Douglas Parsons

Often times when someone wishes to express the rareness of an action or event they mention that it only occurs once in a blue moon.  What does this mean?  Where does this phrase even come from?  Do we perhaps have a second moon that comes into view only every once in a while?

This is not the case. There is only one moon in orbit around the Earth and only one moon within our orbital path around the sun.  So it seems that this phrase does not relate to another moon, but rather it relates to our own moon. 

NGC 891 is a spiral galaxy much like our own.  This galaxy can be seen through medium sized telescopes and appears to be a smear of light with a small dark band down the length of it.