If you see a dude in an old-school hockey mask chasing a werewolf (or vice versa) on Friday, here's why: Friday the 13th coincides with a full moon. Hide your kids.
That said, only those living in Asia, Africa, Europe, South America — and those in the Eastern Time Zone — experienced the full moon on June 13. The full moon showed up at 12:11 a.m. ET, so anyone in time zone west of EDT saw the silver-dollar moon the day before, on June 12.
Friday the 13th hasn't fallen on the same day as a full Moon since Oct. 13, 2000, and it won't happen again until Aug. 13, 2049 — at least, if we're following Coordinated Universal Time (UTC), which is four hours ahead of EDT.
As Slate pointed out, though, anyone in New Zealand or the eastern tip of Asia will see a full moon on Friday, Jan. 13, 2017 (for the rest of the world, it will appear on Thursday, Jan. 12). Most of the United States will see the two events line up again on Friday, Sept. 13, 2019, though that full moon will fall on Saturday, Sept. 14, for most of the world.
To get a better visual picture of how moon visibility changes throughout the year, watch NASA's animation below.
Figuring out when you'll next see the two event coincide requires a moon schedule, a calendar and knowing what time zone you'll be in on a particular day. But you can figure out the basic odds of these events coinciding by doing some simple math.
Three things need to happen for a full moon to fall on Friday the 13th.
- The day has to be a Friday.
- The moon has to be full.
- The date of the month must be the 13th.
Figuring out the odds of a full moon on a Friday the 13th requires a little math.
The odds of any particular day being a Friday are around 14.3% (one in seven). The odds of a full moon on a particular day are about 3.4%. To get that number, divide the average number of full moons per year (12.37) by the average number of days in a year (365.25, thanks to leap year).
Now, to get the odds of any day being a 13th, take the number of 13ths in a year (12) and divide it by the average number of days in a year (365.25). The result is about 3.3%.
The next step is to multiply the following: 3.3% (the odds of any day being a 13th) x 3.4% (the odds of a full moon) x 14.3% (the odds of a day being a Friday). The result is around .016%. But to get the likelihood of such a day occurring in any given year, multiply that number by 365.25.That makes a 5.8% chance that in each year, a Friday the 13th will coincide with a full moon.
It's a small chance, but it's worth noting that it's not any lower than any other date of the year (excluding any 31sts, because not every month is 31 days long, so the calendar has fewer of them). The likelihood of a full moon occurring on, for example, Monday the 19th is also 5.8%.
Superstitions loom over full moons and Fridays that fall on the 13th of the month. To add to the weirdness, Mercury is currently in retrograde — a four-times-per-year phenomenon that makes it seem as though Mercury has stopped moving in space and is said to mess up everyone's ability to think clearly.
The mathematical explanation for why such events happen on the same day is simple enough, but the origin of the superstitions? Less so.
Full moons are said to make people act a bit wacky; perhaps the fluctuations of light crossing the moon's surface have an affect on the human psyche. Friday the 13th is said to bring bad luck. Whether you subscribe to that set of beliefs or not, some believe that people change their behavior on such days in order to minimize risk.
But no firm evidence exists that anything is stranger than usual when the moon looks like a coin in the sky, or that people are down on their luck on Friday the 13th. Scientists have examined both occasions and have come up with zilch.
That said, if you see any werewolves or run into a big scary guy named Jason, please tell us in the comments below.