TORONTO —
Trick-or-treaters around the world will be treated to more than candy this Halloween as a rare blue moon is expected to illuminate the sky Saturday night. 

The spectacle occurs when two full moons coincide in the same calendar month, and this year it will be visible in every time zone.

WHY IS IT CALLED A BLUE MOON?

Despite its name, the moon’s colour is not actually blue. Instead, a blue moon is a common way to refer to the second full moon in October, as the first is typically referred to as a harvest moon.

However, there is also an alternate definition of a blue moon that says it is the third full moon of four occurring in a single season, according to NASA.

HOW RARE IS A BLUE MOON?

Contrary to the saying, blue moons are not all that uncommon as they occur every 2.5 years in different time zones.

What makes this blue moon so rare is that it will be seen in every time zone. The last time that happened was in 1944, according to the Farmer’s Almanac.

On top of that, it’s happening on Halloween, making the phenomenon that much rarer.

On average, blue moons appear on Halloween once every 19 years and are only visible to those who live in certain time zones. The last full moon that appeared on Halloween was on Oct. 31, 2001 and it was only seen by people in central and Pacific time zones.

“Any time the moon is technically full on October 31, it will also have to be a blue moon because the lunar cycle is only 29.5 days long,” the Farmer’s Almanac’s website states.

People who live towards the east of the international date line will see this full moon on Nov. 1 and will have a blue moon at the end of November as it will be their second full moon of the lunar cycle.

Blue moons are a direct result of the lunar cycle. Since the cycle is shorter than most calendar months — lasting only 29.5 days — blue moons are bound to eventually occur in October.
The next blue moon is expected fall on Halloween in 2039.