ASCENSION, RIGHT (Lat. aSCCPSiO, an aris ing; Ger. gerade Aufsteigung). The name given in astronomy to one of the factors which deter mine the position of a heavenly body in the sky, the other being the declination. The vault of the sky is to be regarded as divided into a northern and southern hemisphere by the celestial equator, just as the earth is divided by the terrestrial equator. The location of a heavenly body is then defined by means of right ascension and declina tion, just as that of a city on the earth is fully known from its longitude and latitude. Right ascension is thus quite analogous to longitude on the earth. The initial point for measuring right ascension is one of the points of the celes tial equator where it is intersected by the celip tic (q.v.). and this point is called the vernal equinox, or first point of a Aries (q.v.). The right ascension of a heavenly body is then de fined as the angular distance measured on the celestial equator from the vernal equinox to the foot of a perpendicular circle let fall on the equator from the heavenly body. The right
ascension is ascertained by means of the transit instrument and clock. The transit instrument enables us to observe its meridian passage, and the transit clock gives the time at which this takes place. This clock is set so that when the first point of Aries is in the meridian it stands at 0 hours, 0 minutes, and 0 seconds, and it is so arranged as to indicate 24 sidereal hours, the time that elapses between two successive passages of that point. The reading of the clock at the passage of any heavenly body then enables us to ealeulate that body's right ascension. The right ascension of all the fixed stars down to the ninth nmgnitude has been thus deter mined and published in printed star catalogues.