Residents should take an extra look at the sky this Halloween night as there will be a full moon out. 

This will be the second full moon of the month, making it a "Blue Moon." Though the moon doesn't actually change colour, it is somewhat unique to have two fulls in one month, which is where the expression "once in a Blue Moon" comes from. 

Astrophysicist Neil deGrasse Tyson explained that the moon rotates through phases on a 29.5 day cycle. He adds that this is why some months will have multiple full moons while others only get one. 

"This year, a Full Moon lands on Halloween," wrote Tyson. "With a 29.5-day cycle of phases, any time you squeeze two full moons into a month, the second one is called the 'Blue Moon.'"

"The expression 'Once in a Blue Moon' is colloquial for 'The thing hardly ever happens.' But If you meant it literally, then that thing will occur every two or three years. So, like most cosmic happenings, Blue Moons aren't entirely rare, even if we want to think they are."

Tyson would poke fun at February, noting that with the month having only 29 days at the max, it could never experience a Blue Moon. 

He went on to say the next Blue Moon to land on Halloween would occur until 2039.