August is one of those rare months where there are two full moons. The first occurred on Aug. 1, and the second full moon of the month will take place on Aug. 30. According to modern folklore, the second full moon in a calendar month is called a blue moon.
Usually there is only one full moon each month. Because, however, it takes the moon about 291/2 days to circle Earth once in its orbit, and most months are 30 or 31 days long, it is possible to fit two full moons in a single month. This happens about every 21/2 years. The last blue moon occurred in December 2009, and the next one will happen in July 2015.
The phrase "blue moon" has been around for well over 400 years, and during that time it has had many different meanings. In the 16th century, if someone said, "That's as likely as a blue moon," he meant that something was rare and seldom happens. It had no astronomical meaning. Our modern definition sprang up in the 1940s.
Although a blue moon doesn't really look blue, there have been times in the past when the moon has assumed a bluish color. This can be caused by small airborne particles -- about 1 micron in diameter -- such as ice crystals, volcanic ash or smoke from forest fires. These particles scatter red and green light while allowing other colors to pass. White moonbeams passing through such a particle cloud emerge blue.
In 1883, when the Indonesian volcano named Krakatoa exploded, it shot an ash cloud to the top of Earth's atmosphere. And the moon turned blue. Blue moons persisted for years after the eruption.
Other recent volcanic eruptions have also turned the moon blue. People saw blue moons after the eruption of Mount St. Helens in 1980 and Mount Pinatubo in 1991.