The title is a phrase used to describe something that happens rarely, but most people do not know what a blue moon is. By its older definition, a blue moon is the 3rd moon in an astronomical season that has 4 moons. An astronomical season is the time from one equinox to a solstice and vice versa. By its newer and more recent definition, it is the second full moon in a month. Both of them are rare due to the fact that the period between full moons are approximately 29.5 days. When a moon lands on the first day of a month, the newer definition will be satisfied and can theoretically be called a blue moon. To put into perspective sake, blue moons of each individual definition happens usually in a time difference of 2 to 3 years. In the older definition, the last blue moon was on May 21, 2016 (surprise for those who didn’t know) and the next blue moon (according to this definition) would be on May 18, 2019. For the other definition, the next blue moon would be on January 31, 2018. Because there are two separate definitions for a blue moon, blue moons aren’t exactly too rare. And to be sure, these blue moons are only blue in name only, originating from the “Maine Farmer’s Almanac” 1946 version. But does this mean there is no such thing as a blue moon? Actually no, there exists a real blue moon, that is much more rare than the modern and old definitions.
A legitimate blue moon, that is blue in color (without any editing from photoshop or similar tools) can happen due to some special atmospheric circumstances. These sky conditions are produced from specific sized smoke and particles from either volcano eruptions, dust storms, or forest fires. These particles must be a specific size to scatter the red light reflecting off the moon to appear blue. This specific size is slightly more than 0.7 micron or 70% of a micrometer (1 / 1,000,000 meter) and the reason for that is because the wavelength of red light is 0.7 micron. The particles cannot be in a mixture of other sizes because they may end up scattering blue moon instead. For imagination’s sake, a human blood cell is 5 microns across. Some real life occasions that resulted in a blue moon were: 1883 Krakatoa (Indonesia) eruption, 1950 Sweden forest fire, 1951 Canadian forest fire, 1983 El Chichón (Mexico) eruption, 1980 Mount St. Helen (United States), and 1990 Mt. Pinatubo (Philippines). These visually blue moons are a lovely sight for the eyes, and if the particles stay in the air for long enough, you may even see a lavender or a blue sun too!
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