[Masthead] Mostly Cloudy ~ 72°F  
High: 78°F ~ Low: 59°F
Saturday, Apr. 30, 2016

'Blue moon' not what name implies

Friday, August 31, 2012

Blue in name only.
(NASA Photo)
There will be a blue moon tonight.

Or will there?

It depends on what definition you use. For centuries, the traditional definition of a "blue moon" was the fourth full moon in a calendar season, which normally has only three. By that definition, tonight's full moon is not a blue moon, since it's only the third full moon of the summer.


But according to Sky & Telescope magazine, a magazine article in 1937 was used as a source but misinterpreted by another writer in 1946, leading to the current, but completely accidental, definition of a blue moon as the second full moon in a calendar month, which came into wide use and began to replace the old definition.

In either case, what happens is that our 30-day and 31-day calendar months, and therefore our calendar seasons, don't synch up exactly with the 28-day lunar month.

That means that, about every two and a half years, there's room for a second full moon in a month (or a fourth full moon in a season). Because the event is (relatively) rare, the phrase "once in a blue moon" came to mean something that doesn't happen often.

No real meaning

There's no particular significance to the designation. The name, under its original definition, came about because the schedule for determining the date of Easter has to do with the lunar cycle.

In some cases, according to the Farmer's Almanac, parishoners in the Middle Ages might think that a particular new moon was the one which would be used to determine the date of Easter, but the clergy would tell them it was a "belewe," or betraying, moon, and not the real full moon by which they should mark their Easter preparations. The Old English word "belewe," for "to betray," became confused with "blue," and the name was born.

As you can see, the type of "blue moon" we'll experience tonight has nothing to do with the moon's actual color. There are, however, atmospheric conditions that can make the moon appear more bluish than normal. The moon appeared bluish for months, even years, after the volcano Krakatoa erupted in 1883, according to NASA.

Coincidental honor

As pointed out in Thursday's Times-Gazette, some are noting the appropriateness of a blue moon on the day that the first human being to set foot on the moon Neil Armstrong, is to be buried, and the day that flags will be flown at half-staff in his honor.

If it's cloudy, and you want to be able to say you watched the blue moon, the Slooh Space Camera will have a live feed of the moon, viewable at events.slooh.com.