equatorial coordinate system

noun

A celestial coordinate system in which an object's position on the celestial sphere is described in terms of its north-south declination and east-west right ascension, measured relative to the celestial equator and vernal equinox, respectively.

THE AMERICAN HERITAGE® DICTIONARY OF THE ENGLISH LANGUAGE, FIFTH EDITION by the Editors of the American Heritage Dictionaries. Copyright © 2016, 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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The coordinate system in which a celestial object's position on the celestial sphere is described in terms of its declination and right ascension, measured with respect to the celestial equator. Declination and right ascension correspond directly to geographic latitude and longitude as projected outward onto the celestial sphere. Declination is measured in degrees north or south of the celestial equator, the same as geographic latitude, but right ascension is measured in hours, minutes, and seconds eastward along the celestial equator from the point of the vernal equinox. Because the celestial equator moves among the fixed stars with the precession of the Earth's poles, an object's declination and right ascension change gradually over time, and coordinates in the equatorial system must be specified for particular years. The equatorial system is the system most used in astronomy for describing the position of objects outside the solar system.

Compare altazimuth coordinate system