Tuesday, August 20, 2013

The "Other" Blue Moon

Tonight's full moon is a "Blue Moon".
Spoiler alert: it's not really blue.

Blue Moon on 20 August 2013.

Most of us are familiar with the definition of a Blue Moon as being a second full moon that occurs during the course of a calendar month. Since the moon's phase cycle is 29.5 days, it's easy to see this doesn't happen very often - hence the saying "Once in a Blue Moon". But there are actually 2 kinds of Blue Moon, and tonight's full moon is the second kind.

It's only August 20th, so this can't be the second full moon in August.
Here's what's going on...

Occasionally there are 13 full moons in a year. This means that one of the seasons (spring, summer, autumn, or winter) will have 4 full moons instead of 3. The full moon tonight is the 3rd of 4 full moons this summer. Since the 3 regular full moons have names (in this case: early-summer, mid-summer and late-summer) the odd 4th moon is called "Blue Moon" It is thought that the name Blue comes from the word belewe which means "betrayer".

Photographers note: I took this pic of the moon by holding our point-and-shoot camera up to the eyepiece of our 25 power field scope. We were really surprised at the detail it picked up on the lunar surface. The rays of the crater Tycho show up amazingly well (lower center). The crater Copernicus is evident too (left of center at about 9:00), as well as the crater Kepler towards the left edge at about 3:30.

For a more thorough explanation of the two types of Blue Moon:

"Blue Moon, you saw me standing alone,
  Without a dream in my heart, without a love of my own..."
~ written by Richard Rodgers and Lorenz Hart

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