Blue moon

From Neil deGrasse Tyson (via his facebook feed):

The average time the Moon takes to complete a cycle of phases is 29.5 days. So all months but February can host a second full Moon, if the timing is right. We call these "Blue Moons" and we get one on Friday, August 31. Chances of a full Moon on the first of a month is about 1 in 30, so you'd expect a Blue Moon every 30 months or so -- about once every 2.5 years. Not rare. So when the cosmically literate want to reference something that's uncommon, they **never** say "Once in a Blue Moon". -NDTyson