Blue Moons and the Seasons
Rays emanating from Tycho and other craters are particularly prominent when the Moon is full.
Courtesy António Cidadão.
Several clues point to a strong connection between the almanac's Blue Moons and the four seasons of the year. All of the listed Blue Moons fall on the 20th, 21st, 22nd, or 23rd day of November, May, February, or August. These dates fall about a month before the Northern Hemisphere winter and summer solstices, and spring and fall equinoxes, respectively, which occur on similar day numbers.
Although the idea of a seasonal pattern suggested itself to us immediately, verifying the details required a lot of detective work. We found that the Blue-Moon definition employed in the Maine Farmers' Almanac is indeed based on the seasons, but with some subtle twists.
Instead of the calendar year running from January 1st through December 31st, the almanac relies on the tropical year, defined as extending from one winter solstice ("Yule") to the next. Most tropical years contain 12 full Moons — three each in winter, spring, summer, and fall — and each is named for an activity appropriate to the time of year (such as the Harvest Moon in autumn). But occasionally a tropical year contains 13 full Moons, such that one season has four rather than the usual three.
These tables appeared in the 1939 Maine Farmers' Almanac and show that the beginnings of the seasons were fixed by the 'R.A.M.S.' (right ascension of the mean Sun). The almanac lists separately the 'Turns and Crosses' of the apparent Sun, which executes a 'turn' from northward motion in declination to southward (or vice versa) at the solstices and 'crosses the line' (the celestial equator) at the equinoxes. Click on the table for a larger view.
Courtesy Maine Farmers' Almanac.
Today we usually mark the beginning of the seasons when the Sun's celestial longitude passes 0° (spring), 90° (summer), 180° (autumn), and 270° (winter). The Sun appears to move along the ecliptic at a variable rate because of the Earth's not-quite-circular orbit, so the seasons defined this way are not equal in duration. Another approach uses the dynamical mean Sun or fictitious mean Sun — imaginary bodies that move along the ecliptic and the celestial equator, respectively, at a constant rate and produces seasons of equal length. The Maine almanac defines the seasons using this alternative method.
The almanac also follows certain rules laid down as part of the Gregorian calendar reform in 1582. The ecclesiastical vernal (spring) equinox always falls on March 21st, regardless of the position of the Sun. Lent begins on Ash Wednesday, 46 days before Easter, and must contain the Lenten Moon, considered to be the last full Moon of winter. The first full Moon of spring is called the Egg Moon (or Easter Moon, or Paschal Moon) and must fall within the week before Easter.
When is the Moon 'blue,' in a calendrical sense? According to the Maine almanac, a Blue Moon occurs when a season has four full Moons, rather than the usual three. This type of Blue Moon is found only in February, May, August, and November, one month before the next equinox or solstice. According to modern folklore, a Blue Moon is the second full Moon in a calendar month. This type of Blue Moon can occur in any month but February, which is always shorter than the time between successive full Moons.
S&T illustration.
At last we have the "Maine rule" for Blue Moons: Seasonal Moon names are assigned near the spring equinox in accordance with the ecclesiastical rules for determining the dates of Easter and Lent. The beginnings of summer, fall, and winter are determined by the dynamical mean Sun. When a season contains four full Moons, the third is called a Blue Moon.
Why is the third full Moon identified as the extra one in a season with four? Because only then will the names of the other full Moons, such as the Moon Before Yule and the Moon After Yule, fall at the proper times relative to the solstices and equinoxes.
