What Is A Blue Moon?
QUICK ANSWER
A blue moon is typically the second full moon to occur in a single calendar month. The traditional definition is the third full moon in a season with four. Blue moons happen about every 2-3 years. The term comes from an 1883 volcanic eruption that briefly made the Moon look actually blue.
A blue moon is one of the most familiar full moon names, even though it doesn't refer to a specific month. The phrase has entered everyday language as a synonym for something rare, as in once in a blue moon. The astronomical event itself isn't extremely rare (about every 2-3 years), but the term has a surprising origin involving a massive volcanic eruption in the 1800s.
What makes a moon blue?
Two definitions. According to NASA, the modern popular definition of a blue moon is the second full moon in a calendar month. The older traditional definition refers to the third full moon of the four occurring between an equinox and solstice in that year. Both definitions arise from the fact that the lunar month (about 29.5 days) doesn't quite fit twelve times into a calendar year, so occasionally an extra full moon happens. The popular monthly definition originated from a 1946 magazine article.
Why is it called blue?
Because of an 1883 volcanic eruption. The term blue moon is believed to have originated after the catastrophic eruption of Krakatoa in 1883. The volcano put so much dust and ash into the atmosphere that the Moon actually appeared blue for months afterward. The blue color was unusual enough that the term once in a blue moon was coined to refer to something rare. Other extreme atmospheric events (large forest fires, other volcanic eruptions) can still produce actual blue-colored moons today, but they remain rare.
How often does a blue moon happen?
About every 2-3 years on average. By the modern monthly definition, blue moons occur whenever a calendar month happens to contain two full moons. Since the lunar cycle is about 29.5 days and most months are 30 or 31 days long, a full moon in the first day or two of a month allows for a second one before month's end. February, with only 28 or 29 days, can never have a blue moon. By the older seasonal definition, blue moons are slightly less frequent.
Is the moon actually blue?
Usually no. Despite the name, blue moons by either modern definition look exactly the same as any other full moon. The actual blue color requires very specific atmospheric conditions, with airborne particles of a particular size (about 1 micron across) that selectively scatter red light. Such conditions are rare and unrelated to the calendar timing. Major volcanic eruptions and large forest fires can produce visible blue moons, but they're truly rare events, perhaps once or twice per century globally.
A blue moon is typically the second full moon in a single calendar month, by the modern popular definition. The older traditional definition refers to the third full moon of four in a season. Blue moons happen about every 2-3 years on average. The term comes from an 1883 volcanic eruption (Krakatoa) that briefly made the Moon look actually blue. Despite the name, blue moons usually don't look blue at all.
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