HDATE(1L)

NAME
hdate, hcal - Islamic (Hijri) date and calendar

SYNOPSIS
hdate [day month year]
hdate [-h day month year]

hcal [-h] [[month] year ]

DESCRIPTION
For a given date, hdate prints the corresponding Islamic (Hijri) date. If no arguments are given, it uses the current (today's) date. If the -h command option is specified, hdate takes the given date as a Hijri date, and translates it back to the common calendar. (The common calendar is the Gregorian calendar for years after 1582 and the Julian calendar for years before that.)

hcal prints a calendar with both common and Hijri dates. If no arguments are given, the current month is used. If a year is specified, the program outputs a calendar for that entire year. If a month is also specified, a calendar just for that month is printed. If the -h option is specified, hcal takes the given month/year as a Hijri month/year, and produces a calendar accordingly. If no Hijri month/year is given, a calendar for the current Hijri month is printed. When the -h option is used, the program will also print the time of the start of the new moon whose first visibility marks the beginning of the Hijri month.

The month is a number between 1 and 12. For Christian Era (C.E.) dates, a year must be positive (i.e., A.D.). For Hijri dates, a positive year signifies After Hijrah or Anno Hegirae (A.H.); a negative year signifies Before Hijrah (B.H.). (The year after 1 B.H. was 1 A.H.; there is no year zero.)

The Hijri year consists of 12 (purely) lunar months. They are: (1) Muharram; (2) Safar; (3) Rabi` al-awal; (4) Rabi` al-Thaani; (5) Jumaada al-awal; (6) Jumaada al-Thaani; (7) Rajab; (8) Sha`baan; (9) Ramadan; (10) Shawwal; (11) Zul al-Qi`dah; and (12) Zul al-Hijjah. The most important dates in the Islamic year are: 1 Muharram (new year); 1 Ramadan (first day of fasting); 1 Shawwal (`eid al-Fitr); 8-10 Zul al-Hijjah (the Hajj to Makkah); 10 Zul al-Hijjah (`eid al-'Azha).

EXAMPLES
To find out the current Hijri date, simply enter:

    hdate

To display the Gregorian date corresponding to the 1st of Ramadan (the 9th month) of the Hijri year 1413, type:

    hdate -h 1 9 1413

To find out the Hijri date of the battle of HiTiyn, enter:

    hdate 4 6 1187

To produce a (Gregorian/Hijri) calendar for December 1992, enter:

    hcal 12 1992

To display a (Hijri/Gregorian) calendar for the year 1413 Hijriah, enter:

    hcal -h 1413

NOTES and METHOD DESCRIPTION
For religious reasons, the beginning of a Hijri month is marked not by the start of a new moon, but by a physical (i.e., an actual human) sighting of the crescent moon at a given locale. From the Fiqhi standpoint, one may begin the fast in Ramadan, for example, based on "local" sighting (IKHTILAF AL-MATALE') or based on sighting anywhere in the Muslim World (ITTEHAD AL-MATALE'). Although different, both of these positions are valid Fiqhi positions.

Astronomically, some data are definitive and conclusive (i.e. the time of the BIRTH of the crescent). However, determining the VISIBILITY of the crescent is not as definitive or conclusive; rather it is dependent upon several factors, mostly optical in nature.

Efforts for obtaining an astronomical criterion for predicting the time of first lunar visibility go back the the Babylonian era, with significant improvements and work done later by Muslim and other scientists. These efforts have resulted in the development in a number of criteria for predicting first possible sighting of a crescent. However, there remains a measure of uncertainty associated with all criteria developed thus far. Moreover, there has been little work in the area of estimating crescent visibility on global (as opposed to local) scale. Until this happens, no Hijri calendar software can be 100% reliable, and actual crescent sighting remains essential especially for fixing important dates such as the beginning of Ramadan and the two `eids.

The slight differences in printed Islamic calendars, worldwide, can be traced to two factors: (1) the absence of a global criterion for first visibility; and (2) the use of different visibility criterion (or method of calculation). Weather conditions and differences in the observer's location also explain why there are sometimes differences in the observances of Islamic dates, worldwide.

hcal and hdate use one of the simplest astronomical criterion for earlist lunar visibility. The criterion is a local one and is based on the moon's age since the lunar conjunction (the start of the new moon).

The minimum recorded age of the crescent moon for first visibility is around 13 1/2 hours. hcal and hdate take this into consideration to determine the likelihood of the crescent being sighted just after the first sun set (assumed to occur at 7:30p.m.) following the new moon. And since there is no agreed upon international Lunar date-line, hcal and hdate currently use Makkah (+3 GMT) for the locale.

It is the authors' view that hdate/hcal produces dates/calendars that are more reliable than many of the Islamic calendars which are printed annually in various Muslim communities. The programs, moreover, provide astronomically based information which can be used to support the actual sighting of a crescent moon.

AUTHOR
Waleed A. Muhanna (wmuhanna@magnus.acs.ohio-state.edu).