[tz] Calculating dates of Ramadan

[Tim Thornton]

Generally it is within a day. Off the top of my head I cannot say whether
the error always lies on one side, but the calcs could be tweaked to always
be on the safe side, however that may be defined.
Tim



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-----Original Message-----
From: tz-bounces at iana.org [mailto:tz-bounces at iana.org] On Behalf Of Ian
Abbott
Sent: 16 May 2012 11:29
To: tz at iana.org
Subject: Re: [tz] Calculating dates of Ramadan

On 2012-05-16 10:28, Tim Thornton wrote:
> Regarding calculating Ramadan:
> The Islamic calendar is lunar, not solar, and is about 11 or 12 days 
> shorter than the "normal" Gregorian calendar.
> Months (including Ramadan) start when the first crescent of a new moon 
> is sighted. This varies according to the atmospheric conditions and 
> the exact positions of earth, sun and moon, but can be calculated to 
> within about a day for any given location, which is good enough for tz 
> data in the absence of any further specific information for a location.
> There is also an ongoing debate in Islam on whether the calculated 
> date is good enough, or whether they should stick with past practice 
> of having official observers watching the sky to announce the start of the
month.
> What cannot be done is to say that it always falls between a given 
> pair of Gregorian calendar dates, due to its shifting.
>
> Tim

In practice, how much does the observed (as in astronomical) date differ
from the calculated date?  Does it ever start _before_ the calculated date?

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