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There's a blue moon above...

Arup Chanda in Calcutta

Date: January 31.
Time: Well, make it evening or later.
Venue: The heavens

What exactly is a blue moon? It is a term used to describe the second full moon to occur in a single calendar month. That's why it's so rare. Fewer than three per cent of all full moons are blue moons.

According to Piyush Pandey, assistant director of the Birla Planetarium, Calcutta, "Since a full moon occurs every 29.5 days, and the average length of a month is 30.5 days, it's unlikely to see two full moons in a month."

"If there is a full moon on the 1st or 2nd day of a month, there is a good chance there will be a second full or blue moon that month." And that is exactly what is going to happen on January 31.

1999 is special because it will witness two blue moons in a year. It was 19 years ago, in 1980, that two blue moons came in a year.

Descriptions dealt with, let us provide the details. On January 2, there was a full moon, so there will be another one on January 31. But there will be no full moon in February. March will witness two full moons again -- one on March 2 and the other on March 31.

Pandey said, "Before this we had a blue moon in 1996 on July 30. And after March 31, 1999 there will not be another until November 30, 2001. And then we will have to wait till July 2004. On average, there will be 41 months that have two full moons every century, so you could say that once in a blue moon actually means once every two-and-a-half years."

A month with 31 days is, of course, more likely to have two full moons than one with just 30. February never sees two full moons, because the minimum interval between full moons is always greater than the length of that month, even in a leap year.

After 1999, there will be two blue moons in a year only in 2018 and 2037. Thereafter there will be none till till 2050.

Pandey said 1999 was special because it has two blue moons. There are about four years every century with two blue moons. The first blue moon always occurs in January. The second occurs predominantly in March. In the 10,000 years starting with 1600, this is true in 343 out of 400 cases, or 86 per cent of the time.

"In 37 cases (or nine per cent), the second blue moon is in April. In the remaining 20 cases (five per cent) it is in May.

"Between such years, blue moons happen at intervals like 2 years and 7, 8, 9, or 10 months. The bottom line of all this complexity: just under 3% of all full moons are blue moons." said Pandey.

"There are many examples of the moon actually looking blue," says Pandey.

"When the Indonesian volcano Krakatoa erupted in 1883, its dust turned sunsets green and the moon blue all around the world for almost two years. In 1927 a late monsoon in India caused the moon to appear blue again. And the moon in Newfoundland turned blue in 1951 when huge forest fires in Alberta threw smoke high into the sky.

"Even by the mid-nineteenth century it was clear that although visually blue moons were rare, they did happen from time to time. Which was when the phrase "once in a blue moon" came about. It meant then exactly what it means today -- that an event was fairly infrequent, but not quite regular enough to pinpoint."

"The first appearance in print of this expression goes back to well before the time of Shakespeare -- to 1528. Nobody really believed that the moon ever was blue. So "once in a blue moon" meant never."

Perhaps, but here it is!

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