Which Islamic calendar is commonly used?

[Fusion Future]

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!
>>