Comets are luminous bodies, which appear in the heavens only occasionally, and for a limited period, generally consist ing of a nucleus surrounded by a luminous vapour, some times shooting out into a long train or tail. Comets, be sides the diurnal 'evolution common to all the heavenly bodies, have also a motion peculiar to themselves. In some, this mbtion is in the same direction with that of the planets, and in others it is in the contrary direction. See AsTao NO MY and COMETS.
IF all the inequalities of the earth's surface were remov ed, it would appear to its inhabitants to be a circular plane, fixed in the centre of the celestial sphere. There are two deceptions, however, in this appearance. The earth is not a plane, neither is it at rest. Its true figure, as we have already shown, is spherical, and it is continually in motion, revolving both daily on its own axis, and annually in an or bit round the sun. It must be obvious, on a moment's re flection, that the apparent diurnal motion of the heavenly bodies may be produced, either by the revolution of the celestial sphere from east to west, or by the rotation of the earth on its axis from west to cast. Of the two explanations of the phenomenon, therefore, we arc hound to adopt that which is liable to fewest objections, and this will be found to be the rotation of the earth. The revolution of the hea vens, indeed, presents difficulties both physical and me chanical, which it is impossible to remove, while the other hypothesis is perfectly simple in itself, and in unison with the other phenomena of the universe. The same observa tions will apply, with perhaps still more force, to the an nual revolution of the earth round the sun, which is the true cause of the sun's apparnt motion in the heavens from west to cast. The earth, therefore, is to be ranked among the planets, revolting about the sun in a year from west to east, having the ecliptic for its orbit, and accompanied by the moon as a satellite or secondary planet. But though we have thus noticed the real motions of the earth, as the causes of the apparent revolution of the sun and the celestial sphere, we shall not, in the remaining part of the present article, always confine ourselves to this view of the subject.
In the solution of geographical problems, it is often much simpler to consider the apparent revolution of the celestial sphere, and the motion of the sun in the ecliptic, as real, than as produced by the combined motions of the earth, while the solution is the same in both cases. We shall
not hesitate, therefore, to assume the first of these, when ever by doing so we can render the subject more intelligi ble or concise.
It is obvious from Plate CCLAT. Fig. I. that the planes of all great circles of the celestial sphere form, by their intersections with the surface cif the earth, corresponding great circles on the latter. Thus!, r,zn,p s, x q, &c. on the earth, correspond to IIR, ZN, PS, /EQ, kc. in the hea vens. This transference of the circles from one sphere to the other, may be still more clearly understood, by con ceiving the celestial sphere uniformly contracted in its di mensions, without any change in the relative position of its parts, till it be just sufficient to cover the terrestrial sphere. By this method, the small as well as the great circles of the heavens may be transferred to the same positions on the earth ; thus 'FL would coincide with t 1, and ED with e d.
The points it and s on the earth, immediately under the poles of the world, are called the north and south poles of the earth ; ccq the equator or equinoctial ; hr or HR the rational horizon, to distinguish it from HIV, the limit of an observer's vision at z, and which is called the sensible hori zon ; fi a and // Z r n meridians, or circles of longitude ; and t 1, e c,parallels of latitude. In general, all parallels of de clination on the celestial sphere become parallels of lati tude on the earth, retaining however their proper or indi vidual names. Thus the tropic of Cancer in the celestial sphere, is a parallel of the declination 23° 28' to the north of the equinoctial, and on the earth it is a parallel of lati tude at the same distance from the equator.
Besides the terms already defined in the account of the celestial sphere, there are others peculiar to the earth, which require to be explained.
The first meridian of any country, in modern systems of geography, is the meridian passing through the capital of that country, from which the position of other meridians is determined. The ancients chose for their first meridian that of the Fortunate Isles, which they conceived to be the limit of the habitable world. In later times, the meridian passing through Ferro, one of the Canary Islands, and near ly the same with that of the ancients, was used as the first meridian by geographers of many countries; now eve ry nation generally reckons from the meriian of its own metropolis.