Friggatriskaidekaphobia

Here’s the news from the evening sky:

Not much: when the Young Moon comes out, it will be at its low November angle; and most of the planets are still on the other side of the Sun.

So we could talk instead about Martinmas (Nov. 11, today), which is a cross-quarter day in Scotland, and what is a cross-quarter day? – but that was explained on Groundhog Day.

It’s more urgent to talk about Friday November the 13th, so that you will be on the alert not to make bets or walk under ladders. This is the third Friday the 13th in this year, and I hope you had no ill luck on February 13 or March 13.

When I used to list it in the Astronomical Calendar, my paragraph said: .”..supposed to be doubly unlucky because the day and the number are unlucky. (In South America the unlucky day is Tuesday and in Italy the unlucky number is 17. In Iran women stay outdoors on the 13th day of the year to avoid bad luck.)” I’d be glad for anyone to correct me on any of that or give further sociological information.

In every year there is at least one Friday-the-13th. Years with one of them, and years with two, each occur from 42 to 44 times per century. Of years with three there are only 14 or 15 per century.

2009 Jan 1=Thu
2012 Jan 1=SUN
2015 Jan 1=Thu
2026 Jan 1=Thu
2037 Jan 1=Thu
2040 Jan 1=SUN
2043 Jan 1=Thu
2054 Jan 1=Thu
2065 Jan 1=Thu
2068 Jan 1=SUN
2071 Jan 1=Thu
2082 Jan 1=Thu
2093 Jan 1=Thu
2096 Jan 1=SUN
2099 Jan 1=Thu

As this table I’ve just calculated shows, most of them are, like this year, common (non-leap) years beginning with Thursday; because that makes Feb. 13 a Friday, it has to happen again in March. The rarest kind of 3-Fri-13 year is the leap-year that begins with Sunday, such as 1984, 2012, 2040, 2068, 2096, and 2108.

Jean Meeus in his Mathematical Astronomy Morsels III, page 365, has a chapter called “Friggatriskaidekaphobia,” “Friday-thirteen-fear.” (Frigga is one version of the name of the Norse goddess whose day Friday is; tris-kai-deka is Greek, “three-and-ten.”) In reliable Meeus style he excavates more patterns. For instance in some years, such as 2007 and 2013, two Fridays-13 are 13 weeks apart, and in 2012 all three were. 2012 was a year when Frigga did her most to trip us up, and we’re lucky to have lived through it.