The Doomsday Algorithm

2 Comments

Doesn't that sound like a thriller title? But no, it's a method for calculating the day of the week for any date that you can do in your head. Invented by John Conway (creator of the game Life, among many other things) and streamlined by various other people. I'd heard of it before, but it always sounded too complicated; but the web page lays it out nicely.

I'm still more likely to use a calendar, though, just to be sure. I was very pleased a few years ago to discover that the calendar built into emacs could tell me when Ramadan would be in various years in the 23rd century.

2 Responses to “The Doomsday Algorithm”

  1. Jay Lake

    Not sure how you could calculate Ramadan for future centuries since the Islamic calendar occasionally moves in an unpredictable manner based on the local weather — the day (and at some points month) is defined as beginning when there is sufficient light to distinguish a white thread and a black thread. Historically, unusually cloudy conditions in Mecca have thrown the relationship between Islamic and Christian dating off from the predicted values on several occasions.

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  2. Jed

    Yeah, I did a fair bit of research at the time on the dating system—discussions of how to determine when Ramadan begins at the north pole, or in space, or whatever. But for my purposes I just needed an approximate and plausible date I could point to; I didn’t need to be able to forecast it precisely accurately.

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