They are a rarity, hence the term ‘blue moon’. However, they aren’t actually blue in color – unlike the misleading name! A blue moon is simply an additional full moon that appears in a subdivision of a year. The term has traditionally referred to an ‘extra’ moon, where a year which normally has 12 moons has 13 instead.
One lunation (an average lunar cycle) is 29.53 days. There are about 365.24 days in a tropical year. Therefore, about 12.37 lunations (365.24 days divided by 29.53 days) occur in a tropical year. If you follow the Gregorian calendar, there are 12 months in a year, and normally there is one full moon each month. Each calendar year contains roughly 11 days more than the number of days in 12 lunar cycles. The extra days tend to accumulate, so every two or three years there is an extra full moon.
July 2015 will be featuring two full moons. The first full moon will be July 2, the second will appear on the last day of the month, July 31.
The last known blue moon occurred in 2012, when both September and August boasted two full moons in certain time zones. Another blue moon in expected to appear all the way in January 2018.
Interestingly enough, blue-colored moons can be a possibility. Certain atmospheric conditions can lead to 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.”
To read up more info on the blue moon check out: Blue Moon Info