Dear Calendar People
The wikipedia page http://en.wikipedia.org/wiki/Kuwaiti_algorithm says the Microsoft use a 'Kuwaiti' algorithm for the Islamic calendar and claims that this algorithm is just a variation of the Tabular Islamic calendar. Is this true? If so, what variation? I have a vague memory that this may have been discussed on this list before. Karl 07(15(14 till noon |
Palmen, KEV (Karl) wrote:
> The wikipedia page > http://en.wikipedia.org/wiki/Kuwaiti_algorithm > says the Microsoft use a 'Kuwaiti' algorithm for the Islamic calendar and claims that this algorithm is just a variation of the Tabular Islamic calendar. > > Is this true? > If so, what variation? I made that change to the article. When I first encountered it it was rather biassed on Microsoft's PR, supporting their claim that they had developed an algorithm using statistical analysis (blah blah..) If you follow the first external link on the article, you'll see it's what Van Gent calls Ia whereas the most common tabular islamic calendar is IIc. Also see the errata to Calendrical Calculations: http://emr.cs.iit.edu/home/reingold/calendar-book/second-edition/errata.pdf Basically, the epoch is placed 1 day earlier and the rounding of the accumulated 11/30 values is upwards at .5 which means the 6th leap year of the cycle falls one year earlier. When I first read this, I checked Windows' builtin islamic calendar with my own implementation and sure enough they matched for 1000+ years. Marc. |
In reply to this post by Palmen, KEV (Karl)
Dear Marc and Calendar People
Thank you Marc for the information. -----Original Message----- From: East Carolina University Calendar discussion List [mailto:[hidden email]]On Behalf Of Marc Schoolderman Sent: 15 December 2005 09:47 To: [hidden email] Subject: Re: Microsoft Kuwaiti Algorithm for Islamic Calendar Palmen, KEV (Karl) wrote: > The wikipedia page > http://en.wikipedia.org/wiki/Kuwaiti_algorithm > says the Microsoft use a 'Kuwaiti' algorithm for the Islamic calendar and claims that this algorithm is just a variation of the Tabular Islamic calendar. > > Is this true? > If so, what variation? I made that change to the article. When I first encountered it it was rather biassed on Microsoft's PR, supporting their claim that they had developed an algorithm using statistical analysis (blah blah..) If you follow the first external link on the article, you'll see it's what Van Gent calls Ia whereas the most common tabular islamic calendar is IIc. Also see the errata to Calendrical Calculations: http://emr.cs.iit.edu/home/reingold/calendar-book/second-edition/errata.pdf Basically, the epoch is placed 1 day earlier and the rounding of the accumulated 11/30 values is upwards at .5 which means the 6th leap year of the cycle falls one year earlier. When I first read this, I checked Windows' builtin islamic calendar with my own implementation and sure enough they matched for 1000+ years. KARL SAYS: I'm aware of Van Gent's types I, II, III and IV, but the a and c suffixes are new to me. Reading the linked article reveals that they indicate the epoch. c 15 July 622 (Civil) a 16 July 622 (Astronomical) So the Kuwaiti algorithm has leap years 2, 5, 7, 10, 13, 15, 18, 21, 24, 26 & 29 and epoch 16 July 622 (Julian Calendar). So the epoch is one day LATER. I've recently noticed on http://emr.cs.iit.edu/home/reingold/calendar-book/Calendrica.html that arithmetic Islamic calendar (assumed to be IIc) is presently TWO days ahead of the observational Islamic calendar. This year 1426 is the only year of the 30-year cycle that Ia is TWO days behind IIc and so the Kuwaiti date would actually agree with the observational date today. It is the 16th (not 6th) year of the cycle that begins a day later in I than in II (two days in Ia than in IIc). Karl 07(15(15 |
In reply to this post by Palmen, KEV (Karl)
Dear Calendar People
Oops! I misread it Robert's web page. It is 15 July 622 that is the astronomical 'a' epoch and 16 July that is the civil 'c' epoch. So Ia is one day ahead of IIc every year of the 30-year cycle except this year (the 16th) when it is in sync. Robert's calculator reckons the day of the month of today to be Ic 13 Ia 14 (Kuwaiti algorithm) IIc 14 (usual) IIa 15 IIIc 14 IIIa 15 IVc 14 IVa 15 All without exception are ahead of the observational calendar as calculated at http://emr.cs.iit.edu/home/reingold/calendar-book/Calendrica.html which is day 12 of the month. Karl 07(15(15 -----Original Message----- From: Palmen, KEV (Karl) Sent: 15 December 2005 12:47 To: 'East Carolina University Calendar discussion List' Subject: RE: Microsoft Kuwaiti Algorithm for Islamic Calendar Dear Marc and Calendar People Thank you Marc for the information. -----Original Message----- From: East Carolina University Calendar discussion List [mailto:[hidden email]]On Behalf Of Marc Schoolderman Sent: 15 December 2005 09:47 To: [hidden email] Subject: Re: Microsoft Kuwaiti Algorithm for Islamic Calendar Palmen, KEV (Karl) wrote: > The wikipedia page > http://en.wikipedia.org/wiki/Kuwaiti_algorithm > says the Microsoft use a 'Kuwaiti' algorithm for the Islamic calendar and claims that this algorithm is just a variation of the Tabular Islamic calendar. > > Is this true? > If so, what variation? I made that change to the article. When I first encountered it it was rather biassed on Microsoft's PR, supporting their claim that they had developed an algorithm using statistical analysis (blah blah..) If you follow the first external link on the article, you'll see it's what Van Gent calls Ia whereas the most common tabular islamic calendar is IIc. Also see the errata to Calendrical Calculations: http://emr.cs.iit.edu/home/reingold/calendar-book/second-edition/errata.pdf Basically, the epoch is placed 1 day earlier and the rounding of the accumulated 11/30 values is upwards at .5 which means the 6th leap year of the cycle falls one year earlier. When I first read this, I checked Windows' builtin islamic calendar with my own implementation and sure enough they matched for 1000+ years. KARL SAYS: I'm aware of Van Gent's types I, II, III and IV, but the a and c suffixes are new to me. Reading the linked article reveals that they indicate the epoch. c 15 July 622 (Civil) a 16 July 622 (Astronomical) So the Kuwaiti algorithm has leap years 2, 5, 7, 10, 13, 15, 18, 21, 24, 26 & 29 and epoch 16 July 622 (Julian Calendar). So the epoch is one day LATER. I've recently noticed on http://emr.cs.iit.edu/home/reingold/calendar-book/Calendrica.html that arithmetic Islamic calendar (assumed to be IIc) is presently TWO days ahead of the observational Islamic calendar. This year 1426 is the only year of the 30-year cycle that Ia is TWO days behind IIc and so the Kuwaiti date would actually agree with the observational date today. It is the 16th (not 6th) year of the cycle that begins a day later in I than in II (two days in Ia than in IIc). Karl 07(15(15 |
In reply to this post by Palmen, KEV (Karl)
By the way, regarding your other message, I too get 1 January 2005 for
the Dee-Cecil calendar (using Mozilla 1.7.11). Note that I noticed the calendar also uses a simple Julian formula for the French republican calendar. Haven't checked the other calendars yet. Palmen, KEV (Karl) wrote: > I'm aware of Van Gent's types I, II, III and IV, but the a and c suffixes are new to me. Reading the linked article reveals that they indicate the epoch. > > c 15 July 622 (Civil) > a 16 July 622 (Astronomical) > > So the Kuwaiti algorithm has leap years 2, 5, 7, 10, 13, 15, 18, 21, 24, 26 & 29 and > epoch 16 July 622 (Julian Calendar). So the epoch is one day LATER. It's exactly the reverse: "Of each calendar scheme two variants are possible depending on whether the epoch of the Islamic calendar (1 Muharram, 1 AH) is assumed to be 15 July, 622 CE (known as the “Thursday” or “astronomical” epoch) or 16 July, 622 CE (“Friday” or “civil” epoch)." As an aside, most sources tend to forget that it's Islamic custom to start the day at sunset, I'm wondering if this difference in epoch could be attributed to that as well? > This year 1426 is the only year of the 30-year cycle that Ia is TWO days behind IIc and so the Kuwaiti date would actually agree with the observational date today. No, the year 1426 is actually a year where both algorithms agree with eachother. You can verify this at Van Gent's page. This will lasts until 30 January 2006, which the 'common' tabular Islamic calendar will consider the 30th day of al-Hijja 1426, whereas the Kuwaiti algorithm will consider it 1 Muharram 1427. > It is the 16th (not 6th) year of the cycle that begins a day later in I than in II (two days in Ia than in IIc). I was talking about the 6th *leap* year. Basically, a calendar is a leap year if tabular | (11*y+14) mod 30 < 11 (II) kuwaiti | (11*y+15) mod 30 < 11 (I) Hence my remark about rounding. As a service to the reader, the other two from Van Gent's list indian? | (11*y+11) mod 30 < 11 (III) ? | (11*y+9) mod 30 < 11 (IV) I think I understand the Indian leap year scheme. It's equivalent to: 11*(y+1) mod 30 < 11 And judging from the Hindu epochs, Indians had a penchant for starting to count at zero instead of one, which might explains this. Marc. |
In reply to this post by Palmen, KEV (Karl)
Dear Marc and Calendar People
-----Original Message----- From: East Carolina University Calendar discussion List [mailto:[hidden email]]On Behalf Of Marc Schoolderman Sent: 15 December 2005 14:04 To: [hidden email] Subject: Re: Microsoft Kuwaiti Algorithm for Islamic Calendar Basically, a calendar is a leap year if tabular | (11*y+14) mod 30 < 11 (II) kuwaiti | (11*y+15) mod 30 < 11 (I) Hence my remark about rounding. As a service to the reader, the other two from Van Gent's list indian? | (11*y+11) mod 30 < 11 (III) ? | (11*y+9) mod 30 < 11 (IV) I think I understand the Indian leap year scheme. It's equivalent to: 11*(y+1) mod 30 < 11 And judging from the Hindu epochs, Indians had a penchant for starting to count at zero instead of one, which might explains this. KARL SAYS: I can work out relative meridians of these by taking one unit the K in (11*y+K) mod 30 < 11 to be 12 degrees further East. This places the Indian calendar West of the tabular calendar, opposite to what I'd expect. I think Marc's explanation of the Indian choice of K=11 is a plausible explanation of this. Meridian West of (I)'s Kuwaiti | (11*y+15) mod 30 < 11 (I) 0 tabular | (11*y+14) mod 30 < 11 (II) 12 Indian? | (11*y+11) mod 30 < 11 (III) 48 ? | (11*y+9) mod 30 < 11 (IV) 72 I shall mention again that all four have the property that it has a leap year 10 years after another leap year IN THE SAME 30-YEAR CYCLE. Kuwaiti | (11*y+15) mod 30 < 11 (I) [ 5,15] tabular | (11*y+14) mod 30 < 11 (II) [16,26] Indian? | (11*y+11) mod 30 < 11 (III) [19,29] ? | (11*y+9) mod 30 < 11 (IV) [11,21] Note that K=13 [27,7] and K=10 [30,10] have no leap year 10 years after another leap year in the same 30-year cycle (but in the previous 30-year cycle). Karl 07(15(15 |
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