Buzz
Stockbyte/John Foxx
Prior to the arrival of Curiosity on Mars, the primary subject among amateur stargazers was the Blue Moon expected to appear on August 31.
The commonly accepted explanation for a Blue Moon is a full Moon that happens twice within the span of a single month. However, this definition originates from a mistake made in a 1946 edition of Sky and Telescope magazine.
The misunderstanding arose when the term was used to describe certain lunar phases in older editions of the Maine Farmers' Almanac from the mid-1800s.
What the writer failed to grasp was that the Moon phases referenced in the book followed a seasonal calendar, where the start of each season—spring, summer, fall, and winter—is determined by the dynamical mean Sun. Typically, each season has three full Moons; when an exceptionally rare fourth full Moon occurs (approximately every 2.7 years), the third full Moon is designated as the “blue” Moon.
Wait, the third?
The reason it’s the third Moon that’s singled out as unusual is because it ensures that the fourth Moon aligns with the correct seasonal date, such as the Harvest Moon or Yule Moon. However, explaining the complex ecclesiastical rules for determining the dates of Easter, Lent, and the Vernal Equinox tends to lose most people's attention, which is why the simpler “two full Moons within a month” definition has stuck as the standard for a blue Moon, even though multiple corrections to the original Sky and Telescope article have been published.
Just so you know, blue Moons typically only appear blue when there’s a major atmospheric event like a volcanic eruption or forest fire that affects the sky’s appearance.
