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?
- Sidereal Time (ST; used to know when to
look for comets, planets, constellations, deep sky objects, etc.)