Right Ascension = hours measured along the celestial equator, starting from the March equinox (intersection of the ecliptic with the celestial equator).
- A measurement along the celestial equator is called Right Ascension (RA).
- The units of right ascension are hours, since the celestial equator is divided into 24 equal portions.
- Each hour of Right Ascension is divided into 60 equal minutes. For now, think of these units as measures of length, or distance along the celestial equator, not time.
- By convention, the starting point, or 0 hours of right ascension, is a point on the celestial equator called the vernal or March equinox.
Use a celestial globe to determine the Right Ascension (RA) of the following (remember to indicate units of hours and minutes):
- Mintaka, the top star in Orion's belt
- Altair, in Aquila the Eagle
- The pointer stars of the Big Dipper
- Beta-Cassiopeiae, one of the outside stars of Cassiopeia. Cassiopeia is a double-lobed W shape, and this star is the outside end of the deepest lobe of the W.
Why would it be meaningless to specify a Right Ascension for the north celestial pole?