The AFU and Urban Legend Archive
Misc
 friday the 13th




From: lawson@acuson.com (Drew Lawson)
Subject: Re: Friday the 13th
Date: Tue, 24 Aug 1993 21:09:03 GMT

In article <252s7s$f8h@news.mantis.co.uk>

tony@mantis.co.uk (Tony Lezard) writes:

>Bear in mind that under the current calandar we use (Gregorian) the divide-
>by-four test for a leap year is modified if the year is divisible by 100
>and not by 400, thus 1900 was not a leap year, 2000 will be. The effect of
>this modification is that the number of days in 400 years is a multiple of
>seven. It is thus a simple task (for a computer) to examine the days in any
>400 year period and observe that, indeed, the 13th of the month is more
>likely to fall on a Friday than any other day.

Despite the fact that "It is thus a simple task" tends to translate to, "I heard this and don't want to check it out," I had a go at this one.

Exercising cal, fgrep and shell scripts for about 4 hours last night (execution, not personal time) gave this.

For the years 1800-5799 (4000 years), the 13th distribution is:

Sun 6870
Mon 6850
Tue 6850
Wed 6870
Thu 6840
Fri 6880
Sat 6840

So, it does seem that over the course of 4000 years, you'll have about 23 more Friday the 13ths than an even distribition would give (6857.14).

This is hardly as significant as the original poster made it sound, though.

Drew "triscadecaphobia" Lawson


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