Tomorrow's Friday the 13th full moon is so rare, it won't happen again until 2049

Friday, June 13, 2014, 8:08 AM - Call it what you want - the Strawberry Moon, the Rose Moon or even the Honey Moon - June's full moon has several names, but this particular one is rare enough that we haven't seen one since the year 2000, and we won't see another until the year 2049. What's different about the full moon this month? For anyone living in or eastward of the Eastern time zone, it falls on Friday the 13th.

It's a simple coincidence of numbers, of course. As the Earth rotates on its axis and orbits around the sun, we tick off the days, weeks and months on our calendar. Meanwhile, the moon orbits around the Earth on its own schedule, progressing through its phases each day and taking roughly 29.5 days to get from one full moon to the next.

"Moon phases en" by Orion 8 - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

Our calendar has a steady enough layout that we easily follow it from month to month and year to year. Our days and weeks are consistent enough, but we restart the count of our days (ie: our months) every 28 to 31 days. This causes the factors involved in this coincidence to shift around enough that they only line up every once in a while. 

First, since we consistently count out 7-day weeks, from Sunday to Saturday, regardless of what the rest of the calendar is doing, that makes the specific days of the month fall on different days of the week (except February and March, usually). This makes Friday the 13ths uncommon as it is. Of the 12 times a year when we have the 13th-day-of-the-month, a maximum of three of those will fall on a Friday throughout the year, and while there can be up to 14 months between occurrences of the day, there is always at least one Friday the 13th per calendar year. 

Just going back as far as 2000, this will be the 25th Friday the 13th in that time span. 

Looking at the full moon now, they happen once every 29.5 days (called a synodic month), so we see at least one almost every month (there are a few Februaries that don't have one). That gives us between 12 and 13 full moons every year. However, having it fall on a specific day is a little less common. Since 2000, the full moon has fallen on a Friday a total of 26 times. In that same time, the full moon has occurred on the 13th of the month exactly nine times. 

Bringing it all together, though, to have the 13th fall on a Friday and have the synodic month line up to put a full moon on that same day, the number of occurrences since the year 2000 drops to two (including this one). What's more, is that these three factors won't line up again for another 35 years, and 2 months, when another Friday the 13th full moon comes in August of 2049.

Given that there can be a little as 2-3 years between two occurrences of the Friday the 13th full moon, that's a pretty long period of time to wait to see this happen again. However, it actually isn't the longest. Between January 1922 and November 1970, the moon and the calendar stubbornly refused to line up. That produced a 48 year and 10 month stretch of no Friday the 13th full moons.

Friday the 13th is already viewed as unlucky by certain cultures, so, with the extra coincidence of this full moon, is that going to make this day even more unlucky? Well, beyond the superstition, there's no evidence that Friday the 13th is actually more "unlucky" than other days of the year. In fact, there are some statistics that support the idea that it's actually a slightly safer day than others (mostly because people tend to be more careful on that day, it seems). 

This should probably be seen as a fairly lucky day, though, since all of these separate time factors had to line up just right for us to see it!