The term “Blue Moon” has its roots in folklore and mythology. In the 1940s, the Farmers’ Almanac, a publication that has been in print since 1792, began using the term “Blue Moon” to describe the second full Moon in a calendar month. The almanac’s editors believed that this extra full Moon was a rare event that deserved a special name.

When the Moon is viewed through a layer of particles in the atmosphere, the scattered blue light is what reaches our eyes, giving the Moon a blue appearance. The exact shade of blue can vary depending on the size and type of particles present, as well as the amount of dust or smoke in the atmosphere.

However, the term “Blue Moon” is also often used to describe a more common occurrence: a second full Moon in a single calendar month. This happens because the lunar cycle, also known as a synodic month, is approximately 29.5 days long, while the calendar month is usually 30 or 31 days. As a result, every 2.7 years, an extra full Moon appears, which is sometimes called a Blue Moon.