Glossary

ablation The loss of mass from the surface of a meteoroid by vaporization or as molten droplets. albedo The ratio between the amount of light actually reflected and that which would be reflected by a uniformly scattering disk of the same size, both observed at opposition. aqueous Related to, containing, or dissolvable in water. asteroid Any of a host of rocky small bodies, about 1,000 km (600 miles) or less in diameter, that orbit the Sun. aubrites Easily crumbled stony meteorites made mostly of the magnesium silicate enstatite. basalt A hard, dense, dark volcanic rock composed chiefly of plagioclase, pyroxene, and olivine, which often has a glassy appearance. Bode's law An empirical rule giving the approximate distances of planets from the Sun. chondrites The most abundant type of stony meteorite. chondrules Small spherical bodies found in chondrites, which are believed to have once been freefloating molten droplets in the solar nebula. comet Any of a class of small celestial objects orbiting the Sun that develop diffuse gaseous envelopes and often long luminous tails when near the Sun.

Coriolis force An effect that causes material moving on a rotating planet to appear to be deflected to either the right or the left. eccentric Deviating from a circular path; having an elongated, elliptical orbit. flux tube A tube-shaped area of space that contains a strong magnetic field. fragmentation When an object breaks into pieces, usually as the result of a collision or an explosion. heliocentric Related to the Sun, the center of all solar system activity. homogeneous Of a similar nature, or having the same composition throughout. hyperbola A plane curve formed by the intersection of a plane with both halves of a right circular cone at an angle parallel to the axis of the cone.

incandescent Emitting visible light as the result of being heated. isotropic Identical in all directions. magnetosphere Region in the atmosphere where magnetic phenomena and the high atmospheric conductivity caused by ionization are important in determining the behaviour of charged particles. meteorid A relatively small stony or metallic natural object from space that sometimes enters Earth's atmosphere and heats to incandescence.

meteoritics The branch of astronomy dealing with meteorites. millibar A unit of pressure equal to one thousandth (10-3) of a bar, which is equal to one million (106) dynes per square centimeter. occulation The interruption of light as an object passes in front of a viewed object, as during an eclipse. perihelion The point of a celestial body's orbit that is nearest the sun. prograde Having an orbit that is the same as most other celestial bodies; in the solar system, the prograde orbit is counterclockwise. radiant Point in the sky from which a meteor shower appears to originate. radiometry A technique used to derive the diameter of an asteroid by balancing the infrared radiation

(heat) emitted by the asteroid with the solar radiation it absorbs. radionuclides Radioactive isotopes that decay at characteristic rates. regolith A layer of pulverized rock, produced by bombardment with meteorites, that is thought to cover asteroids. retrograde Moving in the opposite direction than is normal; in the solar system, the retrograde orbit is clockwise. synchronous Moving at the same rate, at the same time. triangulation Measuring the distance to an object by observations from two separate points. ureilites Grainy, dark grey or brown stony meteorites with carbon-rich veins.

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