Introduction to Spherical Astronomy: p. 8

Term: hour circle

Diurnal circles are like lines of constant latitude on the Earth. Hour circles are like lines of constant longitude on Earth. Hour circles are great circles which run from the north celestial pole straight to the equator (which they cross at a right angle) and continue to the south celestial pole, and back around to the north celestial pole. Your meridian is part of an hour circle. Since the celestial sphere is rotating, moment-by-moment different hour circles are lined up with your meridian.