A rare "blue moon" was visible last night. Twitter users tweeted photos of the "blue moon", which began going viral online. The rare celestial phenomenon, which occurs when there are two full moons in one month or is either the fourth full moon in a season, will not happen again until 2015. The "blue moon" is not in really blue-colored The term refers to its rarity.

Blue moons exist due to our calendar months being imperfectly synched to the lunar months. It takes 29.5 days for the moon to orbit Earth, during which sky watchers observe all of its phases - resulting in the occasional two full moons within a single month.

The 2nd full moon of the month though will not in fact be blue, although moons have been known to be portrayed in different colors due to various atmospheric conditions. For example after volcanic activity or forest fires, the moon can appear to take on a bluish or sometimes lavender coloration. This is because of soot and ash particles deposited high up into the Earth's atmosphere.
Already this year there was a Super Moon in May when the full moon appeared much larger than normal due to its closeness to the Earth.

The saying "once in a blue moon" came to light in 1946 when amateur astronomer James Hugh Pruett wrote about the two full moons happening in one month for a magazine He coined the rare happening "once in a blue moon", thus popularizing the term.
The full blue moon coincided with the private funeral for iconic astronomer Neil Armstrong, who died last Saturday aged 82.