a great circle on the celestial sphere that passes through the celestial poles and is therefore perpendicular to the celestial equator.

a great circle of the celestial sphere passing through the celestial poles and a celestial body or the vernal equinox. An hour circle moves with the body as the celestial sphere rotates, unlike the celestial meridian of a point that remains fixed.

An arc, in the Equatorial coordinate system, from the North Celestial Pole to the South Celestial Pole, of constant right ascension. Called an hour circle because right ascension is measured in time units. Analogous to a meridian of longitude, in the Terrestrial coordinate system. Also, in a telescope, a circle used to indicate the right ascension that the telescope is pointing at.