Friday, August 31, 2012
Last blue moon until July 2015
Unless you are in the southeast and Isaac is clouding the sky, go outside tonight and enjoy the last "blue moon" we'll see until July 2015.
"But Ed, you wonder, where'd the phrase "blue moon" come from"?
I'm glad you asked. The answer is in two parts. First is why the color blue? Well, when rare seismic or natural phenomena occur such as volcanoes or massive forest fires, they inject into the atmosphere particulate matter of a specific and consistent size. Those particles are usually in the 0.7microns or 700nanometer range. The wavelength of red light is in the 600nm-700nm range. These particles scatter light that is roughly the same wavelength as the particle size, thus most of the light that reaches our eyes after a volcano or forest fire is blue because it's wavelength is far shorter, down in the 400nm-500nm range.
Part two: volcanoes and forest fires are fairly rare at least a few hundred years ago when the phrase was invented. So the blue moon came to be synonymous with rarity. Today, the "blue moon" phenomenon happens when you have two full moons in the same month, also a rarity. Because the lunar cycle is about 29.5 days and a month is a little over 30 days, the monthly and lunar cycles almost match up, but not exactly. Over time, that small difference accumulates until you have a full moon on the first day of a month and another on on the last day. The accumulation of excess hours combined with a full moon on the 1st day of a month is indeed rare, so that's why the second full moon in a month is called "blue".