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Glossary

altitude

the angular height of a celestial object above the horizon, measured in degrees from 0° at the horizon to 90° at the zenith; one of the coordinates used in the horizon system, the other being azimuth


aphelion

point in a planet's, comet's, or asteroid's orbit around the sun when it is farthest from the sun


asteroid

celestial body that orbits the sun and is mostly rocky in nature; most are located in a region between Mars and Jupiter; various subgroups (based on orbits) have also been identified


azimuth

the angular measure along the horizon starting with 0° at North and measuring 90° at East, 180° at South, and 270° at West; one of the coordinates in the horizon system, the other being altitude


coma

a huge cloud of dust and gas surrounding the nucleus of a comet: the coma is formed when ices in the nucleus sublimate as the comet approaches perihelion


comet

celestial body that orbits the sun in an orbit that is more eccentric than the nearly circular orbits of the planets and consists mostly of various ices; these bodies consist of three major parts: the nucleus, coma and tail


declination

celestial coordinate similar to latitude and used in the equatorial system along with RA; measured from 0° at the celestial equator to ±90° at the celestial poles


ecliptic

the plane of the earth's orbit; the plane of the solar system; the apparent path of the sun through the sky


equatorial system

coordinates, similar to latitude and longitude, used to describe the position of stars, planets and other celestial objects;these coordinates are independent of the observer


horizon

the line between the sky and ground


horizon system

coordinates used to describe the position of a celestial object based on the observer


meteor, meteorite, meteoroid

A very tiny piece of dust, often the dust from a comet. While in space, it is a meteoroid. When it collides with the Earth's atmosphere, it is a meteor (aka shooting star). If the piece is large enough to survive and land, it is a meteorite.


nucleus

central core region of a comet believed to be composed primarily of various ices and dirt


orbit

the path of a celestial object around another object, ie, the planets' path around the sun or a moon's path around it's planet


perihelion

point in the orbit when a planet, comet, or asteroid, is closest to the sun; similarly, perigee is when the Moon or a manmade satellite is closest to the Earth


RA

right ascension, celestial coordinate similar to longitude and used in the equatorial system along with declination; measured in hours from 0 to 24; the 0 hour mark is based on the location of the Vernal Equinox


sublimate

change of state from solid to gas without becoming a liquid


tail

gas and dust that stream away from comet nucleus


zenith

the point in the sky directly overhead an observer


Updated: 30-Jul-2013