Planet Earth Info

Earth is the third planet from the Sun and is the largest of the terrestrial planets. Earth is also referred to as "the Earth", "Planet Earth", "Gaia", "Terra", or "the World". This is the only planet known to have liquid water on the surface and the only place in the universe known to harbor life. Earth has a magnetic field that, together with a primarily nitrogen-oxygen atmosphere, protects the surface from radiation that is harmful to life. The atmosphere also serves as a shield that causes smaller meteors to burn up before they strike the surface. The Earth formed around 4.57 billion years ago and its only known natural satellite, the Moon, began orbiting it around 4.53 billion years ago. At present the Earth orbits the Sun once for every roughly 365.26 times it rotates about its axis. The axial tilt of 23.4° produces seasonal variations on the surface. Atmospheric conditions on Earth have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 71% of the surface is covered in salt-water oceans, and the remainder consists of continents and islands. The outer surface is divided into several tectonic plates that gradually migrate across the surface over geologic time spans. The interior of the planet remains active, with a thick layer of convecting yet solid mantle, a liquid outer core that generates a magnetic field, and a solid-iron inner core. The space environment interacts with the Earth to a significant degree. The relatively large moon provides ocean tides, stabilizes the axial tilt and has gradually modified the length of the planet's rotation period. A cometary bombardment during the early history of the planet played a role in the formation of the oceans. Later, asteroid impacts caused significant changes to the surface environment. Long term periodic changes in the orbit of the planet are believed to have caused the ice ages that have covered significant portions of the surface in glacial sheets. Based on the available evidence, current scientists have been able to reconstruct detailed information about the planet's past. Earth formed 4.57 billion years ago out of the solar nebula, along with the Sun and the other planets. Initially molten, the outer layer of the planet cooled to form a solid crust when water began accumulating in the atmosphere. The moon formed soon afterwards, possibly as the result of a Mars-sized object with about 10% of the Earth's mass, known as Theia, impacting the Earth in a glancing blow. Some of this object's mass merged with the Earth and a portion was ejected into space, but enough material survived to form an orbiting Moon. Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered by comets, produced the oceans. The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later, the last common ancestor of all life existed. The development of photosynthesis allowed the sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and gave rise to the ozone layer. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes. True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth. Over hundreds of millions of years, continents formed and broke up as the surface of Earth continually reshaped itself. The continents have migrated across the surface of the Earth, occasionally combining to form a supercontinent. Roughly 750 million years ago (mya), the earliest known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia, 600-540 mya, then finally Pangaea, which broke apart 180 mya. Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate. Following the Cambrian explosion, about 535 mya, there have been five mass extinctions. The last extinction event occurred 65 mya, when a meteorite collision probably triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the past 65 mya, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright. This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had, affecting both the nature and quantity of other life forms. The present pattern of ice ages began about 40 mya, then intensified during the Pleistocene about 3 mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40-100,000 years. The last ice age ended 10,000 years ago.



Composition and structure of Planet Earth:
The Earth's shape is very close to an oblate spheroid a rounded shape with a bulge around the equator although the precise shape (the geoid) varies from this by up to 100 metres (327 ft). The average diameter of the reference spheroid is about 12,742 km (7,913 mi). More approximately the distance is 40,000 km/p because the metre was originally defined as 1/10,000,000 of the distance from the equator to the north pole through Paris, France. The rotation of the Earth creates the equatorial bulge so that the equatorial diameter is 43 km (27 mi) larger than the pole to pole diameter. The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m [29,028 ft] above local sea level) and the Mariana Trench (10,911 m [35,798 ft] below local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%. For comparison, this is less than the 0.22% tolerance allowed in billiard balls. Because of the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador. Chemical composition The mass of the Earth is approximately 5.98 ×1024 kg. It is composed mostly of iron (32.1%), oxygen (30.1%), magnesium (13.9%), aluminum (1.4%), silicon (15.1%), sulfur (2.9%), calcium (1.5%), and nickel (1.8%), with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.

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Earth's atmosphere:
The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 6 km. It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The atmosphere protects the Earth's life forms by absorbing ultraviolet solar radiation, moderating temperature, transporting water vapor, and providing useful gases.

Orbit and rotation
An animation showing the rotation of the Earth.Relative to the background stars, it takes the Earth, on average, 23 hours, 56 minutes and 4.091 seconds (one sidereal day) to rotate around the axis that connects the north and the south poles. From Earth, the main apparent motion of celestial bodies in the sky (except that of meteors within the atmosphere and low-orbiting satellites) is to the west at a rate of 15°/h = 15'/min. This is equivalent to an apparent diameter of the Sun or Moon every two minutes. Earth orbits the Sun at an average distance of about 150 million kilometres (93.2 million miles) every 365.2564 mean solar days (1 sidereal year). From Earth, this gives an apparent movement of the Sun with respect to the stars at a rate of about 1°/day (or a Sun or Moon diameter every 12 hours) eastward. Because of this motion, on average it takes 24 hours a solar day for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 30 km/s (108,000 km/h or 67,000 mi/h), which is enough to cover the planet's diameter (~12,600 km or ~7,800 mi) in seven minutes, and the distance to the Moon (384,000 km or 238,000 mi) in four hours. Earth seen as a tiny dot by the Voyager 1 spacecraft, four billion miles from EarthThe Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth Moon system's common revolution around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from Earth's north pole, the motion of Earth, its moon and their axial rotations are all counterclockwise. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.5 degrees against the Earth Sun plane (which causes the seasons); and the Earth–Moon plane is tilted about 5 degrees against the Earth-Sun plane (without a tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses). The axial tilt of the Earth causes the seasons. By astronomical convention, the four seasons are determined by the solstices the point in the orbit of maximum axial tilt toward or away from the Sun and the equinoxes, when the tilt is minimized. Winter solstice occurs on about December 21, summer solstice is near June 21, spring equinox is around March 20 and autumnal equinox is about September 23. In an inertial reference frame, the Earth's axis undergoes a slow precession with a period of some 25,800 years, as well as a nutation with a main period of 18.6 years. These motions are caused by the differential attraction of Sun and Moon on the Earth's equatorial bulge because of its oblateness. In a reference frame attached to the solid body of the Earth, its rotation is also slightly irregular from polar motion. The polar motion is quasi-periodic, containing an annual component and a component with a 14-month period called the Chandler wobble. In addition, the rotational velocity varies, in a phenomenon known as length of day variation. In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4. For other eras, see precession and Milankovitch cycles. Coincidentally, the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun. This results in a ~6.9% increase in solar energy reaching the southern hemisphere at perihelion. However, this effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere. The Hill sphere (gravitational sphere of influence) of the Earth is about 1.5 Gm (930,000 miles) in radius. This is maximum distance at which the Earth's gravitational influence becomes stronger than the more distant Sun and planets.



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