This Tuesday's moon is commonly known as a “blue moon” since it marks a second full moon within the same calendar month. The first one occurred on August 2.

In the 1500's belief in a blue moon meant belief in something absurd; that an event, like a blue moon, would never happen. Later the expression “once in a blue moon” entered our literature meaning something rare or special that might happen. A possible explanation for the change is that the moon, under certain atmospheric conditions, does appear to have a slight bluish cast.

Actually, “blue moons” are neither blue nor as rare as they are popularly believed to be. They come along seven times every 19 years.

The last time we had two full moons in the same month was December 1990. A blue moon will occur again in June 1996.

The period between two full moons in the same month averages once every 2 and a half to three years.

The time between one full moon to the next is 29 and a half days. Known as the synodic period, it marks the average length of a lunar month.

Since August has 31 days, it is a likely candidate for a double full moon. The month of February, however, is never long enough to accommodate two full moon phases, even during a leap year.

In fact, February sometimes contains only three lunar phases, and it is even possible for the shortest month to bypass a full moon altogether, as was the case in 1961 and will be the case in 1999.

In addition to referring to the second full moon in a given month, a blue moon also alludes to an extremely rare occurrence when it is possible for the moon's disk to actually take on a bluish tint.

Atmospheric conditions have to be exactly right for the moon to appear blue. The color is produced in part by the scattering of light by dust particles and large molecules in the Earth's atmosphere, in much the same way that blue skies and red sunsets are produced.

The size of the particles helps determine the color of light we see.

Small particles scatter the short wavelength colors of blue and green more efficiently than the long wavelength colors of yellow and red. Our sky overhead appears blue because the shorter blue wavelengths are scattered at greater angles than the red and are reflected back to our eyes.

Red, orange and yellow wavelengths pass through the small particles. When the light has a short distance to travel through the atmosphere — when the sun is overhead — we don't notice the red. But the rays from a setting sun pass through more of the particle-filled atmosphere to reach us. The atmosphere scatters out all the blue and leaves only red and orange to color our sunsets.

Space, on the other hand, has no atmosphere and no particles to scatter sunlight; in space the sky appears black.

Particles the same size as the wavelength of light are the cause of blue moons. These particles scatter out the long red wavelenghts more than the short blue ones, allowing a normally white moon to be seen through the blue end of the spectrum.