CIR'CUMPO'LAR STAR (Lat. eircum, around ± polus, pole, axis). Any star which, in the apparent daily revolution of the sky, does not pass below tire horizon of the observer: or, in familiar language, does not set. It will be remembered that the apparent daily motion of the stars is in reality the result of the actual rotation of the earth upon an axis which passes through the centre of the earth and a point in the sky. near the north or polar star, and that the lines in which the stars seem to move—called lines of diurnal motion—are the circumferences of circles perpendicular to this axis. If an ob server is at the equator. the axis lies in the oh server's horizon. the circles of diurnal motion are all perpendicular to the horizon, and all stars seem to rise and set. If the observer is at a dis tance from the equator—for example, in lati tude 10' N.—the northern end of the celestial axis is raised 10 degrees above the horizon, and any star which is within 10 degrees of the north pole of the sky will not pass below the horizon in its apparent motion about the pole. The
largest circle of the sky that may be drawn about the pole without passing below the hori zon of the observer is called the circle of per petual apparition. A similar circle drawn about the opposite pole. witlifmt mining above the horizon. is called the circle of perpetual occulta tion. and the stars N1 hill!! that circle are never visible to the observer. What to an observer at a given latitude in the Northern Hemisphere is a circle of perpetual apparition, will to an observer in the Southern Hemisphere at any point equally removed from the equator be a circle of per petual occultation: and vice versa. what to the former is a circle of perpetual will to the latter he a circle of perpetual apparition, the stars within it being southern circumpolar stars.