The sky will sport a rare so-called blue moon on Friday. The event occurs only once in every three years or so and will happen next in January 2018.

You can read on the US Naval Observatory's web page that the clue moon is a second full moon within the same calendar month. Lunar cycle completes in 28 days, meaning two full moons can be seen in one month.

Humans know about the blue moon since the birth of the Roman calendar. A full moon was already there on July 2, and as the month is of 31 days, a new lunar period has begun. Andrew Fraknoi, chairman of the astronomy department at Foothill College, said it is nothing other than a mathematical oddity.

Fraknoi said the moon appears to have a blue hue because of certain substances in atmosphere, such as crystals or galactic dust.

An article in Sky and Telescope Magazine in 1946 officially termed it blue moon to rid the confusion among its readers about the name. The science behind a blue moon is very similar to the one that causes a typical full moon.

"The moon is in direct reflection from the sun, so the sun the earth and the moon are roughly in a straight line so the moon is acting like a mirror. It's right behind us so we get the full reflection", said Eugene Chiang, Professor of Astronomy, Earth and Planetary Science at UC Berkeley.