Ascension, RIGHT

Ascension, RIGHT, the name given in astronomy to one of the arcs which determine the position relatively to the equator of a heavenly body on the celestial sphere, the other being the declination (see ARMILLARY SPHERE). It meant originally the difference of time of rising of the first point of Aries (q.v.) and the heavenly body referred to, on a right sphere. Hence it is called Right Ascension. The sphere of the heavens would be right if the poles were on the horizon, as is the case at the earth's equator. At all other places, the axis of the celestial sphere is oblique—i.e. inclined to the horizon—so that the right ascension of a star gives no direct knowledge of its rising time except there. The term right ascension has thus passed into general use as meaning simply the arc of the equator intercepted between the first point of Aries, and the point at which the circle of declination passing through the star cuts the equator. Measured always from west to east, right ascension on the heavens corresponds to longitude on the earth. The right ascension of a heavenly body is ascertained by means of the transit instrument and clock. The transit instrument determines its meridian passage, and the transit clock gives the time at which this takes place. When the first point of Aries is in the meridian, the clock stands at 0 hours, 0 minutes, 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, therefore, at the passage of any heavenly body gives its right ascension in time, and this, when multiplied by 15, gives the same in degrees, minutes, and seconds. The right ascension is usually given, however, in time. The old term, oblique ascension, was given to the right ascension of the point of the equator that rose simultaneously with the heavenly body; and the difference of the oblique and right ascension was called the 'ascensional difference.'