RIGHT ASCENSION AND DECLINATION.
If the telescope be fixed at right angles to the earth's axis, its directional line will trace in the course of one rotation of the earth a circle, called the celestial equator, on the imaginary celestial sphere. The first point of Aries, one of the equinoxes, lies on this equator. All measurements are referred to this point, and since the earth's time of rotation is constant, it reappears in the fixed tele scope at regular intervals, termed sidereal days. A sidereal day is slightly less than a solar day, so that this system of time measurement would not do for ordinary purposes; all astronomical clocks, how ever, keep sidereal time. After an in terval of one sidereal hour from the passage of the first point of Aries over the cross-wires, the telescope will be directed towards a point on the celestial equator of 360° to the left of that reference point. Evidently, the celestial equator may be divided either into degrees or sidereal hours, the latter system of division always giving the time since the transit of the first point of Aries.
If the telescope be rotated round an axis parallel to that of the earth, through 15° to the right, the reference point will again come into view. Angular distance like this 15°, measured along or parallel to the celestial equator, is termed Eight Ascension, and is only measured in one direction in sidereal hours, from the half of the great circle which passes through the two celestial poles and the first point of Aries. This is the circle which would be traced by the directional line of the telescope when rotated in a vertical plane containing the equinoxes. The circles of Right Ascension get smaller nearer the celestial poles. Angular distance measured vertically above or below the celestial equator is termed Declination, and is expressed in degrees. In the diagram (Fig. 803) the star at a has a Right Ascension denoted by y 1,, whilst its Declination is b a and is positive ; y 5 and b a are each arcs of equal circles.