(81.) Several of the terms in use among astronomers have been explained in the preceding chapter, and others used anticipatively. But the technical language of every subject requires to be formally stated, both for consistency of usage and definiteness of conception. We shall therefore proceed, in the first place, to define a number of terms in perpetual use, having relation to the globe of the earth and the celestial sphere.

(82.) Definition 1. The axis of the earth is that diameter about which it revolves, with a uniform motion, from west to east; performing one revolution in the interval which elapses between any star leaving a certain point in the heavens, and returning to the same point again.

(83.) Def. 2. The poles of the earth are the points where its axis meets its surface. The North Pole is that nearest to Europe; the South Pole that most remote from it.

(84.) Def. 3. The earth's equator is a great circle on its surface, equidistant from its poles, dividing it into two hemispheres—a northern and a southern; in the midst of which are situated the respective poles of the earth of those names. The plane of the equator is, therefore, a plane perpendicular to the earth's axis, and passing through its centre.

(85.) Def. 4. The terrestrial meridian of a station on the earth's surface, is a great circle of the globe passing through both poles and through the plane. The plane of the meridian is the plane in which that circle lies.

(86.) Def. 5. The sensible and the rational horizon of any station have been already defined in art. 74.