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Earth Radius
Earth
Earth
radius is the approximate distance from Earth's center to its surface, about 6,371 km (3,959 mi). This distance is used as a unit of length, especially in astronomy and geophysics, where it is usually denoted by R⊕. Strictly speaking, the term "radius" is a property of a true sphere. Since Earth
Earth
is only approximately spherical, no single value serves as its definitive radius. Meaningful values range from 6,353 to 6,384 kilometres (3,948 to 3,967 mi). A distance from the center of Earth
Earth
to some point on its surface might be referred to as Earth’s radius at that point. More commonly, Earth radius means a computed average of distances to the surface or to some idealized surface
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Topography
Topography
Topography
is the study of the shape and features of the surface of the Earth
Earth
and other observable astronomical objects including planets, moons, and asteroids. The topography of an area could refer to the surface shapes and features themselves, or a description (especially their depiction in maps). This field of geoscience and planetary science is concerned with local detail in general, including not only relief but also natural and artificial features, and even local history and culture. This meaning is less common in the United States, where topographic maps with elevation contours have made "topography" synonymous with relief. The older sense of topography as the study of place still has currency in Europe. Topography
Topography
in a narrow sense involves the recording of relief or terrain, the three-dimensional quality of the surface, and the identification of specific landforms
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South Pole
Coordinates: 90°S 180°E / 90°S 180°E / -90; 180The Geographic South Pole. (The flag used on the flagpole is interchangeable.)Image taken by NASA
NASA
showing Antarctica
Antarctica
and the South Pole
South Pole
in 2005.South Geographic Pole South Magnetic Pole
South Magnetic Pole
(2007) South Geomagnetic Pole
South Geomagnetic Pole
(2005) South Pole
South Pole
of InaccessibilityThe South Pole, also known as the Geographic South Pole
South Pole
or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface
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Tidal Force
The tidal force is an apparent force that stretches a body towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for the diverse phenomena, including tides, tidal locking, breaking apart of celestial bodies and formation of ring systems within Roche limit, and in extreme cases, spaghettification of objects. It arises because the gravitational force exerted on one body by another is not constant across its parts: the nearest side is attracted more strongly than the farthest side. It is this difference that causes a body to get stretched
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De Sphaera Mundi
De sphaera mundi
De sphaera mundi
( Latin
Latin
title meaning On the Sphere of the World, sometimes rendered The Sphere of the Cosmos; the Latin
Latin
title is also given as Tractatus de sphaera, Textus de sphaera, or simply De sphaera) is a medieval introduction to the basic elements of astronomy written by Johannes de Sacrobosco
Johannes de Sacrobosco
(John of Holywood) c. 1230. Based heavily on Ptolemy's Almagest, and drawing additional ideas from Islamic astronomy, it was one of the most influential works of pre-Copernican astronomy in Europe.Contents1 Reception 2 Content2.1 The universe as a machine 2.2 Spherical Earth3 References 4 Sources 5 External linksReception[edit] Sacrobosco's De sphaera mundi
De sphaera mundi
was the most successful of several competing thirteenth-century textbooks on this topic
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Geoid
The geoid is the shape that the surface of the oceans would take under the influence of Earth's gravity and rotation alone, in the absence of other influences such as winds and tides. This surface is extended through the continents (such as with very narrow hypothetical canals). All points on a geoid surface have the same effective potential (the sum of gravitational potential energy and centrifugal potential energy). The geoid can be defined at any value of gravitational potential such as within the Earth's crust or far out in space, not just at sea level
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Mean Sea Level
Mean
Mean
sea level (MSL) (often shortened to sea level) is an average level of the surface of one or more of Earth's oceans from which heights such as elevations may be measured. MSL is a type of vertical datum – a standardised geodetic reference point – that is used, for example, as a chart datum in cartography and marine navigation, or, in aviation, as the standard sea level at which atmospheric pressure is measured to calibrate altitude and, consequently, aircraft flight levels. A common and relatively straightforward mean sea-level standard is the midpoint between a mean low and mean high tide at a particular location.[1] Sea
Sea
levels can be affected by many factors and are known to have varied greatly over geological time scales
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Planet
Shown in order from the Sun
Sun
and in true color. Sizes are not to scale.A planet is an astronomical body orbiting a star or stellar remnant thatis massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.[a][1][2]The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System
Solar System
can be seen with the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union
International Astronomical Union
(IAU) officially adopted a resolution defining planets within the Solar System
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Spheroid
A spheroid, or ellipsoid of revolution, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. If the ellipse is rotated about its major axis, the result is a prolate (elongated) spheroid, shaped like an American football
American football
or rugby ball. If the ellipse is rotated about its minor axis, the result is an oblate (flattened) spheroid, shaped like a lentil. If the generating ellipse is a circle, the result is a sphere. A spheroid has circular symmetry. Because of the combined effects of gravity and rotation, the shape of the Earth, and of all planets, is not quite a sphere but instead is slightly flattened in the direction of its axis of rotation. For that reason, in cartography the Earth
Earth
is often approximated by an oblate spheroid instead of a sphere
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Equator
An equator is the intersection of the surface of a rotating sphere (such as a planet) with the plane perpendicular to the axis of rotation and midway between its poles. On Earth, the Equator
Equator
is an imaginary line on the surface, equidistant from the North and South Poles, dividing the Earth
Earth
into Northern and Southern Hemispheres
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North Pole
Coordinates: 90°N 0°W / 90°N -0°E / 90; -0An azimuthal projection showing the Arctic Ocean
Arctic Ocean
and the North Pole. The map also shows the 75th parallel north
75th parallel north
and 60th parallel north. Sea ice
Sea ice
at the North Pole
North Pole
in 2006The North Pole, also known as the Geographic North Pole
North Pole
or Terrestrial North Pole, is (subject to the caveats explained below) defined as the point in the Northern Hemisphere
Northern Hemisphere
where the Earth's axis of rotation meets its surface. The North Pole
North Pole
is the northernmost point on the Earth, lying diametrically opposite the South Pole. It defines geodetic latitude 90° North, as well as the direction of true north
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Angular Frequency
In physics, angular frequency ω (also referred to by the terms angular speed, radial frequency, circular frequency, orbital frequency, radian frequency, and pulsatance) is a scalar measure of rotation rate. It refers to the angular displacement per unit time (e.g., in rotation) or the rate of change of the phase of a sinusoidal waveform (e.g., in oscillations and waves), or as the rate of change of the argument of the sine function. Angular frequency (or angular speed) is the magnitude of the vector quantity angular velocity
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Global Positioning System
The Global Positioning System
System
(GPS), originally Navstar GPS,[1] is a satellite-based radionavigation system owned by the United States government and operated by the United States
United States
Air Force.[2] It is a global navigation satellite system that provides geolocation and time information to a GPS receiver
GPS receiver
anywhere on or near the Earth
Earth
where there is an unobstructed line of sight to four or more GPS satellites.[3] Obstacles such as mountains and buildings block the relatively weak GPS
GPS
signals. The GPS
GPS
does not require the user to transmit any data, and it operates independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the GPS
GPS
positioning information
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Flattening
Flattening
Flattening
is a measure of the compression of a circle or sphere along a diameter to form an ellipse or an ellipsoid of revolution (spheroid) respectively. Other terms used are ellipticity, or oblateness. The usual notation for flattening is f and its definition in terms of the semi-axes of the resulting ellipse or ellipsoid is f l a t t e n i n g = f = a − b a . displaystyle mathrm flattening =f= frac a-b a . The compression factor is b/a in each case. For the ellipse, this factor is also the aspect ratio of the ellipse. There are two other variants of flattening (see below) and when it is necessary to avoid confusion the above flattening is called the first flattening
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Density
The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ (the lower case Greek letter rho), although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume:[1] ρ = m V displaystyle rho = frac m V where ρ is the density, m is the mass, and V is the volume. In some cases (for instance, in the United States oil and gas industry), density is loosely defined as its weight per unit volume,[2] although this is scientifically inaccurate – this quantity is more specifically called specific weight. For a pure substance the density has the same numerical value as its mass concentration. Different materials usually have different densities, and density may be relevant to buoyancy, purity and packaging
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Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet, dwarf planet, or natural satellite. It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distinguished based on its phase (solid crust vs. liquid mantle). The crusts of Earth, Moon, Mercury, Venus, Mars, Io, and other planetary bodies formed via igneous processes, and were later modified by erosion, impact cratering, volcanism, and sedimentation. Most terrestrial planets have fairly uniform crusts. Earth, however, has two distinct types: continental crust and oceanic crust
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