Blue moon

There are 3 uses of the term blue moon:

• The second full moon in one calendar month.
• The third of four full moons in one season (originated in Maine, USA in the 19th century).
• When the moon appears blue from earth, caused by dust in the atmosphere. Often coincides with events such as volcanic eruptions, monsoons and forest fires.

The term has been found in literature dating as far back as 1521 and is a common metaphor for a rare or unusual event - once in a blue moon.

External links

• What's a Blue Moon? by Donald W. Olson, Richard T. Fienberg, and Roger W. Sinnott - Sky&Telescope
• Once in a Blue Moon - What is a blue moon? by Ann-Marie Imbornoni