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What Is a Blue Moon?
The moon has always mystified us since the first time man has set eyes upon it; a little white ball floating in the sky like a brother to the Earth. About once every twenty eight days, the moon gives off its full light, hence the name "full moon," showing to the Earth observer as a perfect white sphere with its craters gleaming elegantly. The term "blue moon" is used when this event occurs four times in one of the year's four seasons or twice in a month. Precisely, a blue moon happens on the third full moon of a season with four full moons or on the second full moon in one particular month. This doesn't mean, though, that you'll find the moon literally giving off blue light at those times of the year.
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How Often Does it Occur?
Depending on the definition of a blue moon that you want to refer yourself to, you will end up with different results for each occurence's frequency. According to the Farmers' Almanac definition of a blue moon - an occurence happening on the third full moon of a season with four full moons - this event occurs just about every two years and nine months. According to the calendar definition of a blue moon - an occurence happening on the second full moon of any month with two full moons - you end up with a blue moon just about once every two and a half years. Remember that a full moon occurs about every 28 days. You can base a calculation on the next blue moon on this.
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When the Moon Turns Blue... Literally...
Although the moon doesn't necessarily appear blue in a "blue moon," you might actually catch the moon giving off a blue light at times. You see this event often because of a large amount of dust on the Earth's atmosphere and other particles such as smoke which fine-tune and interfere the light wave patterns of the moon to give off a deceptively blue-ish color. When you see any body giving off different colors, you are experiencing a phenomenon caused by the Earth's own atmosphere - the same phenomenon that causes stars to twinkle. This disparity in the atmosphere's particles is what prevents us from seeing very far in the sky.
Examples of events that might make the moon appear blue are volcanic eruptions or large smoky fires. Lighting a campfire isn't always enough. Red light travels at a wavelength of about 0.7 microns. If the moonlight shines through many particles slightly wider than this wavelength, the red light will filter out and what you'd see is a large mystic blue moon through a misty atmosphere.