SPHERICAL ASTRONOMY. 72 
The most important Points, Circles, and Terms of the Terrestrial Sphere. 
4. Spherical astronomy determines certain points and circles, as well on 
the terrestrial as on the celestial sphere. If, for instance, in fig. 12, C 
represent the centre of the earth, and NCS its axis of rotation, then, N,S, are 
the poles of the earth, QE the terrestrial equator, and AB the parallel of lati- 
tude of the place of observation, A, on the surface. Consequently the straight 
line, AP, parallel to SCN, is that direction in which the visible pole, P, of the 
heavens, is seen from the place of observation, A. The line AZ, a prolon- 
gation of a radius of the sphere, is the direction of the zenith from the observer 
at A. Furthermore, let NAES be the meridian of A; NGS, a fixed meri- 
dian, as the meridian of Paris; then, GH, or the spherical angle, GNE, is the 
(geographical) longitude of A, and EA, or the angle ECA, the (geographical) 
latitude. (For further particulars respecting geographical longitude and 
latitude, see Section 10.) Finally, if ns be a plane, tangent to the earth’s 
surface at A, it will constitute the visible apparent horizon of the place; 
and the straight line, nAs, produced by the intersection of this plane with 
the meridian, will be the meridian line of A, so that for A, x will be the 
north, and s the south pole of the horizon. 
Miscellaneous Considerations respecting the Apparent Rotation of the Celes- 
tial Sphere, and the attendant Phenomena. 
5. A careful examination of the phenomena exhibited in the apparent 
daily rotation of the starry heavens, shows that in respect to this rotation, 
the size of the earth may be considered as entirely insignificant, that is, the 
observer can be supposed to be situated in the centre of the earth; an 
assumption very allowable when we reflect on the immense distance of the 
fixed stars from the earth. Let pl. 6, fig. 13, represent the celestial sphere, 
i, the observer, Z, his zenith, N, his nadir, then will the circle, HwOeH 
(whose poles are N and Z), be the celestial horizon; Pp represent the 
poles of the heavens, the circle, HZONH, the meridian, and HP, the alti- 
tude of the pole for the observer at 7. This will be readily seen by referring 
to what was said on the subject in§ 2. The circle, EwQeH, perpendicular 
to the axis, Pp, will be the equator, in which the vernal equinox occurs at 
v, Then, as already explained in fig. 2, the arc, "T (fig. 13), will be the 
right ascension, T'S the declination, and PS the polar distance of the star, 
S, projected in the equator by the declination circle, PSTp ; BD will also 
be the diurnal circle (parallel of declination) described by the star in its 
apparent motion about the pole, P. The circle, ZM, perpendicular to the 
horizon, is the vertical circle; the arc, HM, the azimuth; MS, the altitude, 
and ZS the zenith distance of the star, S. Finally, the points, H, w, O, e, 
are respectively the north, west, south, and east points of the horizon. 
If Hh and Oo represent small parallels of declination, touching the north 
point of the horizon at H, and the south point at O, then HA will be the 
circle of perpetual apparition, between which and the visible pole P the 
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