North Polar Sequence
A series of stars within 2° of the north celestial pole that were used to provide standard magnitudes and an arbitrary zero point for the magnitude scale. The original sequence (published 1914–22) included 96 stars with photographic magnitudes down to 20.1; it was extended by a supplementary list of 56 other stars. A North Polar Sequence of Photovisual Magnitudes down to 17th magnitude was subsequently introduced. The sequences have been superseded by the use of UBV photometry to establish the zero point, used in conjunction with lists of accurately measured standard stars.