Blue moon

Blue Moon is a term used in astronomy to describe a phenomenon where there are two full moons in one calendar month. This event is relatively rare, hence the phrase "once in a blue moon" to describe something that doesn't happen often.

Origin of the Term

The term "Blue Moon" has been in use for several centuries, although its meaning has changed over time. The earliest recorded usage of the term is in a pamphlet published in 1528 by William Barlow, Bishop of Chichester, titled "Rede Me and Be Not Wroth". In this context, a "Blue Moon" was used to signify a rare or absurd event.

Modern Usage

In modern usage, a "Blue Moon" is defined as the second full moon in a calendar month. This definition was popularized by the Farmer's Almanac in the 19th century. Because the lunar cycle is approximately 29.5 days, most months only have one full moon. However, if a full moon occurs on the first or second day of a month, there is a chance a second full moon could occur within the same month, resulting in a "Blue Moon".

Frequency

On average, a "Blue Moon" occurs once every 2.7 years. This is because the lunar month is shorter than most calendar months, and some months may have no full moon at all. The next time two full moons will occur in the same month is in August 2023.

Cultural Significance

The phrase "once in a blue moon" is used colloquially to mean a rare event. It has been used in literature, music, and popular culture to signify rarity or specialness. The term has also been used in various scientific contexts, such as in the field of astrophysics to describe rare celestial events.

See Also

• Lunar cycle
• Full moon
• Farmer's Almanac