The term "blue moon," or the second full moon in a single month, does not refer to the moon's color at all. A blue moon occurs, on average, every 2.72 years. (The next blue moon will be this Saturday, July 31.) A bluish-looking moon can occur as a result of the earth's atmosphere, however. For example, a bluish moon was observed in North America on Sept. 26, 1950, the result of Canadian forest fires scattering high-altitude dust.
This period of extremely hot, humid, weather that occurs in the northern hemisphere in late summer got its name from the "dog star" Sirius in the constellation Canis Major ("big dog," in Latin). At this time of year, Sirius, the brightest visible star, rises in the east at the same time as the sun. Ancient Romans believed that the heat of Sirius added to the sun's heat to create hot, sultry weather.
Sources: The Handy Science Answer Book by the Science and Technology Department at the
Carnegie Library; The Encyclopedia of Word and Phrase Origins, by Robert Hendrickson