Once in a Blue Moon
When she's not adorning drafts of my column with red ink, my editor occasionally sings in a local band. Lately, her renditions of Billie Holliday's "Blue Moon," have been inspiring questions from the audience. Isn't there a blue moon this month? Yes, there is, but don't expect to see a moon the color of a robin's egg or the Alaska flag. "Blue moon" is a term of vague origin used to describe two relatively rare phenomena: the second full moon in a month, or when the moon actually appears blue due to particles suspended in air.
July, 1996's blue moon is the calendar variety. July features two full moons--one occurred on the first, and another will show itself on the 30th. This type of blue moon happens every 2.7 years because of a disparity between our calendar and the lunar cycle. The lunar cycle, the time it takes for the moon to revolve around the earth, is 29 days, 12 hours, and 44 minutes. Because our months aren't 29.5 days long, blue moons happen once in a, well, you know.
Babylonians, Egyptians and Greeks based their calendars on the moon. Each month (a word derived from moon) was either 29 or 30 days to correspond to the 29.5 days it takes the moon to orbit the earth. To synchronize their calendar with the sun and the changing seasons, the Romans developed the Julian calendar, which includes months with 28 to 31 days. From the Julian calendar evolved the Gregorian calendar, which we use today. The real blue moon, the one that actually appears blue, is the result of floating particles in the air that mess with moonlight. Microscopic bits of volcanic ash, fire smoke, and dust all have the ability to scatter light rays and make us see the sun and moon dressed in unexpected colors. Blue moons and blue suns were seen following the eruption of Krakatoa Volcano in 1883 and in England during 1950, a year of many forest fires in Canada.
Smoke, ash, dust, or similar particles in air make us see different colors because light energy vibrates at different frequencies, and particles the same size as light wavelengths tend to scatter different colors. The sun emits light in all colors of the rainbow, from violet to red, but we see the sun as yellow or white at midday because all the colors of light are reaching the earth with the same intensity. Add microscopic specks to the mix and some frequencies of light bounce off.
Orangy sunsets, for example, occur when sunlight travels through more atmosphere at the end of the day than when the sun is directly overhead. As the light from the sun travels through more atmosphere and encounters more particles, only light with longer wavelengths, such as red or orange, isn't scattered away.
Blue moons happen when particles floating in the atmosphere are just the right size to scatter red light. The particles act like a filter, allowing only a bluish light to pass.
Blue-colored moons are much rarer than the kind produced by the whims of our calendar, so to say you've seen a blue moon, it's safest to watch the sky on July 30th. If you miss it then, don't dismay. The next blue moon occurs in 1999, when full moons appear January 2 and 31. There's no way you can miss them. It's dark in January, remember?
