Hour Circle
in astronomy, one of the great circles passing through the poles of the sphere, and necessarily perpendicular to the equator. So called, because to note when the sun reaches each of these circles is a method of ascertaining the hour of the day . Hour circles are called also circles of declination. Also a small brass circle fixed to the north pole of a terrestrial globe , divided into 24 hours, and furnished with an index for pointing them out.