Blue moon, you saw me standing alone Without a gleam in my heart Without a love of my own Bom ba bom bom bom bom bom bom ba dang dang dang dinga dong ding Baluuuu moooon You know, they just don't write songs like they used to. Perhaps we should concern ourselves with the other two meanings. Normally, the molecules of the air we breathe scatter the light coming from the Sun in such a way that the sky looks blue and the Sun and moon above look yellow. These molecules are smaller than the wavelengths of visible light (sunlight), and as a result, blue light is scattered more effectively than green or red light. This scattering process is referred to as Rayleigh scattering. In essence, the very small air molecules direct more blue light than red light toward our eyes. However, when the atmosphere contains particles that are comparable in size to the wavelength of visible light , the reverse occurs, and red light is scattered more than green or blue light. When this happens, it's possible for the sky to have a reddish hue and for the Sun or the moon to appear blue or green. This is called non-selective scattering.

This kind of scattering can be produced by ash from volcanic eruptions, by dust from sandstorms, or by smoke particles from forest fires or industrial pollution. For example, following the eruption of Krakatoa in 1883, blue and green Suns and moons were observed in a number of tropical countries. If the fine ash particles were about .00004 inches in diameter (1.1 micron), the moon would look green, but if the particles were about .00003 inches in diameter (0.85 microns), then the moon would look blue. This is because the wavelengths of visible light that result in a green color are ever so slightly longer than the wavelengths that result in a blue color. In the presence of ash, dust or smoke scatterers, the Sun or moon's color is more subtle than brilliant. Even if the all the particles from an erupting volcano were 0.85 microns, for instance, the Sun and moon would not be a pure blue color. The ash particles in combination with the overwhelming number of much smaller air molecules act to make the disk of the Sun and the moon appear a bluish-white color.

The response of the human eye and brain also plays a role in the colors we see or rather perceive. How our brains respond to visual color sensations is very subjective. If only two colors are present in a scene, it is possible for the eye to expand these colors into a broader spectrum. So, if the sky is a spectacular red color at sunset due to volcanic ash and dust, a crescent moon against this vivid background may have an emerald color.

You may be able to duplicate blue and green moons in the classroom. Craig Bohren in his book "Clouds In A Glass Of Beer" (honest) suggests an experiment involving cigarette smoke and a beaker inverted on the glass plate of an overhead projector (great, it sounds like I'm giving our young adults an excuse to sample beer and cigarettes). Anyway, the image of a clear beaker transmitted on a screen is white. With cigarette smoke added, the color should be reddish. However, if you hold the smoke in your mouth a bit longer, the image on the screen may be tinted with blue. The particles of smoke become slightly larger when held momemtarily. The difference in holding time to produce a red screen or a blue screen is only a fraction of a second. If you practice a little, you may be able to play the part of a magician, alternately creating red and blue screens.

Except in a laboratory setting, being able to see a blue moon is a rare event. Hence, the saying "once in a blue moon" is used to express something that happens infrequently. The other meaning of a blue moon is more astronomical in nature. When two full moons occur during the same calendar month, the second full moon is said to be a blue moon. Since the lunar cycle (the time between successive full moons, which is how the word "month" came to be) is about 29 1/2 days, you can see that it's not easy to fit two full moons into the same month. On June 1 there will be a full moon. The moon is also full on July 1. However, in July a second full moon (the blue moon) occurs on July 30th. Why doesn't the 30th day of June have a blue moon? The reason is because on June 1, the exact time when the moon is fully illuminated by the Sun (when the Earth is exactly between the Sun and the moon) occurs during the evening. One lunar cycle later (29 1/2 days) is the early morning hours of July 1. So then the next full moon will occur in the evening of July 30. About every 15 months or so there will be two full moons during the same month. However, if a blue moon occurred during the evening of January 30 (in a non leap year), then March would also have a blue moon. Astronomical blue moons are not near as rare as visual blue moons - maybe once in your lifetime you'll have the chance to see a blue moon or Sun.