Blue moon – level 3

04-08-2015 07:00

When someone says to you,” Yeah, once in a blue moon,” you know what they mean – rare, absurd, or pretty much never going to happen. Well, stop right there because this year it means the end of July!

For the second time this month, the moon is about to become full. There was one on the 2nd  of July and now a second is coming on July 31st.

According to modern folklore, when there are two full moons in a month, the second one is blue. Most blue moons look pale grey and white just like the moon you see on any other night. Simply squeezing a second full moon into a calendar month doesn’t exactly change its colour.

Nevertheless, on rare occasions, the moon can turn blue. According to NASA, a truly blue moon usually requires a volcanic eruption. It says back in 1883, for example, people saw blue moons almost every night after the Indonesian volcano Krakatoa exploded. Some of the plumes of ash were apparently filled with particles that were one micron wide, about the same as the wavelength of red light.

Particles of this size strongly scatter red light whilst allowing blue light to pass through. Krakatoa’s clouds then acted like a blue filter. This was also the case with the number of other eruptions, and forest fires can do the same trick.

On the other hand, though, tomorrow’s moon could turn red. Often, when the moon is low, it looks red for the same reason that sunsets are red. NASA explains that the atmosphere is full of aerosols much smaller than the ones injected by volcanoes. These aerosols scatter blue light whilst leaving the red behind.

Difficult words: squeeze (to fit), plume (a cloud), micron (a very small measurement), wavelength (a measure of light or sound), scatter (to throw over a wide area), aerosol (a gas).



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