A circumpolar object (star, constellation, deep space object) is one that never sets below the horizon during the Earth’s daily rotation. Any point in the sky that has a declination (degrees of separation from the celestial equator) greater than the observer’s latitude (degrees of separation from the terrestrial equator) will never set below the horizon. If for some reason the Sun ceased to illuminate the sky, a circumpolar object could be seen to circle Polaris once in about every 24 hour period. Cepheus, for instance, is a circumpolar constellation that is oriented with the top of the house-like asterism nearest Polaris. In the September evening sky, Cepheus is high above Polaris, and the “house” appears to be upside down. Over the course of 24 hours, Cepheus will circle Polaris. In six hours, the “house” is horizontal with the top pointing west, in 12 hours it appears upright, but below Polaris, in 18 hours it appears vertical again, this time with the top pointing east. Six months from now in March, Cepheus’ orientation at dusk is the same upright appearance as Cepheus’ 12-hour position in September.
The circumpolar region of the celestial sphere is indicated on the annotated sky map below as the nearly circular region. Two points define this region, the north celestial pole, and the north point on the horizon. The north celestial pole is at the center of the region’s circular border, and all stars on the map circle around this central point as a result of the earth’s daily rotation about its axis. Stars near the pole make small tight circles around the pole, while those out near the drawn circle, but still within the circle, make wider sweeping circles. Those stars outside of the circle also circle the pole, but will dip below the the northern horizon, and are therefore not circumpolar stars.
© James R. Johnson, 2015.