| the arc of the celestial equator measured eastward from the vernal equinox to the foot of the great circle passing through the celestial poles and a given point on the celestial sphere, expressed in degrees or hours. |
| right ascension n. Abbr. RA The angular distance of a celestial body or point on the celestial sphere, measured eastward from the vernal equinox along the celestial equator to the hour circle of the body or point and expressed in degrees or hours. |
right ascension
in astronomy, the east-west coordinate by which the position of a celestial body is ordinarily measured; more precisely, it is the angular distance of a body's hour circle east of the vernal equinox, measured along the celestial equator. It is often expressed in units of time rather than degrees of arc. Right ascension and declination define the position of a celestial object. The symbol for right ascension is alpha, the Greek letter a (alpha). See also hour angle.
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