STARGAZERS can look forward to an “unusual” celestial event next month when a Blue Moon rises in the sky.

Blue Moons are defined as the second full moon in a calendar month.

Seeing two full moons in a month is rare as there are about 29.5 days between full moons, according to NASA. 

The cosmic display happens seven times every 19 years, which means the world will not see the next one on October 31 until 2039.

What makes this event even rarer is that it will be seen in all parts of the world for the first time since World War II.

Echo: Our lunar neighbor will not shine blue, but the name is given because it is the second full moon to appear in the same monthOur lunar neighbor will not shine blue, but the name is given because it is the second full moon to appear in the same month

Although blue moons occur once every two or three years, they are even more rare on Halloween, according to AccuWeather meteorologist Brian Lada.

He told “After the blue moon on Oct. 31, 2020, trick-or-treaters will need to wait until 2039 to see the next blue moon on Halloween."

The next full moon on October 1 will be the harvest moon.

What is a Blue Moon?

Despite its name, a blue moon isn't actually blue, it looks like your average grey-coloured full moon. 

But a moon can still appear blue if the atmosphere is filled with dust or smoke particles from a volcanic eruption. 

The idea of a Blue Moon as the second full moon in a month comes from an article in the March 1946 issue of Sky and Telescope magazine, EarthSky reports.

This issue published an an article called Once in a Blue Moon by James Hugh Pruett, who referred to the 1937 Maine Farmer’s Almanac, but with a simpler definition.

"Seven times in 19 years there were – and still are – 13 full moons in a year," he wrote.

Echo: A blue moonA blue moon

"This gives 11 months with one full moon each and one with two.

"This second in a month, so I interpret it, was called Blue Moon."

A moon can turn blue, but the signing is very rare.

EarthSky notes that sky conditions must align perfectly and contain large particles of dust or smoke to reflect a hue – making it unpredictable to know when one will rise.

Altogether, there will be 13 full moons in 2020, another rarity because most years only see 12.

When are the remaining full moons in 2020?

September 21 - Corn moon

October 15 - Harvest moon

October 31 - Blue moon

November 30 - Beaver moon

December 29 - Cold moon