SC002: p. 4

Term: right ascension (RA)

Right ascension is like longitude. Longitude says how far the city is east or west along the Earth's equator. The hour circles of right ascension make a clock-like pattern on the SC002. The map has been oriented so that a right ascension (RA) of 0^h is straight up. The rest of the hour circles increase "clock-wise": 6^h to the right, 12^h straight down, 18^h to the left, 24^h is the same as 0^h so it's back to straight up. The right ascension exactly at the north celestial pole can't be defined: any number seems as good as any other.

Notice that while longitude is usually measured in degrees, we have a new unit (^h or hours) to report RAs. One hour of right ascension (1^h) is 15°. Since 24x15°=360°, there are 24^h of right ascension around the celestial pole. The reason for this oddity is that the celestial sphere makes one full rotation (24^h of RA) in one day (24 hours of time). Thus the celestial sphere advances about 1^h of RA in an hour of time. Looking at the stars of north celestial pole region is like looking at a clock face, except the stars rotate about the pole in the opposite direction as the hands on a clock.