Glossary

accretion Growth by gradual buildup. annulus A ringlike structure or marking.

barycentre The center of mass of two or more bodies that orbit around each other. chromosphere An incandescent, transparent layer of gas that surrounds a star. coalesce To grow together to form a united whole. convection Heat transfer, via liquid or gas, from one region to another. dipole A pair of electric charges or magnetic poles that are of equal magnitude but opposite sign or polarity, separated by a distance. ecliptic plane The plane of Earth's orbit. escarpment A steep slope that results from erosion or faulting, separating two relatively level areas of differing elevations. faculae Large bright spots on the Sun's photosphere, usually near sunspots.

grabens Depressions on a planet's surface that are bounded by fault lines. interstellar Between or among the stars.

lithosphere Where Earth's uppermost mantle and the crust act mechanically as a single rigid layer. lobate Having curved or scalloped edges. magnetosphere A region surrounding Earth in which charged particles are trapped and their behavior is dominated by the planet's magnetic field. mascons Areas where particularly dense lavas rose up from the mantle and flooded into basins. micrometeorites A tiny particle of meteoric dust, especially one that falls to the surface of Earth or the Moon. nebula A mass of interstellar dust or gas, visible as luminous patches or areas of darkness, depending on the way radiation is absorbed. penumbra The grayish outer portion of a sunspot or the outer part of the shadow cast by Earth or the Moon during an eclipse. photosurface The portion of the Sun seen in ordinary light; face of the Sun as viewed from Earth. planetesimals Small bodies thought to have orbited the sun during the formation of the planets. precess Move in a gyrating fashion. prograde Direct motion of the planets around the Sun. regolith An insulating debris layer made of dust and rock fragments ejected from distant impacts on a planet's surface. retrograde Moving backward or in an order that is opposite the usual. solar constant The average density of solar radiation measured outside Earth's atmosphere and at Earth's mean distance from the Sun.

spicules Streams of plasma that appear in the Sun's chromosphere. subduction A geologic process in which one edge of one crustal plate is forced below the edge of another. superior conjunction The position of a celestial body when it is on the opposite side of the Sun from Earth. tenuous Long and thin.

tessarae Geologic regions of Venus that resemble mosaic tiles. transit Event when a small astronomical body passes in front of a larger astronomical body. tropopause The topmost portion of the troposphere. troposphere The region on Earth where nearly all water vapour exists and essentially all weather occurs. vulcanoids A hypothetical remnant population of asteroid-sized objects orbiting the Sun inside Mercury's orbit.

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