Right Ascension

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).
    1. The units of right ascension are hours, since the celestial equator is divided into 24 equal portions.
    2. 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.
    3. 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):

  1. Mintaka, the top star in Orion's belt
  2. Altair, in Aquila the Eagle
  3. The pointer stars of the Big Dipper
  4. 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?