The world is about to witness another rare celestial phenomenon after the transit of Mercury or the passing of the planet across the Sun that happened a few weeks ago. A 'blue moon' will happen this weekend on May 21 and it will be visible to the naked eye.

A blue moon is defined as the second full moon in one calendar month. Normally there is only one full moon in a month. The occurrence of a 'blue moon' rarely happens, because of the rarity this event gave birth to the phrase 'once in a blue moon'. The full moon that will rise in the sky on May 21 is not your typical full moon, but a blue moon according to a report by Space.com.

Although the name confuses a lot of spectators, it doesn't necessarily mean that the moon will turn into a deeper shade of blue. It only means the world will witness two full moons within one lunar calendar.

Blue moon is also defined as an extra full moon within a complete season in between the equinox and solstice. The fourth full moon in one season is also called 'blue moon'.

"There are 235 full moons in 19 calendar years, yet only 228 calendar months (or 76 three-month seasons). Therefore, it's inevitable that 7 out of 19 years will feature two full moons in one calendar month. And it's also inevitable that 7 out of 19 years will have four full moons in one season" said Bruce Mclure an astrology expert, in an interview with Earthsky.Org.

According to astronomers, this will be the third time a 'blue moon' will appear in three years. This event is closely monitored by astronomers and researchers because it is an odd occurrence in the standard lunar calendar. The next 'blue moon' is bound to happen in 2019.The last 'blue moon' was witnessed last July 2015 according to Mirror.

Photos of the 2015 blue moon:

 

This rare celestial phenomenon can be observed by the naked eye and will not require any type of special equipment. But astronomers warn about the presence of clouds which could block the view of the oddly-timed appearance of the extra full moon.

 

Tags blue moon, moon, season, once in a blue moon, Telescope, lunar

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