Encyclopedia of Astrobiology

2015 Edition
| Editors: Muriel Gargaud, William M. Irvine, Ricardo Amils, Henderson James (Jim) CleavesII, Daniele L. Pinti, José Cernicharo Quintanilla, Daniel Rouan, Tilman Spohn, Stéphane Tirard, Michel Viso

Right Ascension

  • Daniel RouanEmail author
Reference work entry
DOI: https://doi.org/10.1007/978-3-662-44185-5_1376


The right ascension is one of the two coordinates of the equatorial system of reference, used by astronomers to locate a celestial object on the sky. It is the coordinate measuring the angular distance along the  celestial equator between that object and the “ Vernal point,” which is in first approximation a fixed point with respect to the stars. The Vernal point is the location on the sky where the Sun crosses the equatorial plane at the March equinox. Right ascension is measured in units of hours, minutes, and seconds, positively toward the east. Because of the  precession of the equinoxes, the Vernal point moves slowly with time, so that the coordinates (right ascension and  declination) must be associated with an indication of the date (the equinox) at which they are valid.

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© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  1. 1.LESIA, Observatoire Paris-Site de MeudonMeudonFrance