Washington, Aug 31 (IBNS): People across the globe witnessed a rare celestial phenomenon on Friday night better known as the 'blue moon'.

A blue moon can refer to either the third full moon in a season with four full moons or the second full moon in a month.

People witnessed the first full moon of the month on August 2.

Sky gazers will have to wait till July 31, 2015 to see the next 'blue moon'.

Most years have twelve full moons that occur approximately monthly.

In addition to those twelve full lunar cycles, each solar calendar year contains roughly eleven days more than the lunar year of 12 lunations.

The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon.

The term "blue moon" comes from folklore.

Different traditions and conventions place the extra "blue" full moon at different times in the year.

In the Hindu calendar, this extra month is called 'Adhik (extra) masa (month)'.

It is also known as purushottam maas, so as to give it a devotional name.