. Astronomy for high schools and colleges . all, and the star will therefore neither risenor set, but will simply perform an apparent diurnal revo-lution around the pole. Below the pole it will appear tomove from west to east, gradually rising up in the north-east and passing toward the west, above the pole. Thedirection of the motion is shown by the arrows on Fig. 6, THE DIURNAL MOTION. 19 The circle within which the stars neitlier rise nor set iscalled tJie circle of perpetual apparition. The radius ofthe circle of perpetual apparition is equal to the altitudeof the pole above the horizon. A
. Astronomy for high schools and colleges . all, and the star will therefore neither risenor set, but will simply perform an apparent diurnal revo-lution around the pole. Below the pole it will appear tomove from west to east, gradually rising up in the north-east and passing toward the west, above the pole. Thedirection of the motion is shown by the arrows on Fig. 6, THE DIURNAL MOTION. 19 The circle within which the stars neitlier rise nor set iscalled tJie circle of perpetual apparition. The radius ofthe circle of perpetual apparition is equal to the altitudeof the pole above the horizon. As a result of this apparent motion, each individualconstellation changes its configuration with respect to thehorizon, tliat part which is highest when the constellationis above the pole being lowest wlien below it. This isshown in Fig. 7, which represents a supposed constellation,at different times of the night, as it revolves around thepole. The same thing may be seen by simply turningFig. G around and viewing it with different sides NORTHFig. 7. If the polar distance of the star exceeds the altitude ofthe pole, it will dip below the horizon duriflg a part ofits diurnal coiuse, and will be longer below it the greaterits polar distance. A star whose polar distance is 90° lies on the celestialequator, and one half its diurnal motion is below and theother half above the horizon. The sun is in the celestialequator about March 21st and September 21st, of eachyear, so that at these times the days and nights are ofequal length. 20 ASTRONOMT. Looking farther south at the celestial sphere, we shallat length see stars which rise a little to the east of south,and set a little to the west, being above the horizon but ashort time. The south pole is as far below our horizon as the northpole is above it. Hence, stars near the south pole neverrise in our latitudes. The circle within which stars neverrise is called the circle of perpetual disappearance. The raeridian.—The plane of the
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