I was always vaguely aware that it took some calculation to establish the date for Easter. However, I really had no idea about paschal moons, golden numbers etc
The popular explanation for how the date for Easter is established each year is that it is the first Sunday after a full moon occuring on or after the vernal equinox.
But this calculation would be affected by time zones and other factors.
In pricipio omnes creature viruerunt, in medio flores floruerunt; postea viriditas descendit. Et istud vir preliator vidit et dixit: Hoc scio, sed aureus numerus nondum est plenus
Hildegard of Bingen appears to have been the first to use the phrase, “golden numbers” in the Ordo Virtutum, a morality play from 1151. The calculation of the Easter date is actually based on the Metonic Cycle, a period of 19 years during which the phases of the moon repeat exactly. This means that Full and New Moons for that period can be predicted. This cycle can then be translated to the Julian calendar, and then used to predict Easter with the calculation of the Golden Number and the Dominical Letter.
The Golden Number is a number assigned to each calendar year that maps on to the Metonic Cycle. There is an actual calculation for the number to be assigned, but my head begins to swim a little. Even more complex, Dominical letters (A,B,C etc.) are assigned to the days. Suffice it to say that there are tables (like algebra and ephemera tables) where these numbers are listed. (see for example http://home.telepath.com/~hrothgar/lunar_almanac.html)
Maybe I should let Wikipedia do the heavy lifting:
- Nineteen civil calendar years are divided into 235 lunar months of 30 and 29 days each (the so-called “ecclesiastical moon”.)
- The period of 19 years (the metonic cycle) is used because it produces a set of civil calendar dates for the ecclesiastical moons that repeats every nineteen years while still providing a reasonable approximation to the astronomical facts.
- The first day of each of these lunar months is the ecclesiastical new moon. Exactly one ecclesiastical new moon in each year falls on a date between March 8 and April 5, both inclusive. This begins the paschal lunar month for that year, and thirteen days later (that is, between March 21 and April 18, both inclusive) is the paschal full moon.
- Easter is the Sunday following the paschal full moon.
So, the paschal moon is one of the ecclesiastical moons, this one associated with Easter, ‘paschal’ from the Greek and related to the Hebrew word, ‘pesach’ meaning ‘passover’.
Sometimes, a muddled mind produced by confusing numbers can bring on an altered state akin to enlightenment, but I would rather listen to Hildegarde:
Ordo text translation and source: In the beginning all creation was verdant, flowers blossomed in the midst of it; later, greenness sank away. And the champion saw this and said:”I know it, but the golden number is not yet full). ( from http://www.oxfordgirlschoir.co.uk/hildegard/ordovirtutumtext.html)
Attribution: Last Supper fresco from Kremikovtsi Monastery, Bulgaria, 16th century AD By Edal Anton Lefterov (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)%5D, via Wikimedia Commons
Full Moon: By Kevin Burkett from Philadelphia, Pa., USA (Full Moon) [CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0)%5D, via Wikimedia Commons