Blue moon is the second full moon in a month. This event isn’t very common (remember the phrase “Once in a blue moon,” meaning a rare event). It occurs every two to three years, seven times in a Metonic cycle. Because of the extra full moon in a year, there isn’t a total of 12 full moons, but a total of 13. The next blue moon will be on 31st August, 2012. When there is a blue moon in January, so-called “double” blue moon occurs. That’s because January is followed by a short month, February, which enables the second blue moon in March, April or May. This event happens every 19 years or so. The last “double” blue moon was on 31st March, 1999, with 31st January having the first blue moon. The next “double” blue moon will be on 31st March, 2018, with 31st January having the first blue moon.
While blue moon is the second full moon in a month, the black moon is the second new — also called dark — moon in a month. Black moon may not be as known as the blue moon, but it definitely isn’t less important. The next black moon will occur on 31st January, 2014. Just like “double” blue moon, “double” black moon happens around every 19 years. The next one will be on 30th March, 2014, with 31st January having the first dark moon.
P. S.: I’m very sorry for not posting in the last month, I was very busy. And I won’t be able to create much posts in the future either, because of the stuff at school. I hope you understand.