STARGAZERS can look forward to an “unusual” celestial event later this 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.

Although blue moons occur once every two or three years, they are even more rare on Halloween.

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.

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?

  • October 31 - Blue moon
  • November 30 - Beaver moon
  • December 29 - Cold moon