What Is a Blue Moon, and How Rare Is It?


Stockbyte/John Foxx

Before Curiosity landed on Mars, the main topic of discussion among amateur astronomers was the blue Moon that is scheduled to make an appearance on August 31.

The widely accepted definition of a blue Moon is a full Moon that appears twice within a calendar month. But that description is actually based on an error that was printed in a 1946 issue of Sky and Telescope magazine.

The writer had misinterpreted the term when it was used to describe lunar phases in some old issues of the Maine Farmers' Almanac dating back to the mid-1800s.

What that writer didn’t understand was that the Moon phases detailed in the book were based on a seasonal calendar, in which the beginnings of spring, summer, fall, and winter are determined by the dynamical mean Sun. Each season traditionally has three full Moons; during the rare season where a fourth full Moon made an appearance (usually approximately every 2.7 years), the third such Moon was termed the “blue” Moon.

Wait, the Third?

As for why it’s the third Moon that gets called out as the oddball, it’s simply so that the fourth Moon still falls on the appropriate seasonal date relative to the equinox (Harvest Moon, Yule Moon, etc.). But explaining the ecclesiastical rules of determining the dates of Easter, Lent and thus the Vernal Equinox tends to make laymen’s eyes glaze over, so even though several corrections to the original Sky and Telescope piece have since been printed, the easier-to-understand “two full Moons within a month” explanation has remained the blue Moon standard.

By the way, blue Moons usually only have an azure tint when there is some sort of major event that affects the atmosphere, such as a volcanic eruption or forest fire.