Saturday, August 1, 2015

Lunation Calendar

There's usually one fully sunlit moon per month. Yesterday a second full moon appeared before July ceded our days to August.

Full Moons of July 2015

An occasional extra full moon is a function of the astronomical regularity of lunation and the papally decreed Gregorian Calendar introduced by Pope Gregory XIII in 1582.
Lunation - One lunar cycle with average duration of 29.53 days.
Our word moon comes from the Old English word mona.
"Even the word month is likely derived from the word moon and an Indo-European word, mê, which means to measure."
Howard Markel
By measuring diurnality from the dark new moon to the sunlit full moon, our ancestors developed monthly calendars. Religious rituals like the Christian Easter were dutifully computed and observed around predictable lunar cycles. The Islamic calendar is purely lunar. The Christian Gregorian calendar is quasi-lunar.

Widely adopted, the Gregorian calendar has 12 months in a year. By comparison there are 12.37 lunations per year.
365.24 days/year ÷ 29.53 days/lunation = 12.37 lunations/year
The Gregorian calendar has exactly 12 months in a year, so an accumulation of .37 lunations occurs each Gregorian year. The accumulations of fractional lunations beyond the counting number 12, accounts for the occasional extra moon in a year.

An extra moon, whether considered over the time frame of a year, or over a season, or over a month, is colloquially called a blue moon to signify its rarity.
The blue moon isn't really an extra moon or blue in color, rather it follows as a side effect from a human construct of time - the Gregorian calendar.

Moon and Antelao
Image: Marcella Giulia

“Her antiquity in preceding and surviving succeeding tellurian generations: her nocturnal predominance: her satellitic dependence: her luminary reflection: her constancy under all her phases, rising and setting by her appointed times, waxing and waning: the forced invariability of her aspect: her indeterminate response to inaffirmative interrogation: her potency over effluent and refluent waters: her power to enamour, to mortify, to invest with beauty, to render insane, to incite to and aid delinquency: the tranquil inscrutability of her visage: the terribility of her isolated dominant resplendent propinquity: her omens of tempest and of calm: the stimulation of her light, her motion and her presence: the admonition of her craters, her arid seas, her silence: her splendour, when visible: her attraction, when invisible.”
James Joyce, Ulysses