Did you know that tonight, July 31st is a blue moon? It is! And for those astronomically challenged, a "blue moon" is the third full moon in a astronomical period (season).
A normal year has four astronomical periods (seasons) of spring, summer, fall, and winter. Each are ~3 months long and have three full moons. When one of the seasons has four full moons (about every 2-3 years) the extra full moon is called a Blue Moon. Hence, the old expression. "That only happens in a blue moon." That is, not often.
A visually blue-colored moon is actually pretty rare. Rather, the atmosphere through which a moon rises affects the way it looks. When the atmosphere is filled with dust or smoke particles wider than 0.7 microns, they scatter red light making the Moon look blue. This can happen after a dust storm, forest fire or a volcanic eruption.
Eruptions like on Mt. Krakatoa, Indonesia (1883), El Chichon, Mexico (1983), Mt. St. Helens (1980) and Mount Pinatubo (1991) resulted in blue moons.
So all being said, go outside tonight and enjoy the blue moon. You can even howl if you like. Go science!
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