(Encyclopedia) circle, closed plane curve consisting of all points at a given distance from some fixed point, called the center. A circle is a conic section cut by a plane perpendicular to the axis? (Encyclopedia) hour circle, in astronomy, a secondary axis in the equatorial coordinate system. The hour circle of a celestial body is the great circle on the celestial sphere that passes through? (Encyclopedia) vertical circle, in astronomy, the great circle on the celestial sphere that passes from the observer's zenith through a given celestial body. In the altazimuth coordinate system? (Encyclopedia) Antarctic Circle, imaginary circle on the surface of the earth at 661?2S lat., i.e., 231?2 north of the South Pole. It marks the southernmost point at which the sun can be seen at? (Encyclopedia) Arctic Circle, imaginary circle on the surface of the earth at 661?2N latitude, i.e., 231?2 south of the North Pole. It marks the northernmost point at which the sun can be seen at? (Encyclopedia) meridian circle: see transit instrument. (Encyclopedia) Avebury Circle: see under Avebury. (Encyclopedia) squaring the circle: see geometric problems of antiquity. (Encyclopedia) diurnal circle d?rn?l, apparent path followed by a star due to the earth's rotation on its axis. The stars appear to move on the celestial sphere in concentric circular paths? (Encyclopedia) Knights of the Golden Circle, secret order of Southern sympathizers in the North during the Civil War. Its members were known as Copperheads. Dr. George W. L. Bickley, a Virginian who?