Most of us have heard the phrase "once in a blue moon" at one time or another. This phrase is used to describe an occurrence or event that is considered rare. It turns out that blue moons are fairly rare, but the definition of what one is has evolved over the decades.
A full moon is seen approximately every 29.5 days. This makes it so that each month of the year will usually see one full moon occur within it. This also means that each season of the year will have 3 full moons, given that each season lasts 3 months.
The first “accepted” definition of a blue moon happens when an extra or fourth full moon occurs during a season of the year. In a season where there are 4 full moons, the 3rd full moon is deemed the blue moon. This definition came to be during the late 1930’s. However, in recent years, the term blue moon has transformed into the times when a second full moon occurs within one month. Both definitions are acceptable, but the second one has become the more popular version and will be used for the rest of this article.
How rare is a blue moon? A blue moon occurs once every 2 or 3 years. The last blue moon occurred on March 31st of 2018. The next blue moon occurs on Halloween this year. The last time a blue moon occurred on Halloween in all the time zones in the U.S. was back in 1944, but the Central and Western time zones saw one back in 2001.
Unlike the name suggests, a blue moon is not actually blue, but it will instead be its normal color. The moon appears in different hues when there is an increased amount of particulate matter or thin cloud cover hanging in the atmosphere.