WASHINGTON, July 1 (UPI) -- Blue moons aren't actually blue, that's just the name for the second full moon in a single calendar month, an uncommon occurrence.
July 2015 will feature two full moons, the second being a blue moon. The first full moon will be July 2, the second will appear on the last day of the month, July 31. The last blue moon occurred in 2012, when both September and August boasted two full moons in certain time zones. Another blue moon will appear in January 2018.
The definition of a blue moon as the second full moon in a calendar month is a modern invention. Only a few years ago, blue moons were the name given to the third of four full moons in a single season.
The phrase "once in a blue moon," of course, connotes rarity. But with so many ways to term a full moon blue, they're relatively common. The only truly rare blue moon is the moon that actually appears blue -- which isn't an impossibility.
Certain atmospheric conditions can produce a blue-tinted moon, the most likely of which is caused by a sizable volcanic eruption. In 1883, when the Indonesian volcano Krakatoa violently erupted, plumes of ash ascended to the top of Earth's atmosphere, turning the moon blue (and sometimes green).
High-altitude layers of ash particles absorbed light from the red end of the spectrum, causing moonbeams to appear blue and green. Bluish moons persisted for several years.
According volcanologist Scott Rowland of the University of Hawaii, the phenomenon also caused the sun to appear lavender in color, and precipitated "such vivid red sunsets that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration."
The Farmer's Almanac refers to July's full moons as thunder moons, named so for the month's propensity to host intense afternoon rain storms. The almanac also calls the first full moon a buck moon, as June marks the time of year when young deer begin to sprout antlers.