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( Armillary sphere)
An armillary sphere (variations are known as spherical astrolabe, armilla, or armil) is a model of the celestial sphere. The exterior parts of this machine are a compages of brass rings, which represent the principal circles of the heavens. In the north pole of the ecliptic is a nut b, to which is fixed one end of the quadrantal wire, and to the other end a small sun Y, which is carried round the ecliptic B—B, by turning the nut&_160; and in the south pole of the ecliptic is a pin d, on which is another quadrantal wire, with a small moon ? upon it, which may be moved round by hand&_160; but there is a particular contrivance for causing the moon to move in an orbit which crosses the ecliptic at an angle of 5? degrees, in to opposite points called the moon's nodes; and also for shifting these points backward in the ecliptic, as the moon's nodes shift in the heaven. Within these circular rings is a small terrestrial globe J, fixed on an axis K, which extends from the north and south poles of the globe at n and s, to those of the celestial sphere at N and S. On this axis is fixed the flat celestial meridian L L, which may be set directly over the meridian of any place on the globe, so as to keep over the same meridian upon it. This flat meridian is graduated the same way as the brass meridian of the common globe, and its use is much the same. To this globe is fitted the movable horizon M, so as to turn upon the two strong wires proceeding from its east and west points to the globe, and entering the globe at the opposite points off its equator, which is a movable brass ring set into the globe in a groove all around its equator. The globe may be turned by hand within this ring, so as to place any given meridian upon it, directly under the celestial meridian L. The horizon is divided into 360 degrees all around its outermost edge, within which are the points of the compass, for showing the amplitude of the sun and the moon, both in degrees and points. The celestial meridian L passes through two notches in the north and south points of the horizon, as in a common globe both here, if the globe be turned round, the horizon and meridian turn with it. At the south pole of the sphere is a circle of 25 hours, fixed to the rings, and on the axis is an index which goes round that circle, if the globe be turned round its axis.
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