Friday, November 13, 2015


Today is Friday the 13th. Many people fear this day, thinking that the number 13 brings bad luck. The fear of the number 13 is so pervasive that I've seen some elevators that do not have a button for the 13th floor. That fear has a name: Triskaidekaphobia.

Triskaidekaphobia is a well known phobia, an unreasonable fear of the number 13. We've seen it in popular culture in the movies, on TV and even in the press. But the number 13 is no different than any other number except for its meaning and how people perceive it's meaning.

While some people fear this day, I tend to think of it as a lucky day, or just another day. There are some people who will not get out of bed when the day of the week is Friday and the date of the month is 13. People will change their behavior in significant ways when those two conditions are present. They even make plans well in advance for days like today. If you'd like a forecast, here is a list of months and years with Friday the 13th, going into the future to about 2020. You can even extend the list out to 2050 so you can schedule your days off if you want to.

One thing I like about that list is that there is a recurring pattern in the occurrences of Friday the 13th. On that page, we have notice that between 2010 and 2020, Friday lands on the 13th day of the month 20 times. I also find it interesting that a week has 7 days, 2 weeks 14 days, so the probability that Friday will land on the 13th is not exactly remote. Couldn't we pick a better number to be afraid of?

What about the people with birthdays that are on the 13th of the month? "Sorry, I won't be able to attend your birthday this year. I just won't be getting out of bed that day." What do we say to the hapless souls with a birthday on the 13th and a fear of the number 13? "Hey, it's just another day."

Here is a list of 157 historical events that happened on November 13th, calendar errors permitting. The list shows how history goes on blithely without a care for the number 13. Here is a sample:

1553 - English Lady Jane Grey and Archbishop Thomas Cranmer accused of high treason (bad day)
1789 - Ben Franklin writes "Nothing . . . certain but death & taxes" (ok, maybe still a bad day)
1843 - Mt Rainier in Washington State erupts (if you're a geologist this is pretty cool)
1849 - Peter Burnett elected first governor of California (good day for at least one man)
1921 - "Sheik", a silent film starring Rudolph Valentino, is released (movie fans rejoice)

Wait. What about those calendar errors? Wikipedia has some interesting history on the evolution of the calendar. Most notably we see the Julian calendar being replaced by the Gregorian calendar. According to that same page, "The Julian calendar gains against the mean tropical year at the rate of one day in 128 years. For the Gregorian the figure is one day in 3,226 years." That led to some significant efforts to compensate for the length of the year to ensure that the months match the tropical year. The tropical year is based on the relative position of the tilt of the rotational axis of the earth with respect to the sun as the earth orbits the sun.

See how much things change? We try to make things the same with the calendar and our clocks, but the earth, the sun and the moon keep changing on us. Did I mention that time dilates with respect to the influence of gravity? If you think that time only dilates in staff meetings, you might want to sit down for the next paragraph.

Albert Einstein's theory of general relativity predicted that time goes slower when we get closer to a source of gravitational attraction. You know, like the earth. I remember this from watching the television series, Nova, in the 1970s. They demonstrated this with two atomic clocks. On on the ground and one taken into the air long enough to show a difference. The phenomenon is known as gravitational time dilation. Just how we will keep our clocks straight when we colonize another planet?

So much for the human effort to make their conception of time fit to reality. Even with the way the Universe squirms around us, I doubt that I will ever convince anyone to let go of their fear of Friday the 13th. Have a fine day. :)
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