The azimuth of a body is its distance from the meridian of the place measured on the horizon, or it is the arch of the horizon intercepted between the meridian of the place, and the vertical circle passing through the body. It is call ed eastern or western azimuth, according as the body is on the east or west side of the meridian.
A body is said to rise, culminate, and set, when its centre is in the eastern side of the horizon, the meridian of the place, and the western side of the horizon respectively.
The eastern and western amplitude of a body is the arch of the horizon intercepted between the point where the bo dy rises or sets, and the east or west point of the horizon.
The zodiac is a zone or circular space of the celestial sphere, extending about 9° on each side of the ecliptic.
The preceding definitions will be rendered more intelli gible, perhaps, by referring to Plate CCLXV. Fig. I. where IIZRN represents the celestial sphere, and hzrn the earth, situated in its centre. Then if Z be the zenith of a place z, P and S the north and south poles of the world, and C the vernal equinox, 'IR will be the horizon, ,EQ the equi noctial, EL the ecliptic, and P' its pole, PS the equinoctial colure, HZRN the solstitial colure, TL the tropic of Can cer, and ED the tropic of Capricorn. Also, since the circle HZRN passes through Z, P and P', the poles of the hori zon, equinoctial and ecliptic, it represents a vertical circle, a meridian, and a circle of celestial longitude. if there fore, A be a given star, the arch RA will be its altitude, and ZA its zenith distance to an observer at z, or rather at the centre of the earth, QA will be its declination, LA its lati tude, CQ its right ascension, and CL its longitude. In this case, its azimuth is nothing, the star being on the meridian of the place. It is hardly necessary to observe, that though CL and CQ, as represented in the Figure, appear to be straight lines, they are nevertheless arches of great circles, the point C being the pole of the hemisphere HZRN. The same is true of all the other straight lines passing through the point C.
Having thus explained some of the principal points and lines, which geographers have imagined to be traced out on the celestial sphere, we shall briefly mention the differ ent classes and characters of the heavenly bodies them selves. These are all comprehended under three heads,
Fixed Stars, Planets, and Comets.
The fixed stars are those heavenly bodies, that have al ways the same situation relatively to one another, having no other motion than what they derive from the apparent diurnal revolution of the celestial sphere from east to west. In order to distinguish them more easily from one another, they are divided into classes, according to their brilliancy and apparent magnitude, the brightest and largest being de nominated stars of the first magnitude, the next largest of the second magnitude, and so on to the sixth. Stars less than those of the sixth magnitude, are not in general visible by the naked eye, and are called telescopic stars. Besides this classification, the fixed stars are also divided into groups called Constellations, each being named after some animal, or other object, to which the stai F., in their arrangement, bear a fancied resemblance. There are twelve such con stellations in the zodiac, and it is Irom them that the signs of the ecliptic take their names. The stars in each constel lation are distinguished by the letters of the Greek alpha bet, the first letter denoting the largest star of the constel lation, whatever may be its magnitude. Stars not included in any of the constellations, arc called Utj'ormed Stars.
The planets, as we have already observed, arc bodies which, besides their diurnal revolution, have also a motion of their own in a direction contrary to the other. They arc eleven in number, and arc found to revolve about the sun as a centre, in different.periods, and at different distances. These eleven arc denominated Primary Planets, to distin guish them from the Secondary or Satellites, with which some of them arc accompanied, and to which they serve as centres of revolution. The path which a planet describes about its centre is called its Orbit. The primary planets gxe written and expressed as follows : For the distances of the planets from the sun, the pe riods of their revolutions, magnitudes, &c. see AST EON() My Index.