blue moon: is define..??

hi blogs86. to add to what written, generally when we say once in a blue moon, we mean "rarely happens". my take on this (based on what i read from the net just seconds ago) is blue moon happens when in a month the 2nd moon appears. generally, it's possible to have two full moons in a month. the first full moon is called red moon. and the second full moon ("if" it happens) is tagged as a blue moon. thus, a rare phenomenon. another explanation i got is, if there's a volcanic eruption, the sulphur and other dust particles caught in the air give an illusion of color blue in the sky, making the moon appear color blue. this happened in Krakatoa in 1883. after this eruption, the moon looked blue for several days. since volcanoes don't erupt everyday, blue moon becomes (yet again) a rare phenomenon. =)

According to wikipedia blue moon is a name given to an irregularly timed full moon. Usually in a year there are twelve full moons which occur monthly, but each calendar year contains those twelve full lunar cycles. Wikipedia also added that the most literal meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event, thus, in the saying "once in a blue moon".