We all are pretty much aware of this phrase, ‘once in a Blue Moon.’ It comes from the fact that a Blue Moon is super rare considering it’s the second full moon that occurs in a month.
Blue moons are uncommon, rising once every two or three years, but a blue moon in 2020 seems to fall exactly on Halloween, October 31.
It’s not called a ‘Blue Moon’ because of the colour but when it occurs. The time between one full moon and the next is close to the length of a calendar month. So, the only time one month can have two full moons is when the first full moon happens in the first few days of the month. This happens every two to three year. The last Blue Moon by this definition of the term happened on March 31, 2018.
Squeezing a second full moon into a calendar month doesn’t change the physical properties of the moon itself, so the color remains the same.”
Sometimes, however, the moon does look blue. But it has to do with atmospheric conditions and not calendar time. A famous example occurred in 1883, after the volcano Krakatoa erupted. The moon appeared blue because of all the dust from the volcano that rose into the air.