Just when is the 12th?

The Orange parades yesterday passed without serious sectarian trouble. I mentioned some of the myths around this celebration yesterday, particularly that it was a celebration of the Battle of the Boyne on the 12th July 1690.

But it wasn’t on 12 July. So how did this confusion arise?

At that time, there were two calendars in use in Europe. The original was the Julian calendar, but as this “ran slow” compared to easily observable celestial events, such as the solstices and equinoxes, it gave the “wrong” days for religious ceremonies. In particular, Easter, the most important event in the Christian year was clearly incorrect. Most religious festivals were on fixed days, such as 25 December for the Nativity. But Easter is different, and has to be calculated.

The Council of Nicea in 325 AD, set the method: Easter is on the Sunday after the paschal full moon, that is the full moon which falls on or soon after the spring equinox. The Church decided that the spring equinox is on 21 March (though more often it is on 20 March), and the full moon is 14 days after a new moon.

To solve the difficulty of Easter falling on the “wrong” date needed reform of the calendar; this was undertaken by Pope Gregory, and is named after him. A method of calculating Easter was then devised, and remains current. Eastern Orthodox churches still use the Julian calendar for dating Easter, so the same event is celebrated on different days. Though suggestions for a fixed date for Easter are regularly advanced, a fixed date has not been achieved.

When Britain changed to the Gregorian calendar in 1752, a method of determining Easter devised on Golden Numbers and Sunday Letters was devised, though it gives identical results to the Gregorian method. You could say that it uses entirely artificial full moons.

Anyway, it was the Easter problem that required sorting using a reformed calendar. The differences between the old Julian calendar and the new Gregorian calendar are not fixed, rather they increase by 1 day every 128 years.

So, in 1690 there were 10 days difference, with 1 July being then equivalent to 11 July. In 1752, there were 11 days difference, so 1 July in the Old Style was equivalent to 12 July in the New Style. This coincidence with the Battle of Aughrim on 12 July 1691 Old Style and the New Style date then for the Battle of the Boyne probably accounts for the amalgamation of the two events, and the widespread belief that the Boyne was actually on 12 July.

There are now 13 days difference; so to recalculate the date of the Boyne today would mean it was on (the equivalent of) 14 July.

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