1. a great circle on the celestial sphere passing through the zenith and perpendicular to the horizon;

The Collaborative International Dictionary of English v.0.48:

Vertical \Ver"ti*cal\, a. [Cf. F. vertical. See Vertex.] [1913 Webster] 1. Of or pertaining to the vertex; situated at the vertex, or highest point; directly overhead, or in the zenith; perpendicularly above one. [1913 Webster] Charity . . . is the vertical top of all religion. --Jer. Taylor. [1913 Webster] 2. Perpendicular to the plane of the horizon; upright; plumb; as, a vertical line. [1913 Webster] Vertical angle (Astron. & Geod.), an angle measured on a vertical circle, called an angle of elevation, or altitude, when reckoned from the horizon upward, and of depression when downward below the horizon. Vertical anthers (Bot.), such anthers as stand erect at the top of the filaments. Vertical circle (Astron.), an azimuth circle. See under Azimuth. Vertical drill, an upright drill. See under Upright. Vertical fire (Mil.), the fire, as of mortars, at high angles of elevation. Vertical leaves (Bot.), leaves which present their edges to the earth and the sky, and their faces to the horizon, as in the Australian species of Eucalyptus. Vertical limb, a graduated arc attached to an instrument, as a theodolite, for measuring vertical angles. Vertical line. (a) (Dialing) A line perpendicular to the horizon. (b) (Conic Sections) A right line drawn on the vertical plane, and passing through the vertex of the cone. (c) (Surv.) The direction of a plumb line; a line normal to the surface of still water. (d) (Geom., Drawing, etc.) A line parallel to the sides of a page or sheet, in distinction from a horizontal line parallel to the top or bottom. Vertical plane. (a) (Conic Sections) A plane passing through the vertex of a cone, and through its axis. (b) (Projections) Any plane which passes through a vertical line. (c) (Persp.) The plane passing through the point of sight, and perpendicular to the ground plane, and also to the picture. Vertical sash, a sash sliding up and down. Cf. French sash, under 3d Sash. Vertical steam engine, a steam engine having the crank shaft vertically above or below a vertical cylinder. [1913 Webster]The Collaborative International Dictionary of English v.0.48:

Azimuth \Az"i*muth\, n. [OE. azimut, F. azimut, fr. Ar. as-sum?t, pl. of as-samt a way, or perh., a point of the horizon and a circle extending to it from the zenith, as being the Arabic article: cf. It. azzimutto, Pg. azimuth, and Ar. samt-al-r[=a]'s the vertex of the heaven. Cf. Zenith.] (Astron. & Geodesy) (a) The quadrant of an azimuth circle. (b) An arc of the horizon intercepted between the meridian of the place and a vertical circle passing through the center of any object; as, the azimuth of a star; the azimuth or bearing of a line surveying. [1913 Webster] Note: In trigonometrical surveying, it is customary to reckon the azimuth of a line from the south point of the horizon around by the west from 0[deg] to 360[deg]. [1913 Webster] Azimuth circle, or Vertical circle, one of the great circles of the sphere intersecting each other in the zenith and nadir, and cutting the horizon at right angles. --Hutton. Azimuth compass, a compass resembling the mariner's compass, but having the card divided into degrees instead of rhumbs, and having vertical sights; used for taking the magnetic azimuth of a heavenly body, in order to find, by comparison with the true azimuth, the variation of the needle. Azimuth dial, a dial whose stile or gnomon is at right angles to the plane of the horizon. --Hutton. Magnetic azimuth, an arc of the horizon, intercepted between the vertical circle passing through any object and the magnetic meridian. This is found by observing the object with an azimuth compass. [1913 Webster]The Collaborative International Dictionary of English v.0.48:

Circle \Cir"cle\ (s[~e]r"k'l), n. [OE. cercle, F. cercle, fr. L. circulus (Whence also AS. circul), dim. of circus circle, akin to Gr. kri`kos, ki`rkos, circle, ring. Cf. Circus, Circum-.] [1913 Webster] 1. A plane figure, bounded by a single curve line called its circumference, every part of which is equally distant from a point within it, called the center. [1913 Webster] 2. The line that bounds such a figure; a circumference; a ring. [1913 Webster] 3. (Astron.) An instrument of observation, the graduated limb of which consists of an entire circle. [1913 Webster] Note: When it is fixed to a wall in an observatory, it is called a mural circle; when mounted with a telescope on an axis and in Y's, in the plane of the meridian, a meridian circle or transit circle; when involving the principle of reflection, like the sextant, a reflecting circle; and when that of repeating an angle several times continuously along the graduated limb, a repeating circle. [1913 Webster] 4. A round body; a sphere; an orb. [1913 Webster] It is he that sitteth upon the circle of the earth. --Is. xi. 22. [1913 Webster] 5. Compass; circuit; inclosure. [1913 Webster] In the circle of this forest. --Shak. [1913 Webster] 6. A company assembled, or conceived to assemble, about a central point of interest, or bound by a common tie; a class or division of society; a coterie; a set. [1913 Webster] As his name gradually became known, the circle of his acquaintance widened. --Macaulay. [1913 Webster] 7. A circular group of persons; a ring. [1913 Webster] 8. A series ending where it begins, and repeating itself. [1913 Webster] Thus in a circle runs the peasant's pain. --Dryden. [1913 Webster] 9. (Logic) A form of argument in which two or more unproved statements are used to prove each other; inconclusive reasoning. [1913 Webster] That heavy bodies descend by gravity; and, again, that gravity is a quality whereby a heavy body descends, is an impertinent circle and teaches nothing. --Glanvill. [1913 Webster] 10. Indirect form of words; circumlocution. [R.] [1913 Webster] Has he given the lie, In circle, or oblique, or semicircle. --J. Fletcher. [1913 Webster] 11. A territorial division or district. [1913 Webster] Note: The Circles of the Holy Roman Empire, ten in number, were those principalities or provinces which had seats in the German Diet. [1913 Webster] Azimuth circle. See under Azimuth. Circle of altitude (Astron.), a circle parallel to the horizon, having its pole in the zenith; an almucantar. Circle of curvature. See Osculating circle of a curve (Below). Circle of declination. See under Declination. Circle of latitude. (a) (Astron.) A great circle perpendicular to the plane of the ecliptic, passing through its poles. (b) (Spherical Projection) A small circle of the sphere whose plane is perpendicular to the axis. Circles of longitude, lesser circles parallel to the ecliptic, diminishing as they recede from it. Circle of perpetual apparition, at any given place, the boundary of that space around the elevated pole, within which the stars never set. Its distance from the pole is equal to the latitude of the place. Circle of perpetual occultation, at any given place, the boundary of the space around the depressed pole, within which the stars never rise. Circle of the sphere, a circle upon the surface of the sphere, called a great circle when its plane passes through the center of the sphere; in all other cases, a small circle. Diurnal circle. See under Diurnal. Dress circle, a gallery in a theater, generally the one containing the prominent and more expensive seats. Druidical circles (Eng. Antiq.), a popular name for certain ancient inclosures formed by rude stones circularly arranged, as at Stonehenge, near Salisbury. Family circle, a gallery in a theater, usually one containing inexpensive seats. Horary circles (Dialing), the lines on dials which show the hours. Osculating circle of a curve (Geom.), the circle which touches the curve at some point in the curve, and close to the point more nearly coincides with the curve than any other circle. This circle is used as a measure of the curvature of the curve at the point, and hence is called circle of curvature. Pitch circle. See under Pitch. Vertical circle, an azimuth circle. Voltaic circuit or Voltaic circle. See under Circuit. To square the circle. See under Square. Syn: Ring; circlet; compass; circuit; inclosure. [1913 Webster]WordNet (r) 3.0 (2006):

vertical circle n 1: a great circle on the celestial sphere passing through the zenith and perpendicular to the horizon