A Blue Moon can be the second full moon in a calendar month. Or it can be the third of four full moons in a single season. The November 21, 2010 Blue Moon is the third of four full moons between the September 2010 equinox and December 2010 solstice. This celestial event occurs only 7 times within a 19 year cycle...Happy Blue Moon!
Earthshine on dark side of moon.