Hour circle

Hour circle, in astronomy, any great circle (similar to longitude) on the celestial sphere that passes through the celestial poles—i.e., is perpendicular to the celestial equator. The declination of a celestial object is measured along its hour circle. The hour circle that at any moment is passing through an observer’s zenith is his meridian.

The term also refers to a scale on the mounting of an equatorial telescope, indicating the hour angle at which the telescope is aimed.