# April Runs

### … hikes, kickbikes, skis and more ## JUD13 – Friday 13th

Apparently a special day as the combination of Friday the 13th plus full moon only comes once in several decades (be prepared for the next one in 2049!).

Some arithmetic follows (thanks to Joseph Stromberg):

For a full moon to fall on a Friday the 13th, you need a confluence of three independent conditions — the moon phase (full), the day of the week (Friday), and the day of the month (13th). To get the probability of all three happening, you multiply the probably of each by each other.

On average, there are 12.37 full moons in a year and 365.25 days in a year (because of leap years). So the odds of any one day being a full moon are 3.39 percent. There are seven days in a week, so the odds of any one day being a Friday are 1 in 7, or 14.29 percent. There are 12 13th’s in a year (each month has exactly one). So the odds of any one day being a 13th is 12/365.25, or 3.29 percent.

To get the odds of any one day being all three of these things, simply multiply the percentages by each other. The result is .0159 percent. Then, to get the odds of a year having this happen, you multiply by 365.25 — so the odds of a year having a Friday the 13th full moon are just 5.81 percent. That means it happens, on average, every 20 years or so. Eveving run: 14.5 km