Thirteen percent chance of a full moon on the 13th?

To quote Stevie Wonder, “Superstition ain’t the way.” This past Friday the 13th, was also the night of a full moon. This is the first full moon since January 13, 2006. That was 13 years ago. Coincidence? Not really, but it’s also not something too strange. 

For your sanity, you may want to skip this next paragraph as numbers are about to start flying everywhere. This is to the superstitious, or maybe even those that are just a little superstitious. The probability of this event having happened is mathematically fairly straightforward…

Math

There are 52 Fridays in a year, however, there can be 53 during a leap year. The average number of days in a month is 30.42. So, 52.07 (counting for the chance of fifty-three Fridays in a year) divided by 365.25 days is a 14.26 percent chance of a Friday on any given day of the year. There are only twelve ’13th’ days, so the chance of a day being the 13th is found in a similar fashion, and it comes out to be 3.29 percent. Multiplying these percents together gives 0.47 percent chance of having a Friday the 13th as a day on the calendar year or, an average of 1.711 a year. 

Plus more math

The math is not done. There are roughly twelve full moons in a calendar year, but sometimes there are thirteen. By a quick search (because there’s already enough math in this article), there are roughly 12.37 full moons in a year. That’s a 3.39 percent chance of any given day having a full moon. The chance of the 13th day of the month coinciding with a Friday, coinciding with a full moon? 0.02 percent, or an average of 0.06 days a year. That is once every 17.21 years, or a 13 percent chance of it happening every 2.24 years (that last fact really does not matter; in hindsight, most of these statistics are pretty useless). 

Disclaimer: there is a decent chance that the math here is not great.

Equals not so bad

If you skipped the last paragraph, here’s a summary: a Friday the 13th on the same day as a full moon is bound to happen approximately every 17 years. Having one after 13 years? Not that outlandish, but still fascinating. The next time it will happen is 2049. That’s before considering the fact it’s a Harvest Moon, which would fall on Friday the 13th once every 206 years or so.

If you are superstitious, maybe this can give you some post-Friday the 13th comfort. If no ‘cat-astrophe’ prevents this article from being published, then maybe Friday the 13th is not as bad as it seems.

Onwards to 2049!