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Since 1st March, 1999
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Once-in-a-while blue moon
- The phenomenon that enriched English is on its way back

New Delhi, June 14: This June 30 night, brace yourself for a once-in-a-blue-moon experience. Step out of home and stare hard at the shiny disc in the sky.

It won’t be blue, of course. It’ll be the same silver or greyish-white — yet different from any moon in the past three years.

As the second full moon in the same month, it will be a rare enough occurrence that the world has accepted as the proverbial “blue moon”.

How does a month come to have two full moons' A lunar cycle (full moon to full moon) is roughly 29.5 days, shorter than every month except February.

Which means every month other than February can possibly squeeze in two full moons — one at the beginning, one at the end.

Put in another way, since a lunar year is shorter than a solar year (which we follow), there’s an extra lunar month or cycle every two-and-a-half (solar) years and, therefore, an extra full moon to fit in somewhere.

So, if you miss it on June 30, you have to wait only till December 2009 for the next one.

“In a century, 41 blue moons are possible on an average,” Professor Ramana Shetty at the Indian Institute of Astrophysics said.

The last one occurred in July 2004, and the Americas and New Zealand saw one on May 31 this year, after a full moon on May 2. But the May 31 full moon fell on June 1 for Asia, Africa, Europe and Australia, delaying the blue moon by a month.

A blue moon can even happen twice in the same calendar year — in January and March with February missing out entirely — once every 19 years or so. The last time was in 1999.

Some experts, though, junk this modern definition of the blue moon. To them, a blue moon is the third full moon in a season that has four full moons — a much older definition.

Why' The Christian ecclesiastical calendar, where full moons helped identify festival dates, had a problem with the extra, 13th full moon. The only way of saving the results was to rule out the third moon in the season with four. The last blue moon by this older definition occurred on August 2005.

These extra moons are no less important to Hindutva – without some 30 of them in his 82-year-old life, Atal Bihari Vajpayee would not be the Man of the Thousand Moons.

But why call it blue even though it isn’t' There hardly seems a satisfactory answer.

And why should “once in a blue moon” mean “very rarely” when a blue moon is hardly all that rare'

The answer seems to be that the phrase refers not to the astronomical blue moons – by modern or old definition – but to a moon that is literally blue.

According to the Oxford English Dictionary, the first recorded reference to a blue moon dates to a 1528 medieval England proverb: “If they say the moon is blue/We must believe that it is true.”

The suggestion is clear: it’s absurd to talk about a (literal) blue moon.

How did “absurdity” become “rarity”' When the Krakatoa volcano erupted in Indonesia in 1883, the moon appeared an exotic blue for several years at a stretch.

“The ash from large volcanoes reaches the upper parts of the atmosphere. The particles absorb the red light, giving the moon a bluish hue,” earth scientist Professor Duncan Wingham of the University College of London said.

The British colonial experience of this blue moon, some experts believe, led to the proverb. Thank your lucky stars that the June 30 moon won’t be a literal blue.

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