The Full Moon of February occurred at 8:49 a.m. CST this morning. Known as the Snow Moon, it is the second Full Moon of the northern hemisphere’s winter. As far as the lunar calendar is concerned, February is unique in that it is the only month in which is it not possible to have two Full Moons and it is that prospect of two Full Moons in a month that brings up an interesting historical oddity referred—incorrectly, as I’ll discuss in a few moments—as a Blue Moon. It is impossible for February to have two Full Moons, incidentally, due to the 29.5 day synodic period of lunar phases. Thus, the interval between Full Moons is over one day longer than the length of a typical February (and one-half day longer than a leap year February.)
But just what is a Blue Moon, anyway? Not what most people believe when you ask them for a definition. The earliest known usage of the phrase “blue moon”, however, appears to be a 1528 pamphlet entitled Rede Me and Be Not Wrothe in which the author wrote “Yf they say the mone is belewe We must beleve that it is true.” The expression seems to refer to something so absurd as to be unbelievable. According to common lore, however, a Blue Moon is usually described as the second Full Moon in a month, but in reality they are not so rare as to warrant the phrase “Once in a Blue Moon.” In fact, there are two Full Moons in a month about once every two years—hardly a rarity. Believe it or not, this particular usage of the phrase appears to stem from a misinterpretation of the original meaning and can be traced to the original Trivial Pursuit game of the 1980s.
The modern usage of Blue Moon appears to have its origin in the Maine Farmer’s Almanac. Editors at Sky & Telescope, with the assistance of several librarians, obtained 40 copies of the Almanac dating back to the early 1800s and found numerous citations that referred to Blue Moons, but not one of them referred to the second Full Moon of the month. In fact, the Blue Moons always occurred on the 20th – 23rd days of February, May, August, or November! These Full Moons always occur about one month before a seasonal change. A further nuance discovered with additional research revealed that the almanac definition relied on the use of the tropical year, which is measured from one winter solstice (Yule) to the next, instead of using the traditional calendar year. Most tropical years have 12 Full Moons—three per season—but occasionally there are 13 Full Moons with one season having four Full Moons.
Why is the third Full Moon significant? Because historically the name of the fourth Full Moon must be in accordance with the seasonal change as the name of that Full Moon relates to the impending equinox or solstice. Thus, the Full Moons of this winter are: the January (Moon After Yule) Full Moon, the February (Snow) Full Moon, and the March (Lenten) Full Moon. Occasionally, however, the first Full Moon of winter occurs just hours after the winter solstice and is therefore in December and not January. In those years, the occurrence of Full Moons will be: the December (Moon After Yule) Full Moon, the January (Snow) Full Moon, the February (Blue) Full Moon, and the March (Lenten) Full Moon. In both cases, the sequence of Full Moons occurs prior to the arrival of the vernal equinox and the start of spring. Thus, Easter won’t occur until after the first Full Moon of spring—referred variously as the Egg, Grass or Hare Full Moon.
A complete description of how Blue Moons are determined is quite convoluted, but is an interesting example of how calendar reform, religious observances (namely Lent and Easter), and astronomical events are joined together.
Read the full Sky & Telescope article for a detailed explanation of the calculation and how the editors traced the history of the Blue Moon definition.