If you saw the full moon last night… it was associated with something unusual… and so the idea behind the blog for today.
You may know the expression "once in a blue moon" means something is not common or it rarely happens…. right?
Well… the second full moon of a single month is commonly called the "blue moon"… so it is a special full moon. Two full moons in one month happen about every 3 years.
But can there be a moon really be blue in color? The answer is yes… the last known blue moon was observed in Edinburgh, Scotland, in September 1950… truly a rare event. One of the observers was an astronomer… Robert Wilson of the Royal Observatory.
Mr. Wilson took measurements and concluded the moon was blue because the moon light was going through a cloud of small particles from forest fires in Alberta, Canada… which had come across the Atlantic ocean.
Normally… the color of the moon is essentially white or a combination of all colors. The moon observed high in the sky is almost always white… because there path of the moon light through the atmosphere goes through the fewest amount of atmospheric particles.
The more particles… air molecules… water vapor and pollutants the light comes in contact with… the more light is scattered away.
Most atmospheric particles scatter blue light first… because the particles are smaller than the wavelength of the light (about 500 nanometers or 1/100,000 inch). So the moon on the horizon—where the light has gone through more atmospheric particles — has lost most of its blue color and thus appears many times an orange or red color.
Most atmospheric particles are smaller than the wavelength of visible light… so the blue light is scattered first. Particles with sizes greater than the wavelength of light (for example… cloud droplets) will scatter all the colors of light equally. Yet, particles about the same size have the wavelength of light will scatter red light first. Very few particles in the atmosphere are the same size as the wavelength of light. So to see a blue moon is rare indeed.
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