It’s not exactly a math article, but Professor Homunculus likes it anyway. Check it out.

My wife and I will celebrate our tenth anniversary (my god she must be a patient woman!) on the 27th of August 2009, the day Mars will be largest to our eyes. Hotcha! (I’ll write a post about this astronomical phenomenon soon.) (No- wait! I’d better write about it now. See the mea culpa at the bottom of the post )

We seem to be a astronomically significant couple. I proposed to her on a Double Blue Moon (the second blue moon in one calendar year). Those happen about 4 times a century. I just googled them, and there doesn’t seem to be a name for that astronomical phenomenon (I mean, “Double Blue Moon” doesn’t seem as romantic as “Harvest Moon”, “Full Wolf Moon”, or “Full Beaver Moon? (hmmmm…)) I thought I’d christen it myself.

From now on the “Double Blue Moon” shall be known as the “Foley Moon“ (why not?)

Hotcha!

What is a Blue Moon?

There are several definitions for a blue moon.

In general, a full moon that does not occur at an expected interval (say, more than twelve per year, or three per season) was considered a “blue moon.”

Another definition is for when the moon (even if it isn’t necessarily a full moon) actually appears to be blue. This can happen because of smoke particles in the air. Apparently this was the case after the eruption of the volcano at Krakatoa in the 1800s, and reportedly at least one time since, in 1950 in Sweden and Canada.

If two full moons occur in any calendar month, the second is called a blue moon.

The above definition is the most frequently used, contemporary one. The two contemporary definitions of blue moon, the second full moon in a calendar month (the astronomical definition) and something that rarely happens, are not equivalent. Astronomical blue moons happen fairly often, once every 2.5 years on average.

There are 29.53 days between any two Full Moons (Moon’s synodic period) . There are 365.25636 days in the year. That equals 12.37 Lunar Months. The extra 10 or so days allows for an occasional extra Full Moon, because after about 2.7 years that builds up to over one extra period of more than 29.53 days.  Here is a  list of Blue Moons for the first half of the 21st century.

• November 30 2001
• July 31 2004
• June 30 2007
• December 31 2009
• August 31 2012
• July 31 2015
• January 31 2018
• March 31 2018
• October 31 2020
• August 31 2022
• May 31 2026
• December 31 2028
• September 30 2031
• July 31 2034
• January 31 2037
• October 31 2039
• August 31 2042
• May 30 2045
• January 31 2048
• September 30 2050

The phrase, “Once in a Blue Moon”

According to Erich Weissman’s World of Astronomy:

The earliest use of “blue moon” meant an obvious absurdity which everyone knew never happened. However, the moon does occasionally turn blue as a result of smoke from forest fires or particles from a volcanic eruption. Since these blue-looking moons were rare but did happen from time to time, the phrase “once in a blue moon” was coined, meaning that an event is unusual, but can happen occasionally (Kibbey).

According to this article about blue moons:

The recent use of the word Blue Moon to describe the second full moon of a month can be traced to J. Hugh Pruett’s April, 1946, article in Sky and Telescope magazine entitled Once in A Blue Moon.

In his article, Hugh mentioned an old Maine Farmers’ Almanac for the year 1937. He wrote, “In effect . . . at one time the various full moons of the year were given names according to the order in which they occurred — provided there was only one per month. These names were as follows: Moon after Yule, Wolf Moon, Lenten Moon, Egg Moon, Milk Moon, Flower Moon, Hay Moon, Grain Moon, Fruit Moon, Harvest Moon, Hunters’ Moon and Moon Before Yule. But seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon, and was considered unlucky and a real nuisance as it occurred at various times of the year and upset scheduling of church festivals.”

You can read the full story of the origin of the phrase “Once in a Blue Moon” here.

What is a Double Blue Moon?

If two blue moons occur in the same calendar year, the second blue moon is a “double blue moon.”

There are approximately 4 Double Blue Moons in each century. Can you figure out why?

There were double full moons in 1961 and 1999, and the next ones will be in 2018 and 2037.

The song, “Blue Moon”

As you can imagine, “Blue Moon” is “our song.”

Blue Moon

Now Im no longer alone

Without a dream in my heart

Without a love of my own

-  Rogers and Hart

This song was first recorded in 1934, but was unreleased. Since then it appeared in several movies, and was covered by  many artists, the most famous of which were:

Ella Fitzgerald with Tony Bennet, Elvis Presley, Frank Sinatra, Bob Dylan, The Marcels, Sam Cooke, Frankie Laine, Louis Armstrong, Dean Martin, Django Reinhardt, The Supremes, Mel Torme, The Ventures, Sha Na Na, Dizzie Gillespie,  and my personal favorite version by Cybill Shephard (although as a Doo-Wop fan, the Marcels come in a close second – the “double” blue moon!)

References:

http://www.farmersalmanac.com/full-moon-names

http://science.nasa.gov/current/event/ast30mar99_1.htm

http://scienceworld.wolfram.com/astronomy/BlueMoon.html

Do the math:

Is there a month in which a blue moon cannot occur?

Hint:  The Moon’s synodic period is 29.531 days.

Mea culpa:

Ever wonder what happens when a know-it-all amateur recreational-mathematician dives in out of his league? Well, you don’t have to wonder anymore. I goofed up, big. Normally I am a big skeptic, but I guess I didn’t have my skeptic’s hat on when my science-teacher friend sent me the viral e-mail that’s been going around about Mars and the Moon.

It turns out that it’s a hoax. A very astute reader (see the comment below) sent me the heads-up on it. Sorry about that, dear readers. Please pardon me while I wipe the egg off my face. For the skinny on the scam, see http://www.universetoday.com/2006/07/27/no-mars-wont-look-as-big-as-the-moon-in-august/

The lesson? Don’t assume a damned thing until you’ve checked it with snopes.com (or your friendly, neighborhood astronomy teacher.) Thanks, Mike!