There are blue moons, an uncommon additional full moon in a month, and there is the hunter's blue moon - an even rarer occurrence.
For most years, we only experience one full moon per month. In the event of a blue moon, one of the months experiences two full moons, bringing the total to thirteen a year. The term traces its origins from the Maine Farmer's Almanac, a guidebook that contains meteorological and astronomical cues to help the farmers track the season.
In the book, it explains that the blue moon is actually the third moon in a season that had four full moons, instead of the usual three per quarter. It offers a correction in estimating the length of the season, and thus the planting schedules.
Unfortunately, a misunderstanding in a March 1946 article simplifies the rule, incorrectly, as the second moon in the month.
A Hunter's Blue Moon For Whole World
Blue moons occur at least once every two or three years. However, astronomers have been following a 19-year cycle for the moon's movement around the Earth. Known as the Metonic cycle, this period predicts the recurrence of the moon's position for every day of the year. Greek and Hebrew calendars follow the cycle in computing for the date of Easter, with the Babylonians following a similar 19-year cycle since the sixth century BC.
Now that a blue moon is expected to appear on Halloween, October 31, the Metonic cycle proposes that it should happen once every nineteen years. This full moon that will fall on Halloween is also traditionally called the Hunter's Moon - the autumn moonlight guiding hunters in finding food stocks to last them throughout the winter.
Unfortunately, the Metonic cycle is not perfectly synchronized with the Gregorian calendar we use today. Precisely speaking, the Metonic cycle is off by 1 hour, 27 minutes, and 33 seconds - requiring a leap day once ever 219 years. In comparison, the Gregorian calendar requires correction in the form of a leap day once every four years.
Although a Hunter's Blue Moon was visible for some parts of Earth nineteen years ago, 2001, the last full moon of this kind that appeared to most parts of the world was actually back in 1944 - 76 years, or four Metonic cycles ago. Unfortunately, South Pacific regions such as parts of Australia and New Zealand, won't be a part of this event. They will, however, get their own blue moon after another 19 years - in 2039!
An Actual Blue Moon
It is important to note that the term "blue moon" simply refers to the second full moon in a month, not necessarily that the moon we will see will be bluish in color. However, with the right atmospheric conditions - mostly the presence of particles in the air - has made the moon appear blue a couple of times throughout recorded history.
One of the oldest, and most significant events to create a literal blue moon was the eruption of Krakatoa in 1883. The gigantic volcano exploded in what was among the deadliest and most destructive events in history. The entire island of Krakatoa, as well as its neighboring islands, were all wiped off the face of the earth - leaving a large caldera in its place. Aside from shockwaves felt all around the world, the explosion spewed materials into the atmosphere that turned the moon blue for almost two years after the event.