2.O GL®enAPHY AND ASTRONOMY. As the word altitude is used to signify the height of the sun or star above the horizon, so the depression of the sun orstar is itsdistance from or below the horizon. The azimuthof the sun or star is its distancefrom any of the four cardinal points, east, west, north and south, measured by thedegrees of the horizon. Note, When we speak of the sun's azimuth in general, we visually mean his distance from the south ; but when his distance from the north, east, or west, is intended, we say, his azimuth from the north, the east, or the west. Great circles cutting every degreeof the horizon at right angles, and meeting in the zenith and nadir arecalled azimuthal or vertical circles. They direct theeye to the point of the sun or star's azimuthon the horizon, though the sun or star may be far above, or below the horizon. Note, vertical circles are the same with regard to the zenith, nadir,, and the horizon, as meridians or hour circles are with re- gard to the two poles of the world and the equator. But these vertical circles can never he actually drawn on a globe, because ,zenith, nadir, and horizon are ever variable. See them repre- sented figurevi. by the lines z u N, z a N, e N, z, &c. supposing n a to be the horizon. Note, 'rhe giladrant of altitudes being moveable when one end of it is fastened at' thezenith, the graduated edgeof it may he laid over the place of the sun or star, and brought down to the horizon ; then it represents any azimuth or vertical circle, in which the sun or star is ; and thus it shews the degreeof its azi- muth ou the horizon. Note, The azimuth of the sun or star from the east or west points of the horizon at its rising or setting, is called its amplitude. Note, The sun is always in the south at noon, or 12 o'clock, and in Cite north at midnight, viz. in Europe and all places on this side on the equator. Bet it is not at the east or west at six o'clock any other dap in the year besides the two equi- noctial days, as will easily oppear in an oblique position of the sphere, (of which see the next section) and especially in the last section where the analemma shall be more fully de- scribed. Yet the relation which the parallels of altitude bear to the vertical circles, and which these vertical or azimuthal cir- cles, bear to the meridians or hour circles, may be respresented to theeye in figure vi. andvii. In figure vi. suppose the outermost circle to be the meridian, n a, thehorizon, z the zenith, N the nadir ; then d b, f g, k m, will beparallels of altitude: and z a N, z e N, z o N, z c N, &c. will be vertical circles, or circles of azimuth crossing the others at right angles.
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