ARMIL'LARY SPHERE (armilla, a ring), an instrument intended to give a just con ception of the constitution of the heavens, and of the motions of the heavenly bodies, as seen by an observer on the earth. It consists of a number of rings fixed together so as to represent the principal circles of the celestial sphere, and these are movable round the polar axis within a meridian and horizon, as in the ordinary celestial globe. It was by means of such rings furnished with sights that Ilipparchus, Ptolemy, and other an cient astronomers made many of their observations, and we find even Tycho Brahe mak ing most of his planetary observations with the help of such an instrument. The A. S. is, however, now only used as an aid to instruction in astronomy, and in this respect is generally supplanted by the celestial globe. The object aimed at in the A. S. will be better understood by reference to the celestial globe represented in Sup posing the observer on the earth to be in the center of the sphere, the earth on which be stands shuts out from his view the lower half of the heavens, or the part lyiug below the horizon 1111. The hemisphere above him may be. regarded as divided into two equal portions, an eastern and a western, by the meridian 3LII, which passes through the pole P, and the zenith Z, of which the eastern z half is shown in the figure. The north pole is supposed to be elevated above e :..1#4221111111414AIIIIIIPIP"—.04111\k u . , 4 0 „. the horizon, and its elevation is meas red by the arc P, or the height above the n. point ; and the heavens ap pear to rotate round au axis P of which P is one extremity; the south pole, Q, the other extremity, being be low the horizon. The meridian .N1.3I,
0 and the horizon HH, are the only circles 1.
p ..*:0-41#4 which maintain a fixed position with ;C -7- regard to the observer. Of the other • -.4 4, NI leading celestial circles, the equator or , .
'' = vim - ' ' equinoctial LL, extending from the e.
to the w. point of the horizon, the trop - :(.5 ics of Cancer and Capricorn, respective ly BB and CC, and the arctic circle AA, • although rotating with the stars, main ' % tain the same position with regard to . the horizon ; while the ecliptic, KK, is constantly changingits inclination and position ,towards it, Circles which ex I tend from pole to pole, cutting the equator at right angles, are called circles of declination. The circle which passes through the vernal ecminox T . (see AnrEs), is denominated the equinoctial colure: and that passing through the summer solstice 0 (see SOLSTICE), the solstitial colure. The circles just named, together with the antarctic circle, are represented by corresponding rings in the A. S. If S be a star, the following are the names given to the arcs which determine its position with regard to these circles: V, right ascension: SV, declination; SP, polar distance; SZ, senith distance; XS, altitude; (XN +180°), azimuth reckoned from the south pole westward.