Wednesday 12 September 2012

Science, Vocabulary & Song: Once In A Blue Moon

In this Aug. 31, 2012, photo, a small plane passes in front of a "blue moon" 
over the Westchester County airport in Armonk, N.Y. Blue moons are
commonly defined as the presence of a full moon for a second time within a 
single month. This one took place on the day Neil Armstrong, the first man
 to walk on the moon, was laid to rest.
Image and caption from mlive

"One small step for [a] man, one giant leap for mankind." -- Neil Armstrong

Cambridge Dictionary defines  "once in a blue moon" means "not very often", Oxford Dictionary says "very rarely", Hiscock (1999) collected many definitions and sums up the meaning as "incredibly rare", but what is a "blue moon"? Is a blue moon really blue?

The blue moon is not actually blue in colour.

According to Wikipedia's detailed calculated example,

One lunation (an average lunar cycle) is 29.53 days. There are 365.26 days in a solar year. Therefore, about 12.37 lunations (365.26 days divided by 29.53 days) occur in a solar year. In the widely used Gregorian calendar, there are 12 months (the word month is derived from moon) in a year. Each calendar year contains roughly 11 days more than the number of days in 12 lunar cycles. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. The extra moon necessarily falls in one of the four seasons, giving that season four full moons instead of the usual three, and, hence, a blue moon.

So, a blue moon is the appearance of the third full moon in a season that has four full moons, instead of the usual three. In folklore, it has a different definition. Instead, it is the second full moon in a calendar month.

But can a blue moon be blue?

Blue coloured blue moon, taken using a camera filter
Image from NASA

The answer is YES!!!

NASA Science News has an article talking about the appearance of an actual blue moon in Indonesia back in 1883, when volcano Krakatoa's erupted and shot ashes into the atmosphere,

Some of the ash-clouds were filled with particles about 1 micron (one millionth of a meter) wide--the right size to strongly scatter red light, while allowing other colors to pass. White moonbeams shining through the clouds emerged blue, and sometimes green.

Flour suspended in water appears to be blue
because only scattered light reaches the
viewer and blue light is scattered by the flour
particles more strongly than red.
Image and caption from Wikipedia

I'm suspecting this is the same concept as why the sky is blue -- the Tyndall Effect, and I will talk about that in a future post. For now, I just want to dedicate a song to Mr. Neil Armstrong, the first man on moon.

Neil Armstrong -- First Man on the Moon
Image from littlegreenfootballs
R.I.P. Mr. Neil Armstrong. I will wink at the moon whenever I think about you.

Once In A Blue Moon by Mabel Mercer, from the House M.D. original soundtrack.

Image from tunewiki

"It suddenly struck me that that tiny pea, pretty and blue, was the Earth. I put up my thumb and shut one eye, and my thumb blotted out the planet Earth. I didn't feel like a giant. I felt very, very small." -- Neil Armstrong
once in a blue moon -- not very often
folklore -- (n)[U] the traditional stories and culture of a group of people
erupt -- (vb)[I] When a volcano erupts, it explodes and flames and rocks come out of it
micron  -- (n)[C] one millionth of a meter


Hiscock, P. (1999) Folklore of the "Blue Moon"

Clayton Hardiman urges us all to wink at the moon @

Clayton Hardiman: #Wink at the Moon. ICY Neil Armstrong @

Blue moon @ Wikipedia

Blue Moon @ NASA Science

House M.D. - Once In A Blue Moon by Mabel Mercer @ YouTube

Once in a blue moon - Mabel Mercer @ Tunewiki

Apnea @ Wikipedia

Study Shows Half of All Women Found to Have Mild-to-Severe Sleep Apnea @ksee24 news