The moment at which a celestial object lies on an observer’s meridian (the north–south line in the sky); also known as a transit or a meridian passage. Upper culmination and lower culmination refer to the instants of maximum and minimum altitude respectively for a circumpolar object. At upper culmination (also known as culmination above pole) it has an hour angle of 0h; at lower culmination (or culmination below pole) it is passing between the pole and the horizon, and has an hour angle of 12h. Lower culmination for non-circumpolar objects occurs below the horizon and is thus unobservable. When used without qualification, ‘culmination’ means upper culmination.