Re: Islamic Calendar Stuff

[rms39 columbia edu (Russell Steinthal)]

On Wed, 16 Feb 2000 14:42:13 +0300, Ali Abdin wrote:

>This site also explains the way emacs calculates its dates. They 
call it
>a "crude system" and i don't fully understand it, and thats just not 
how
>the calendar works. 
>
>the whole point of the islamic calendar is that its not based on the 
>actual 'new moon' stuff (that would be easy to calculate) but it is 
based
>on the /sighting/ of the new moon. The sighting is the unpredictable 
part
>(and they only need the sighting during 3 times in the year, two 
eids and
>1st of ramadan). There is a program out there (DOS-only - no source 
code)
>that measures the probability of a moon sighting during a specific 
time
>(based on some astro code) which could be used as a basis for the 
>calendar - can libastro do this? any intended functionality for it? 
The
>URL is: http://www.ummah.org.uk/ildl/
>
>I'll think i'll just do the following: sync the islamic date with 
the
>gregorian date using a newspaper from sometime soon :) of course - 
this
>will cause inaccuracies when you view 'past' dates (they could be 
off by
>a few days). There is no way to accurately detect this unless we 
store
>the number of days for each month for all the prior years (well 
actually
>for two months only)).

The problem, as I see it, is that a calendar system which is actually based on observation, rather than algorithmic or astronomical calculation is almost inherently non-computer friendly.  Are there *any* applications which "correctly" handle this calendar system?  If so, I suggest you look at them and try to figure out how.

Manually syncing things doesn't seem to be a particularly easy thing to have to remember to do, but it may be the best answer.  As you note, it makes working in the past or the future quite complicated.  (Side note: I don't understand why you only need to keep two months of past date counts...)

Finally, even if you could figure out an astronomical algorithm for computing the probability of new moon sighting at a particular place, aren't you still just guessing?  Unless you can find a probability for cloudiness?

-Russell