the Earth's outer core: these convection currents are caused by heat escaping from the core, a natural process called a geodynamo. The magnitude of the Earth's magnetic field at its surface ranges from 25 to 65 microteslas (0.25 to 0.65 gauss). As an approximation, it is represented by a field of a magnetic dipole currently tilted at an angle of about 11 degrees with respect to Earth's rotational axis, as if there were an enormous bar magnet placed at that angle through the center of the Earth. The North geomagnetic pole, which was in 2015 located on Ellesmere Island, Nunavut, Canada, in the northern hemisphere, is actually the south pole of the Earth's magnetic field, and conversely.

While the North and South magnetic poles are usually located near the geographic poles, they slowly and continuously move over geological time scales, but sufficiently slowly for ordinary compasses to remain useful for navigation. However, at irregular intervals averaging several hundred thousand years, the Earth's field reverses and the North and South Magnetic Poles respectively, abruptly switch places. These reversals of the geomagnetic poles leave a record in rocks that are of value to paleomagnetists in calculating geomagnetic fields in the past. Such information in turn is helpful in studying the motions of continents and ocean floors in the process of plate tectonics.

The magnetosphere is the region above the ionosphere that is defined by the extent of the Earth's magnetic field in space. It extends several tens of thousands of kilometers into space, protecting the Earth from the charged particles of the solar wind and cosmic rays that would otherwise strip away the upper atmosphere, including the ozone layer that protects the Earth from harmful ultraviolet radiation.

The Earth's magnetic field serves to deflect most of the solar wind, whose charged particles would otherwise strip away the ozone layer that protects the Earth from harmful ultraviolet radiation. One stripping mechanism is for gas to be caught in bubbles of magnetic field, which are ripped off by solar winds. Calculations of the loss of carbon dioxide from the atmosphere of Mars, resulting from scavenging of ions by the solar wind, indicate that the dissipation of the magnetic field of Mars caused a near total loss of its atmosphere.

The study of the past magnetic field of the Earth is known as paleomagnetism. The polarity of the Earth's magnetic field is recorded in igneous rocks, and reversals of the field are thus detectable as "stripes" centered on mid-ocean ridges where the sea floor is spreading, while the stability of the geomagnetic poles between reversals has allowed paleomagnetists to track the past motion of continents. Reversals also provide the basis for magnetostratigraphy, a way of dating rocks and sediments. The field also magnetizes the crust, and magnetic anomalies can be used to search for deposits of metal ores.

Humans have used compasses for direction finding since the 11th century A.D. and for navigation since the 12th century. Although the magnetic declination does shift with time, this wandering is slow enough that a simple compass can remain useful for navigation. Using magnetoreception various other organisms, ranging from some types of bacteria to pigeons, use the Earth's magnetic field for orientation and navigation.

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         In 1990, a geological exploration began to find out more about the Earth's crust. A hole drilled into the ground in the Kola Peninsula, Russia, has reached a depth of around 15km (9.3 miles). Nobody has been down it, and it is still well short of the Earth’s centre.

         Travelling to the Earth's center is a popular theme in science fiction. Some subterranean fiction involves traveling to the Earth's center and finding either a Hollow Earth or Earth's molten core. Planetary scientist David J. Stevenson suggested sending a probe to the core as a thought experiment. Humans have drilled over 12 kilometers (7.67 miles) in the Sakhalin-I. In terms of depth below the surface, the Kola Superdeep Borehole SG-3 retains the world record at 12,262 metres (40,230 ft) in 1989 and still is the deepest artificial point on Earth.

         The idea of a so-called "Hollow Earth", once popular in fantasy adventure literature, is that the planet Earth has a hollow interior and an inner surface habitable by human beings. Although the scientific community has made clear that this is pseudoscience, the idea nevertheless is a less popular feature of many fantasy and science fiction stories and of some conspiracy theories.

         The most famous example of a hollow-Earth fantasy is Jules Verne's 1864 science-fiction novel Journey to the Center of the Earth, which has been adapted many times as a feature film and for television.

         The 2003 film The Core, loosely based on the novel Core, tells the story of a team that has to drill to the center of the Earth and detonate a series of nuclear explosions in order to restart the rotation of Earth's core. The drilling equipment, dubbed Virgil, includes a powerful, snake-like laser drill, a small nuclear reactor for power, a shell (of "unobtainium", a fictional material) to protect against intense heat and pressure (and generate energy to drive the engine), a powerful x-ray camera for viewing outside, and a system of impellers for movement and control. The only part of the Earth that turns out to be hollow is a gigantic geode, and soon after the drill moves through it, the hole it created fills with magma.

          The 1986 animated television show Inhumanoids featured regular visits to the Inner Core in most of its 13 episodes. Each of the three villainous creatures theoretically ruled over certain layers of the inner Earth, and their evil schemes were thwarted by the human Earth Corps, who often allied with various races of subterranean beings equally threatened by the Inhumanoids.

          During season 3 of the Teenage Mutant Ninja Turtles cartoon the Technodrome is located at the Earth's core, and transport modules are used to drill up to the streets. This season also features the episode "Turtles at the Earth's Core", where a dinosaur lives in a deep cave, and a crystal of energy that works like the Sun to keep the dinosaurs alive. As Krang, Shredder, Bebop and Rocksteady steal the crystal to power the Technodrome, the trouble begins.

          Don Rosa's 1995 Uncle Scrooge story The Universal Solvent imagines a way to travel to the planet's core using 1950s technology, although this would be impossible in reality. The fictional solvent referred to in the story's title has the power to condense everything except diamonds into a kind of super-dense dust. The solvent is accidentally spilled and, as it dissolves everything in its path, it bores a cylindrical shaft into the center of the planet. As part of a recovery effort, a makeshift platform is constructed that descends into the shaft in free fall, automatically deploying an electric motor and wheels as it approaches zero gravity, then using rocket engines to enable it to ascend again to the Earth's surface. The author Rosa describes this fantasy journey in great detail: the supposed structure of the Earth is illustrated, and the shaft is kept in a vacuum to protect against the lethal several thousand kilometers of atmosphere that it would otherwise be exposed to. The ducks must wear space suits and go without food for several days, and they are not entirely certain that the super-dense heat shield will hold. The author maintains continuity with Carl Barks, explaining that the earthquakes in the story are created by spherical Fermies and Terries.

          In Tales to Astonish #2 (1959) "I Fell to the Center of the Earth", an archaeologist named Dr. Burke who is on an expedition to Asia travels to the center of the Earth (and also, as he later finds out, backwards in time)--and encounters neanderthals and dinosaurs.

         In the Doctor Who episode, "The Runaway Bride", a Racnoss warship is found at the center of the planet.

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          The outer core begins at a depth of 2935km (1822 miles) below the Earth's surface. It is a further 3432km (2134 miles) to the very centre of the Earth.

          Earth’s core is the very hot, very dense center of our planet. The ball-shaped core lies beneath the cool, brittle crust and the mostly-solid mantle. The core is found about 2,900 kilometers (1,802 miles) below Earth’s surface, and has a radius of about 3,485 kilometers (2,165 miles).

          Planet Earth is older than the core. When Earth was formed about 4.5 billion years ago, it was a uniform ball of hot rock. Radioactive decay and leftover heat from planetary formation (the collision, accretion, and compression of space rocks) caused the ball to get even hotter. Eventually, after about 500 million years, our young planet’s temperature heated to the melting point of iron—about 1,538° Celsius (2,800° Fahrenheit). This pivotal moment in Earth’s history is called the iron catastrophe.

          The iron catastrophe allowed greater, more rapid movement of Earth’s molten, rocky material. Relatively buoyant material, such as silicates, water, and even air, stayed close to the planet’s exterior. These materials became the early mantle and crust. Droplets of iron, nickel, and other heavy metals gravitated to the center of Earth, becoming the early core. This important process is called planetary differentiation.

          Earth’s core is the furnace of the geothermal gradient. The geothermal gradient measures the increase of heat and pressure in Earth’s interior. The geothermal gradient is about 25° Celsius per kilometer of depth (1° Fahrenheit per 70 feet). The primary contributors to heat in the core are the decay of radioactive elements, leftover heat from planetary formation, and heat released as the liquid outer core solidifies near its boundary with the inner core. 

          Unlike the mineral-rich crust and mantle, the core is made almost entirely of metal—specifically, iron and nickel. The shorthand used for the core’s iron-nickel alloys is simply the elements’ chemical symbols.

          Elements that dissolve in iron, called siderophiles, are also found in the core. Because these elements are found much more rarely on Earth’s crust, many siderophiles are classified as “precious metals.” Siderophile elements include gold, platinum, and cobalt. 

          Another key element in Earth’s core is sulfur—in fact 90% of the sulfur on Earth is found in the core. The confirmed discovery of such vast amounts of sulfur helped explain a geologic mystery: If the core was primarily, why wasn’t it heavier? Geoscientists speculated that lighter elements such as oxygen or silicon might have been present. The abundance of sulfur, another relatively light element, explained the conundrum.

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          The crust is the hard, outer layer of the Earth that forms the land and the ocean floor. The continental crust (the lane masses) is the oldest and thickest part and made up mostly of silica and aluminium. The oceanic crust, made up mostly of silica and magnesium, is around 200 million years old.

          Early on in Earth’s history, minerals began to form. Lighter minerals floated up toward the surface and formed a thin crust of rock around the outside of the planet (which we now live on top of). If Earth was the size of a plum, the rocky crust would be a bit like the thin purple skin. If we want to see below the surface, we can drill down into the crust for thousands of meters.

          The crust is mostly made of minerals such as quartz, feldspar and mica. These are the shiny crystals in granite rocks, which you can see in the southwest of Kenya. Over long periods of time these minerals break down into small pieces and are carried around by winds, currents and waves to form soft sediments like sand. Look out for sediments when you are by a river, a lake or a beach.

          The crust is made up of huge blocks of rock that move around the Earth’s surface very slowly – as slowly as your fingernails grow. The movement of these plates over millions of years causes continents to split apart and smash together. Right now, East Africa is splitting into two pieces along the Great Rift Valley and one day in the distant future, the rift may be flooded by the sea.

          In between the core and the crust is a hot, squishy body of rock called the mantle. The mantle is mostly made of a mineral called olivine, which is a beautiful shade of green. The hot mantle has currents that flow like treacle. These slow currents push the plates of rock around at the surface.

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          The surface of the Earth, the crust, makes up a very small part of the whole planet. While it is relatively straightforward to find out about the Earth's surface, investigating deep within the Earth is part science, part guesswork. What is known is that there are three main layers: the crust, the mantle and the core, and that these consist of rocks and metals in various states and forms.

          The Earth started out as a ball of very, very hot liquid. This liquid was mostly made of two elements called oxygen and silica. But there were small amounts of other elements too. In fact, it was a mixture of almost every element in existence. This all happened around 4.6 billion years ago – that’s a really long time, so long that we can’t even imagine it.

          Over time, Earth began to cool down. The heavier elements, like iron and nickel, sank into the centre of the planet (the core). And it’s hot: the Earth’s core is as hot as the surface of the sun, so hot that we wouldn’t be able to go near it, let alone touch it. But you don’t have to worry about getting too close. Wherever you are, whether in Kenya, China or Brazil, the core is around 1800 miles below your feet. This means we will never be able to visit it.

          Even though we can’t actually go to the Earth’s core, we know some things about it. We know, for example, that the core is full of iron, because Earth acts like a giant magnet, drawing some elements to it. This magnetic core is very useful: it means we can use a compass to find our way, like sailors in the ocean.

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          The rotation of the Earth causes it to bulge slightly in the middle. Centrifugal force makes the Earth's material move away from the centre — the faster the spin, the greater the force. As places at the Equator are moving faster than places at the poles, the centre of the Earth pushes out slightly more than the rest.

          The simplest model for the shape of the entire Earth is a sphere. The Earth's radius is the distance from Earth's center to its surface, about 6,371 kilometers (3,959 mi). While "radius" normally is a characteristic of perfect spheres, the Earth deviates from spherical by only a third of a percent, sufficiently close to treat it as a sphere in many contexts and justifying the term "the radius of the Earth".

          The concept of a spherical Earth dates back to around the 6th century BC, but remained a matter of philosophical speculation until the 3rd century BC. The first scientific estimation of the radius of the Earth was given by Eratosthenes about 240 BC, with estimates of the accuracy of Eratosthenes’s measurement ranging from 2% to 15%.

          The Earth is only approximately spherical, so no single value serves as its natural radius. Distances from points on the surface to the center range from 6,353 km to 6,384 km (3,947 – 3,968 mi). Several different ways of modeling the Earth as a sphere each yield a mean radius of 6,371 kilometers (3,959 mi). Regardless of the model, any radius falls between the polar minimum of about 6,357 km and the equatorial maximum of about 6,378 km (3,950 – 3,963 mi). The difference 21 kilometers (13 mi) correspond to the polar radius being approximately 0.3% shorter than the equator radius.

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          The earth spins as a result of things colliding with each other when the Solar System was formed. Some scientists believe that the Earth started spinning after a direct collision with the Moon. Kept moving by the force of momentum, the Earth takes one day to make one full rotation.

          It can’t be a coincidence. Look down on the Earth from above, and you’d see that it’s turning in a counter-clockwise direction. Same with the Sun, Mars and most of the planets.

          It’s the conservation of angular momentum. Think about the individual atoms in the cloud of hydrogen. Each particle has its own momentum as it drifts through the void. As these atoms glom onto one another with gravity, they need to average out their momentum. It might be possible to average out perfectly to zero, but it’s really unlikely.

          Which means, there will be some left over. Like a figure skater pulling in her arms to spin more rapidly, the collapsing proto-Solar System with its averaged out particle momentum began to spin faster and faster. As the Solar System spun more rapidly, it flattened out into a disk with a bulge in the middle. We see this same structure throughout the Universe: the shape of galaxies, around rapidly spinning black holes, and we even see it in pizza restaurants.

          Over the course of a few hundred million years, all of the material in the Solar System gathered together into planets, asteroids, moons and comets. Then the powerful radiation and solar winds from the young Sun cleared out everything that was left over. Without any unbalanced forces acting on them, the inertia of the Sun and the planets have kept them spinning for billions of years.

          And they’ll continue to do so until they collide with some object, billions or even trillions of years in the future. The Earth spins because it formed in the accretion disk of a cloud of hydrogen that collapsed down from mutual gravity and needed to conserve its angular momentum. It continues to spin because of inertia.

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          The size of the Earth depends upon how you measure it. If you were to circumnavigate the world (on land and sea) following the Equator, you would travel 40,075km (24,902 miles). Starting at one pole and visiting the other, you would travel 67km (42 miles) less. The Earth's diameter, pole to pole, is 12,714km (7900 miles), while the distance through the Earth at the Equator is a further 43km (27 miles).

          The radius of Earth at the equator is 3,963 miles (6,378 kilometers), according to NASA's Goddard Space Flight Center. However, Earth is not quite a sphere. The planet's rotation causes it to bulge at the equator. Earth's polar radius is 3,950 miles (6,356 km) — a difference of 13 miles (22 km).

          Earth's density is 5.513 grams per cubic centimeter, according to NASA. Earth is the densest planet in the solar system because of its metallic core and rocky mantle. Jupiter, which is 318 more massive than Earth, is less dense because it is made of gases, such as hydrogen.

          Earth's mass is 6.6 sextillion tons (5.9722 x 1024 kilograms). It volume is about 260 billion cubic miles (1 trillion cubic kilometers).

          The total surface area of Earth is about 197 million square miles (510 million square km). About 71 percent is covered by water and 29 percent by land.

          Mount Everest is the highest place on Earth above sea level, at 29,028 feet (8,848 meters), but it is not the highest point on Earth — that is, the place most distant from the center of the Earth. That distinction belongs to Mount Chimaborazo in the Andes Mountains in Ecuador, according to the National Oceanic and Atmospheric Administration (NOAA). Although Chimaborazo is about 10,000 feet shorter (relative to sea level) than Everest, this mountain is about 6,800 feet (2,073 m) farther into space because of the equatorial bulge.

          The lowest point on Earth is Challenger Deep in the Mariana Trench in the western Pacific Ocean, according to the NOAA. It reaches down about 36,200 feet (11,034 meters) below sea level.

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          It takes the Earth one whole year to make one full orbit of the Sun.

          Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi), and one complete orbit takes 365.256 days (1 sidereal year), during which time Earth has traveled 940 million km (584 million mi). Ignoring the influence of other solar system bodies, Earth's orbit is an ellipse with the Earth-Sun barycenter as one focus and a current eccentricity of 0.0167; since this value is close to zero, the center of the orbit is close, relative to the size of the orbit, to the center of the Sun.

          As seen from Earth, the planet's orbital prograde motion makes the Sun appear to move with respect to other stars at a rate of about 1° eastward per solar day (or a Sun or Moon diameter every 12 hours). Earth's orbital speed averages 29.78 km/s (107,208 km/h; 66,616 mph), which is fast enough to cover the planet's diameter in 7 minutes and the distance to the Moon in 4 hours.

          From a vantage point above the north pole of either the Sun or Earth, Earth would appear to revolve in a counterclockwise direction around the Sun. From the same vantage point, both the Earth and the Sun would appear to rotate also in a counterclockwise direction about their respective axes.

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          The axis of the Earth (the imaginary line along which it spins) is set at a slight angle. This tilt affects the Earth's climate because it causes the poles to point towards and away from the Sun at different times of the year. We divide these times into the seasons.

          In ancient times, the scholars, seers and magi of various cultures believed that the world took a number of forms – ranging from a ziggurat or a cube to the more popular flat disc surrounded by a sea. But thanks to the ongoing efforts of astronomers, we have come to understand that it is in fact a sphere, and one of many planets in a system that orbits the Sun.

          Within the past few centuries, improvements in both scientific instruments and more comprehensive observations of the heavens have also helped astronomers to determine (with extreme accuracy) what the nature of Earth’s orbit is. In addition to knowing the precise distance from the Sun, we also know that our planet orbits the Sun with one pole constantly tilted towards it.

          This is what is known axial tilt, where a planet’s vertical axis is tilted a certain degree towards the ecliptic of the object it orbits (in this case, the Sun). Such a tilt results in there being a difference in how much sunlight reaches a given point on the surface during the course of a year. In the case of Earth, the axis is tilted towards the ecliptic of the Sun at approximately 23.44° (or 23.439281° to be exact).

          This tilt in Earth’s axis is what is responsible for seasonal changes during the course of the year. When the North Pole is pointed towards the Sun, the northern hemisphere experiences summer and the southern hemisphere experiences winter. When the South Pole is pointed towards the Sun, six months later, the situation is reversed.

          In addition to variations in temperature, seasonal changes also result in changes to the diurnal cycle. Basically, in the summer, the day last longer and the Sun climbs higher in the sky. In winter, the days become shorter and the Sun is lower in the sky. In northern temperate latitudes, the Sun rises north of true east during the summer solstice, and sets north of true west, reversing in the winter. The Sun rises south of true east in the summer for the southern temperate zone, and sets south of true west.

          The situation becomes extreme above the Arctic Circle, where there is no daylight at all for part of the year, and for up to six months at the North Pole itself (known as a “polar night”). In the southern hemisphere, the situation is reversed, with the South Pole oriented opposite the direction of the North Pole and experiencing what is known as a “midnight sun” (a day that lasts 24 hours).

          The four seasons can be determined by the solstices (the point of maximum axial tilt toward or away from the Sun) and the equinoxes (when the direction of tilt and the Sun are perpendicular). In the northern hemisphere, winter solstice occurs around December 21st, summer solstice around June 21st, spring equinox around March 20th, and autumnal equinox on or about September 22nd or 23rd. In the southern hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.

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          Although planets surround stars in the galaxy, how they form remains a subject of debate. Despite the wealth of worlds in our own solar system, scientists still aren't certain how planets are built. Currently, two theories are during it out for the role of champion. 

          The first and most widely accepted theory, core accretion; works well with the formation of the terrestrial planets like Earth but has problems with giant planets. The second, the disk instability method, may account for the creation of these giant planets. 

          Scientists are continuing to study planets in and out of the solar system in an effort to better understand which of these methods is most accurate.

          Approximately 4.6 billion years ago, the solar system was a cloud of dust and gas known as a solar nebula. Gravity collapsed the material in on itself as it began to spin, forming the sun in the center of the nebula.

          With the rise of the sun, the remaining material began to clump up. Small particles drew together, bound by the force of gravity, into larger particles. The solar wind swept away lighter elements, such as hydrogen and helium, from the closer regions, leaving only heavy, rocky materials to create smaller terrestrial worlds like Earth. But farther away, the solar winds had less impact on lighter elements, allowing them to coalesce into gas giants. In this way, asteroids, comets, planets, and moons were created.

          Earth's rocky core formed first, with heavy elements colliding and binding together. Dense material sank to the center, while the lighter material created the crust. The planet's magnetic field probably formed around this time. Gravity captured some of the gases that made up the planet's early atmosphere. 

          Early in its evolution, Earth suffered an impact by a large body that catapulted pieces of the young planet's mantle into space. Gravity caused many of these pieces to draw together and form the moon, which took up orbit around its creator.

          The flow of the mantle beneath the crust causes plate tectonics, the movement of the large plates of rock on the surface of the Earth. Collisions and friction gave rise to mountains and volcanoes, which began to spew gases into the atmosphere.

         Although the population of comets and asteroids passing through the inner solar system is sparse today, they were more abundant when the planets and sun were young. Collisions from these icy bodies likely deposited much of the Earth's water on its surface. Because the planet is in the Goldilocks zone, the region where liquid water neither freezes nor evaporates but can remain as a liquid, the water remained at the surface, which many scientists think plays a key role in the development of life.

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          The amount of time that the Earth has been in existence is immense, and it is impossible to give it an exact age. However, around 5000 million years ago our planet was nothing more than part of a cloud of dust and gas, spinning around in space. Around this time, something caused the material in this enormous cloud to contract, forming the Sun, the Earth and the other planets in our Solar System.

          The age of the Earth is estimated to be 4.54 ± 0.05 billion years. This age may represent the age of the Earth's accretion, of core formation, or of the material from which the Earth formed. This dating is based on evidence from radiometric age-dating of meteorite material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.

          Following the development of radiometric age-dating in the early 20th century, measurements of lead in uranium-rich minerals showed that some were in excess of a billion years old. The oldest such minerals analyzed to date—small crystals of zircon from the Jack Hills of Western Australia—are at least 4.404 billion years old. Calcium–aluminium-rich inclusions—the oldest known solid constituents within meteorites that are formed within the Solar System—are 4.567 billion years old, giving a lower limit for the age of the Solar System.

          It is hypothesized that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites. Because the time this accretion process took is not yet known, and predictions from different accretion models range from a few million up to about 100 million years, the difference between the age of Earth and of the oldest rocks is difficult to determine. It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages.

          Studies of strata - the layering of rocks and earth - gave naturalists an appreciation that Earth may have been through many changes during its existence. These layers often contained fossilized remains of unknown creatures, leading some to interpret a progression of organisms from layer to layer.

          Nicolas Steno in the 17th century was one of the first naturalists to appreciate the connection between fossil remains and strata. His observations led him to formulate important stratigraphic concepts (i.e., the "law of superposition" and the "principle of original horizontality"). In the 1790s, William Smith hypothesized that if two layers of rock at widely differing locations contained similar fossils, then it was very plausible that the layers were the same age. William Smith's nephew and student, John Phillips, later calculated by such means that Earth was about 96 million years old.