Blue Moon
My daughter was telling me about the blue moon that appeared this past Monday evening. Have you heard the phrase, Once in a Blue Moon? That is because a blue moon doesn’t happen very often. Once in a blue moon, I feel like cooking. Once in a while there is a blue moon.
Most years have twelve full moons that occur approximately monthly. In addition to those twelve full lunar cycles, each solar calendar year contains roughly eleven days more than the lunar year of 12 lunations. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon.
Why is it called a blue moon? I don’t know! The most literal meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden and Canada in 1950 and 1951, or after there has been a volcanic eruption. Red moons are far more common than blue moons. The difference in the coloration is caused by the size of smoke or dust particles in the atmosphere. When there are particles slightly wider than the wavelength of red light (0.7 micrometre)–and no other sizes present a blue moon is present. Particles smaller than 1 micrometre, tend to scatter blue light and cause the moon to look red.






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