May 30, 2005

The Doomsday Rule

Slow news night? Or am I just this lazy?

I haven't posted in a while so I'm just lobbing this softball so you know I'm still around.

A couple days ago my oldest daughter asked me which day of the week she was born, errrrr, 13 years ago. Well, I know I know things. "That was a Wednesday afternoon." And I know I get these things wrong all the time. I've only fooled myself into thinking I know them.

So I headed straight to the computer to Google the date. Seriously. And the stuff I found about dates...

First, I confirmed quite quickly that it was a Wednesday since putting Tuesday or Thursday ahead of the date got no results while Wednesday lit up the board, so to speak.

Then I found stuff about the Gregorian Calendar and the Julian Calendar and moving New Year's Day forward etc etc.

Then I found this: The Doomsday Rule.

It looked at first like a quick and/or easy math trick to calculate exactly the weekday of any given date. Well, yes, it was "math". Quick and/or easy was not in the stars, so to speak.

It was a lot of this:

Computing Rosh Hashana

Rosh Hashana is the Jewish New Year's Day. In a Gregorian year Y A.D. (i.e., the first day of the Jewish year Y+3761), it happens on September N, where

{[ Y/100 ] - [Y/400] - 2}+ 765433
(12G) mod 19 + 1
(Y) mod 4 - 313 Y + 89091
= N + fraction

and G is the Golden number, except that it must be postponed by one or two days in the following circumstances....

Now, yes, there was in fact a trick to memorize, and yes, now I can actually figure any weekday based on a given date, in about 5 minutes, as long as we're not talking BC dates.

Or, Rosh Hashana.

Or for instance, when the Vernal Equinox ought to occur in 2027. I will be Googling this one.

Hope you had a good Memorial Day Weekend.


Post a Comment

<< Home