right ascension

right ascension (plural right ascensions)

1. (astronomy) The angular distance east of the vernal point (“the solar zenith at the March equinox”); the celestial equivalent of longitude.
   • 1809, William Nicholson, “ASCENSION”, in The British Encyclopedia, or Dictionary of Arts and Sciences; […], volume I (A … B), London: Printed by C[harles] Whittingham, […]; for Longman, Hurst, Rees, and Orme, […], →OCLC, column 1:

     The sun's right ascension in time is useful to the practical astronomer in regular observatories, who adjusts his clock by sidereal time.

   • 1826, Nathaniel Bowditch, Jonathan Ingersoll Bowditch, American Practical Navigator, US Hydrographic Office, page 280:

     The right ascensions and declinations obtained by the preceding calculations, are the mean values, to which must be applied the corrections for the Nutation and Aberration Tables XLII. XLIII. in cases where great accuracy is required, as is now done in the Nautical Almanac for 24 of the brightest stars for 10 days in the year, and those numbers in the Nautical Almanac are to be preferred.

   • 1988, Peter Duffett-Smith, Practical Astronomy with Your Calculator, page 35,

     The hour-angle, H, and the right ascension, α, are related by the simple formula

     H = LST − α,

     where LST is the local sidereal time.

Along with declination, right ascension determines the direction of a point on the celestial sphere in the equatorial coordinate system.

It is alternatively defined as the ascension (point on the celestial equator which rises with a specified celestial object) as seen from any point on the Earth's equator, where the celestial equator intersects the horizon at a right angle.

• celestial coordinate system
• equatorial coordinate system
• hour angle
• oblique ascension