Answer to the Question 12/97
The question was:Once in a blue moon the moon looks blue or green or purple. Why?
(12/97) Oren Melinger correctly noted that the change in the color of the moon is due to scattering of the light (reflected from the moon) by drops or particles. A slighly more detailed solution is provided below:
We are used that the light scattering in the atmosphere is more effective for shorter wavelengths, and therefore the color of, say, sun is shifted in the "red direction". However, sometimes the size of the water drops is of the same order as the visible light wavelength. In such case the scattering cross section is very sensitive to wavelength. In particular 600 to 1000 nanometer drops will scatter stronger the red light than the blue light. Thus the white light reflected from the moon, passing through a cloud will have red scattered out, and will become blue or green.
For detailed explanation see H.C. van de Hulst Light scattering by small particles, Wiley, NY (1957). See also D.K. Lynch and W. Livingston Color and light in nature, Cambridge U. Press (1995).
p.s. O. Melinger also remarked that the current usage of the term Blue Moon means appearance of two full moons in a single calendar month.
p.p.s. The common usage of the term "Blue Moon" as something extremely rare or nonexistent can be traced as far back as Shakespeare (1528):
Yf they say the mone is blewe
We must believe that it is true.
p.p.p.s. (3/2000) Few years after publication of our "QUIZ" question about blue moon, a similar question has been asked and answered in "The Last Word" section of the New Scientist - see the last page of the 11 March 2000 No.2229 issue. In addition of mentioning various recorded case of blue moon due to atmospheric dust, some of the answers describe a case of clearly blue SUN! In particular, D. Harper mentions a report of R. Wilson in Monthly Notices of Royal Astronomical Society, 111, p. 477 (1951) about "deep indigo blue sun" which was observed in Edinburgh on Sept. 26, 1950 few days after extensive forest fires in Canada. (Spectrogram confirmed that this is NOT an optical illusion - red color was missing from the spectrum.) Apparently, the same case was reported by A. Watson (a pilot); RAF checked and found that at 11 km height there was a layer of smoke, above which sun looked normal.
See all those questions and answers in the on line edition of the New Scientist. A local copy of those questions and answers can be found here.
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