If Andromeda is overhead at midnight, what is the right ascension of the sun? (andromeda is ar RA =1hr, Dec = +40).

I thought I understood what Dec and RA were, but I can't think of how to start this question?


  • $\begingroup$ Think about what the RA is of something completely opposite in the sky of an object. $\endgroup$ – rgettman Oct 23 '14 at 21:14
  • $\begingroup$ It really depends on what is the definition of midnight you're using (i.e. the time system: there's sidereal time, solar time, local mean time, universal, standard, GMT etc.), and what is your definition of "overhead". In more precise terms, exactly overhead should mean crossing the zenith, but you could also have meant just crossing the meridian. $\endgroup$ – SkydivingPanda Feb 21 '15 at 16:02

If you mean solar time midnight than the answer is simple (as commented by @rgettman). The Sun should then be at the opposite side of the sky at $\alpha = 13^{\textrm{h}}$, where $\alpha$ is the right ascension. There is a problem, however, when you use local time (using a timezone). If, for instance, you mean midnight GMT in Liverpool, which is at 3° west then solar midnight will happen at 12 minutes past midnight GMT and the Sun would have $\alpha \approx 13^{\textrm{h}}12^\prime$.


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