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 the altitude ...

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 both the bo...

Antarctic Circle, imaginary circle on the surface of the earth at 661/2°S lat., i.e., 231/2° north of the South Pole. It marks the southernmost point at which the sun can be seen at the winter...

Arctic Circle, imaginary circle on the surface of the earth at 661/2°N latitude, i.e., 231/2° south of the North Pole. It marks the northernmost point at which the sun can be seen at the winte...

Diurnal circle, 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 centered at the celestia...

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 had move...

Chord, in geometry, straight line segment both end points of which lie on the circumference of a circle or other curve; it is a segment of a secant. A chord passing through the center of a cir...

Celestial meridian, vertical circle passing through the north celestial pole and an observer's zenith. It is an axis in the altazimuth coordinate system.

Sphere, in geometry, the three-dimensional analogue of a circle. The term is applied to the spherical surface, every point of which is the same distance (the radius) from a certain fixed point...

Pi, in mathematics, the ratio of the circumference of a circle to its diameter. The symbol for pi is π. The ratio is the same for all circles and is approximately 3.1416. It is of great import...