The phrase "once in a blue moon" is used to describe something that rarely happens. The term "blue moon" is associated with an extra full moon in a year or a season or a month. The time from one full moon to the next is approximately one month. (In fact, the word month and the word moon come from the same Old English word mona.) However, the time between full moons is actually a little shorter than an average month, so it takes a few extra days after 12 full moons to equal a full year. This means that most calendar years have 12 full moons, but every two or three years there will be 13 full moons in the year. One of these full moons is called a "blue moon".
Centuries ago, an extra full moon before the beginning of Lent was called a "belewe" moon. "Belewe" is an Old English word that can mean either "betrayer" or "blue". The date of Easter is calculated based on the full moon, so an extra full moon before Lent was probably intended to be called the "betrayer moon", but the meaning could easily have changed to "blue moon" among the largely uneducated populace of the time.
For centuries, full moons have also been given names such as the "planting moon" and "harvest moon" according to the time of year. The one that is called a "blue moon" varies depending on the source. A very popular source is the Farmer's Almanac. A season, summer for example, usually has three full moons. If a season had four full moons, the Farmer's Almanac called the third full moon in the season a "blue moon" so that the other names for the full moons would continue to apply to the correct time of year.
Many modern dictionaries define a blue moon as the second full moon in a calendar month. This is a fairly recent definition that is based on a misinterpretation of the Farmer's Almanac by Sky and Telescope magazine around the middle of the 20th century. An article was published that defined a "blue moon" as the second full moon in a calendar month. Around the end of the 20th century, Sky and Telescope discovered the error and published a correction, but after 50 years, the mistaken definition had become widely enough used that it had entered the dictionary.
Centuries ago, an extra full moon before the beginning of Lent was called a "belewe" moon. "Belewe" is an Old English word that can mean either "betrayer" or "blue". The date of Easter is calculated based on the full moon, so an extra full moon before Lent was probably intended to be called the "betrayer moon", but the meaning could easily have changed to "blue moon" among the largely uneducated populace of the time.
For centuries, full moons have also been given names such as the "planting moon" and "harvest moon" according to the time of year. The one that is called a "blue moon" varies depending on the source. A very popular source is the Farmer's Almanac. A season, summer for example, usually has three full moons. If a season had four full moons, the Farmer's Almanac called the third full moon in the season a "blue moon" so that the other names for the full moons would continue to apply to the correct time of year.
Many modern dictionaries define a blue moon as the second full moon in a calendar month. This is a fairly recent definition that is based on a misinterpretation of the Farmer's Almanac by Sky and Telescope magazine around the middle of the 20th century. An article was published that defined a "blue moon" as the second full moon in a calendar month. Around the end of the 20th century, Sky and Telescope discovered the error and published a correction, but after 50 years, the mistaken definition had become widely enough used that it had entered the dictionary.
