Which Islamic calendar is commonly used?

[Zayed Al-Saidi]

Digging more in how ICU implementing the astronomical Islamic calendar (see
https://github.com/unicode-org/icu/blob/030fa1a4791ee7c2f58505ebb61253c3032916ec/icu4c/source/i18n/islamcal.cpp
), I find out they depend on general astronomical calculations (from "Practical
Astronomy with your Calculator" book) without any criteria for crescent
visibility base in our location on earth.

I think it's worth to put some info to the end user saying that "This
calendar is base on pure astronomical calculation. It doesn't consider any
crescent visibility criteria".

Regards,
Zayed

On Fri, Jan 13, 2023 at 12:35 PM Fusion Future <qydwhotmail at gmail.com>
wrote:

> The official libicu does not have clear documents, but I have found
> some information on Android Developers website.
>
> https://developer.android.com/reference/android/icu/util/IslamicCalendar
>
> So Islamic calendar in libicu only uses approximations of the true
> astronomical calculations.
>
> ```
> The Islamic religious calendar and Saudi Arabia's Umm al-Qura
> calendar, however, are based on the observation of the crescent moon.
> It is thus affected by the position at which the observations are
> made, seasonal variations in the time of sunset, the eccentricities of
> the moon's orbit, and even the weather at the observation site. This
> makes it impossible to calculate in advance, and it causes the start
> of a month in the religious calendar to differ from the civil calendar
> by up to three days.
>
> Using astronomical calculations for the position of the sun and moon,
> the moon's illumination, and other factors, it is possible to
> determine the start of a lunar month with a fairly high degree of
> certainty. However, these calculations are extremely complicated and
> thus slow, so most algorithms, including the one used here, are only
> approximations of the true astronomical calculations. At present, the
> approximations used in this class are fairly simplistic; they will be
> improved in later versions of the code.
>
> Like the Islamic religious calendar, Umm al-Qura is also based on the
> sighting method of the crescent moon but is standardized by Saudi
> Arabia.
> ```
>
> Zayed Al-Saidi <zayed.alsaidi at gmail.com> 於 2023年1月13日 週五 下午1:54寫道：
> >
> > Thank you for your effort.
> >
> > As of now, there is no agreed method to follow in a cross Islamic world
> to determine the beginning of the Hijri months. The agree one is moon
> sighting by naked eyes for each country.
> >
> >  However, there many attempt to develop one. The easiest one are Tabular
> methods (arithmetic rules). There are two system using way:
> > 1- Tabular Islamic calendar
> > 2- Microsoft's Kuwaiti algorithm
> > Please refer to this page for more details:
> https://en.wikipedia.org/wiki/Tabular_Islamic_calendar
> >
> > The second type is astronomical calculations. In this category, the
> algorithm sets crescent criteria such as SAAO, Yallop, Odeh etc.  Here is
> the most used one:
> > 1- Umm Al-Qura (used in KSA)
> > 2- Universal Hejric Calendar, (used in Jordan and Algeria, see
> https://www.astronomycenter.net/uhc.html)
> > But most islamic country developed their calendar base one crescent
> criteria. For example, in Oman we have "Omani Calendar" which could differ
> a little bit of form Umm Al-Qura calendar.
> >
> > As of ICU's Islamic Calnder (Astronomical), I'm not sure which crescent
> criteria they are following.
> >
> > Regards,
> > Zayed
> >
> >
> >
> > On Wed, Jan 11, 2023 at 5:38 PM Fusion Future <qydwhotmail at gmail.com>
> wrote:
> >>
> >> Hello,
> >>
> >> I see there are different kinds of Islamic calendars in
> >>
> https://cldr.unicode.org/development/development-process/design-proposals/islamic-calendar-types
> >>
> >> But I couldn't understand the difference. Here I would like to know
> >> which Islamic calendar is commonly used nowadays so I can implement
> >> alternate calendar support for it in Plasma.
> >>
> >> Thanks in advance!
> >>
>
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