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 0h is straight up. The rest of the hour circles increase "clock-wise": 6h to the right, 12h straight down, 18h to the left, 24h is the same as 0h 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 (1h) is 15°. Since 24x15°=360°, there are 24h of right ascension around the celestial pole. The reason for this oddity is that the celestial sphere makes one full rotation (24h of RA) in one day (24 hours of time). Thus the celestial sphere advances about 1h 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.