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accretion The accumulation of celestial gas, dust, or smaller bodies by gravitational attraction into a larger body, such as a planet or an asteroid albedo The light reflected by an object as a fraction of the light shining on an object; mirrors have high albedo, while charcoal has low albedo anorthite A calcium-rich plagioclase mineral with compositional formula CaAl2Si2O8, significant for making up the majority of the rock anorthosite in the crust of the Moon anticyclone An area of increased atmospheric pressure relative to the surrounding pressure field in the atmosphere, resulting in circular flow in a clockwise direction north of the equator and in a counterclockwise direction to the south aphelion A distance; the farthest from the Sun an object travels in its orbit apogee As for aphelion but for any orbital system (not confined to the Sun)

apparent magnitude The brightness of a celestial object as it would appear from a given distance—the lower the number, the brighter the object atom The smallest quantity of an element that can take part in a chemical reaction; consists of a nucleus of protons and neutrons, surrounded by a cloud of electrons; each atom is about 10-10 meters in diameter, or one angstrom atomic number The number of protons in an atom's nucleus AU An AU is an astronomical unit, defined as the distance from the Sun to the Earth; approximately 93 million miles, or 150 million kilometers. For more information, refer to the units and measurements appendix basalt A generally dark-colored extrusive igneous rock most commonly created by melting a planet's mantle; its low silica content indicates that it has not been significantly altered on its passage to the planet's surface bolide An object falling into a planet's atmosphere, when a specific identification as a comet or asteroid cannot be made bow shock The area of compression in a flowing fluid when it strikes an object or another fluid flowing at another rate; for example, the bow of a boat and the water, or the magnetic field of a planet and the flowing solar wind breccia Material that has been shattered from grinding, as in a fault, or from impact, as by meteorites or other solar system bodies chondrite A class of meteorite thought to contain the most primitive material left from the solar nebula; named after their glassy, super-primitive inclusions called chondrules chondrule Rounded, glassy, and crystalline bodies incorporated into the more primitive of meteorites; thought to be the condensed droplets of the earliest solar system materials clinopyroxene A common mineral in the mantle and igneous rocks, with compositional formula ((Ca,Mg,Fe,Al)2(Si,Al)206) conjunction When the Sun is between the Earth and the planet or another body in question convection Material circulation upward and downward in a gravity field caused by horizontal gradients in density; an example is the hot, less dense bubbles that form at the bottom of a pot, rise, and are replaced by cooler, denser sinking material core The innermost material within a differentiated body such as a planet or the Sun

Coriolis force The effect of movement on a rotating sphere; movement in the Northern Hemisphere curves to the right, while movement in the Southern Hemisphere curves to the left craton The ancient, stable interior cores of the Earth's continents crust The outermost layer of most differentiated bodies, often consisting of the least dense products of volcanic events or other buoyant material cryovolcanism Non-silicate materials erupted from icy and gassy bodies in the cold outer solar system; for example, as suspected or seen on the moons Enceladus, Europa,Titan, and Triton cubewano Any large Kuiper belt object orbiting between about 41 AU and 48 AU but not controlled by orbital resonances with Neptune; the odd name is derived from 1992 QB, the first Kuiper belt object found cyclone An area in the atmosphere in which the pressures are lower than those of the surrounding region at the same level, resulting in circular motion in a counterclockwise direction north of the equator and in a clockwise direction to the south differential rotation Rotation at different rates at different latitudes, requiring a liquid or gassy body, such as the Sun or Jupiter differentiated body A spherical body that has a structure of concentric spherical layers, differing in terms of composition, heat, density, and/or motion; caused by gravitational separations and heating events such as planetary accretion dipole Two associated magnetic poles, one positive and one negative, creating a magnetic field direct (prograde) Rotation or orbit in the same direction as the Earth's, that is, counterclockwise when viewed from above its North Pole distributary River channels that branch from the main river channel, carrying flow away from the central channel; usually form fans of channels at a river's delta eccentricity The amount by which an ellipse differs from a circle ecliptic The imaginary plane that contains the Earth's orbit and from which the planes of other planets' orbits deviate slightly (Pluto the most, by 17 degrees); the ecliptic makes an angle of 7 degrees with the plane of the Sun's equator ejecta Material thrown out of the site of a crater by the force of the impactor element A family of atoms that all have the same number of positively charged particles in their nuclei (the center of the atom) ellipticity The amount by which a planet's shape deviates from a sphere equinox One of two points in a planet's orbit when day and night have the same length; vernal equinox occurs in Earth's spring and autumnal equinox in the fall exosphere The uppermost layer of a planet's atmosphere extrasolar Outside this solar system garnet The red, green, or purple mineral that contains the majority of the aluminum in the Earth's upper mantle; its compositional formula is ((Ca,Mg,Fe Mn)3(Al,Fe,Cr,Ti)2(SiO4)3) graben A low area longer than it is wide and bounded from adjoining higher areas by faults; caused by extension in the crust granite An intrusive igneous rock with high silica content and some minerals containing water; in this solar system thought to be found only on Earth half-life The time it takes for half a population of an unstable isotope to decay hydrogen burning The most basic process of nuclear fusion in the cores of stars that produces helium and radiation from hydrogen igneous rock Rock that was once hot enough to be completely molten impactor A generic term for the object striking and creating a crater in another body inclination As commonly used in planetary science, the angle between the plane of a planet's orbit and the plane of the ecliptic isotope Atoms with the same number of protons (and are therefore the same type of element) but different numbers of neutrons; may be stable or radioactive and occur in different relative abundances lander A spacecraft designed to land on another solar system object rather than flying by, orbiting, or entering the atmosphere and then burning up or crashing lithosphere The uppermost layer of a terrestrial planet consisting of stiff material that moves as one unit if there are plate tectonic forces and does not convect internally but transfers heat from the planet's interior through conduction magnetic moment The torque (turning force) exerted on a magnet when it is placed in a magnetic field magnetopause The surface between the magnetosheath and the magnetosphere of a planet magnetosheath The compressed, heated portion of the solar wind where it piles up against a planetary magnetic field magnetosphere The volume of a planet's magnetic field, shaped by the internal planetary source of the magnetism and by interactions with the solar wind magnitude See apparent magnitude mantle The spherical shell of a terrestrial planet between crust and core; thought to consist mainly of silicate minerals mass number The number of protons plus neutrons in an atom's nucleus mesosphere The atmospheric layer between the stratosphere and the thermosphere metamorphic rock Rock that has been changed from its original state by heat or pressure but was never liquid mid-ocean ridge The line of active volcanism in oceanic basins from which two oceanic plates are produced, one moving away from each side of the ridge; only exist on Earth mineral A naturally occurring inorganic substance having an orderly internal structure (usually crystalline) and characteristic chemical composition nucleus The center of the atom, consisting of protons (positively charged) and neutrons (no electric charge); tiny in volume but makes up almost all the mass of the atom nutation The slow wobble of a planet's rotation axis along a line of longitude, causing changes in the planet's obliquity obliquity The angle between a planet's equatorial plane to its orbit plane occultation The movement of one celestial body in front of another from a particular point of view; most commonly the movement of a planet in front of a star from the point of view of an Earth viewer olivine Also known as the gem peridot, the green mineral that makes up the majority of the upper mantle; its compositional formula is ((Mg, Fe)2SiO4) one-plate planet A planet with lithosphere that forms a continuous spherical shell around the whole planet, not breaking into plates or moving with tectonics; Mercury, Venus, and Mars are examples opposition When the Earth is between the Sun and the planet of interest orbital period The time required for an object to make a complete circuit along its orbit parent body The larger body that has been broken to produce smaller pieces; large bodies in the asteroid belt are thought to be the parent bodies of meteorites that fall to Earth today perigee As for perihelion but for any orbital system (not confined to the Sun)

perihelion A distance; the closest approach to the Sun made in an object's orbit planetesimal The small, condensed bodies that formed early in the solar system and presumably accreted to make the planets; probably resembled comets or asteroids plate tectonics The movement of lithospheric plates relative to each other, only known on Earth precession The movement of a planet's axis of rotation that causes the axis to change its direction of tilt, much as the direction of the axis of a toy top rotates as it slows prograde (direct) Rotates or orbits in the same direction the Earth does, that is, counterclockwise when viewed from above its North Pole protoplanetary disk The flattened nebular cloud before the planets accrete radioactive An atom prone to radiodecay radio-decay The conversion of an atom into a different atom or isotope through emission of energy or subatomic particles red, reddened A solar system body with a redder color in visible light, but more important, one that has increased albedo at low wavelengths (the "red" end of the spectrum) refractory An element that requires unusually high temperatures in order to melt or evaporate; compare to volatile relief (topographic relief) The shapes of the surface of land;

most especially the high parts such as hills or mountains resonance When the ratio of the orbital periods of two bodies is an integer; for example, if one moon orbits its planet once for every two times another moon orbits, the two are said to be in resonance retrograde Rotates or orbits in the opposite direction to Earth, that is, clockwise when viewed from above its North Pole Roche limit The radius around a given planet that a given satellite must be outside of in order to remain intact; within the Roche limit, the satellite's self-gravity will be overcome by gravitational tidal forces from the planet, and the satellite will be torn apart rock Material consisting of the aggregate of minerals sedimentary rock Rock made of mineral grains that were transported by water or air seismic waves Waves of energy propagating through a planet, caused by earthquakes or other impulsive forces, such as meteorite impacts and human-made explosions semimajor axis Half the widest diameter of an orbit semiminor axis Half the narrowest diameter of an orbit silicate A molecule, crystal, or compound made from the basic building block silica (SiO2); the Earth's mantle is made of silicates, while its core is made of metals spectrometer An instrument that separates electromagnetic radiation, such as light, into wavelengths, creating a spectrum stratosphere The layer of the atmosphere located between the troposphere and the mesosphere, characterized by a slight temperature increase and absence of clouds subduction Movement of one lithospheric plate beneath another subduction zone A compressive boundary between two lithos-pheric plates, where one plate (usually an oceanic plate) is sliding beneath the other and plunging at an angle into the mantle synchronous orbit radius The orbital radius at which the satellite's orbital period is equal to the rotational period of the planet; contrast with synchronous rotation synchronous rotation When the same face of a moon is always toward its planet, caused by the period of the moon's rotation about its axis being the same as the period of the moon's orbit around its planet; most moons rotate synchronously due to tidal locking tacholine The region in the Sun where differential rotation gives way to solid-body rotation, creating a shear zone and perhaps the body's magnetic field as well; is at the depth of about one-third of the Sun's radius terrestrial planet A planet similar to the Earth—rocky and metallic and in the inner solar system; includes Mercury, Venus, Earth, and Mars thermosphere The atmospheric layer between the mesosphere and the exosphere tidal locking The tidal (gravitational) pull between two closely orbiting bodies that causes the bodies to settle into stable orbits with the same faces toward each other at all times; this final stable state is called synchronous rotation tomography The technique of creating images of the interior of the Earth using the slightly different speeds of earthquake waves that have traveled along different paths through the Earth tropopause The point in the atmosphere of any planet where the temperature reaches a minimum; both above and below this height, temperatures rise troposphere The lower regions of a planetary atmosphere, where convection keeps the gas mixed, and there is a steady decrease in temperature with height above the surface viscosity A liquid's resistance to flowing; honey has higher viscosity than water visual magnitude The brightness of a celestial body as seen from Earth categorized on a numerical scale; the brightest star has magnitude —1.4 and the faintest visible star has magnitude 6; a decrease of one unit represents an increase in brightness by a factor of 2.512; system begun by Ptolemy in the second century b.c.e.; see also apparent magnitude volatile An element that moves into a liquid or gas state at relatively low temperatures; compare with refractory

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