What is a blue moon?

A blue moon rises over the Temple of Apollo in Ancient Corinth, Greece on August 30, 2023. | Photo credit: AFP

The blue moon is a common example of an invented tradition — something that someone has claimed is an old, time-honored custom, but in fact was invented more recently. The second full moon in the same month is usually called a blue moon. But there are other ways the moon can be blue, such as literally.

For example, on April 5, 1815, Mount Tambora in contemporary Indonesia produced the most powerful volcanic eruption in history. The previous year, the Mayon volcano in the Philippines had a powerful eruption. The effects of these volcanoes, combined with other climate factors, lowered the Earth’s temperature by 0.4-0.7 degrees Celsius in 1816, making it the year without a summer. Dust and other small particles in the air could cause the moon to appear blue, as described in the poem ‘Alastor’ written by Percy Bysshe Shelley that fateful year.

Following the eruption of the Krakatoa caldera in 1883, the London-based Royal Society recorded that the Moon appeared blue and the Sun green in many parts of the world.

The use of ‘blue moon’ as a long-term metaphor appeared in the early 1820s, and was first used to describe the second full moon of a month in a Farmer’s Almanac published in the US in 1937. One such blue moon will occur in August 2023.