When an event is extremely rare, we sometimes hear people say, “That only happens once in a blue moon.” On January 31st this year we will have a triple rare event: a super moon, when the moon is in closest proximity to earth; a total eclipse of the moon and a blue moon. But what’s a blue moon? How rare is it, and is it really blue? The common definition is: A blue moon is the second full moon that appears in a single month. We see one at least every few years. Folklore holds that this second full moon got its name because early almanacs used blue ink to indicate when to expect one. Strangely, though, history says our moon has looked blue. When the volcano Krakatoa erupted in the 1800s, it put so much ash into the atmosphere that it changed Earth’s seasons for a year. Early on, sunsets were so vivid that New York called out its fire department. And, for weeks, the sun looked green and the moon was blue.
Mary Knapp, Weather Data Library