SPHERICAL ASTRONOMY. det 
other circles are evident from the figure without further explanation. The 
hour circle fastened to the north pole of the fixed meridian has a movable 
index, which, when fastened, revolves with the axis. The artificial sphere 
known as the celestial globe has the advantage over the armillary sphere in 
allowing the representation "of the stars; but ilie latter exhibits to the senses 
far more clearly the relation of the most important points and circles of the 
celestial sphere to the inclosed terrestrial globe. 
2. Fig. 2 gives likewise an explanatory representation of the most import- 
ant points of spherical astronomy. The circle EHZT is the fixed meridian 
or noon circle; if its surface represent the western hemisphere of the celes- 
tial globe, then HRT is half the horizon, H its north, R its west, and T its 
south point. Z is the zenzth, the visible highest point above the horizon, 
standing perpendicularly above the centre of the sphere, while N, the nadir, 
is the invisible lowest point below the horizon; the straight line ZN con- 
necting these points is called the axis of the horizon, and corresponds to the 
direction of the plummet. The arc ACQ represents the semi-equator, ECK 
the semi-ecliptic (path of the sun). The equator ACQ passes through the 
east and west points (R) of the horizon. The point C, where the ecliptic 
and equator intersect, is called the vernal equinox. ‘The spherical angle 
ACE, or KCQ, gives the amount of inclination of the ecliptic to the equator, 
that is, the obliquity of the ecliptic, 23° 27’, measured also by the are AE 
or QK. The visible point N, everywhere 90° distant from the equator 
ACQ, is the north pole, the invisible one N”, directly opposite, is its south 
pole; the visible point, P, distant from N about 23° 27’, and 90° distant from 
every point of the ecliptic, is the north pole of the ecliptic, P’ its corres- 
ponding and invisible south pole. 
Let S’ be the place of any star in the celestial sphere; and from the 
zenith Z, draw through the star 8’ the arc ZS’T’ of a great circle, perpendi- 
cular to the horizon HRT, then the circle of which ZS’T’ is only the fourth 
part is the vertical circle of the star 8’; and the arc T’S’ the altitude of this 
star, which is expressed in degrees, reckoning from the horizon ; finally the 
arc ZS’ is the zenith distance. In the horizon the altitude is 0° and the 
zenith distance 90°. while in the zenith the altitude is 90°, and the zenith 
distance 0°. The arc TT’ of the horizon lying between the meridian and 
the quadrant ZS’T” of the vertical circle passing through the star 8’ is called 
its azimuth, also measured by the spherical angle T’ZT. The azimuth is 
reckoned from 0° to 180°, positively from the south point T, eastwardly as 
far as the north point H, and negatively from T to H, in the opposite direc- 
tion by the west; as is very evident, the azimuth and altitude of a star 
completely fix its position in the celestial sphere with respect to the 
horizon. 
If from the north pole, N, of the equator an arc, NS’'Q’, be drawn through 
the star 8’, perpendicular to the equator, the circle of which NS’Q’ is the 
quadrant is called the declination circle of the star, and the arc Q'S’ the 
declination. This is called north or south as the star is north or south of 
the equator, consequently as it stands in the northern or southe:n hemi- 
sphere of the heavens. The declination is estimated in degrees from the 
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