Waleed A. Muhana :
The Islamic Calendar, which is based purely on lunar cycles, was first introduced in 638 C.E. by the close companion of the Prophet (Pbuh) and the second Caliph, Umar ibn Al-Khattab (Ra.) (592-644 C.E.) He did it in an attempt to rationalise the various, at times conflicting, dating systems used during his time. Umar consulted with his advisors on the starting date of the new Muslim chronology. It was finally agreed that the most appropriate reference point - for the Islamic calendar was the Hijrah. The actual starting date for the Calendar was chosen (on the basis of purely lunar years, counting backwards to be the first day of the first month (1 Muharram) of the year of the Hijrah. The Islamic (Hijri) calendar (with dates that fall within the Muslim Era) is usually abbreviated AH. in Western languages from the latinised Anno Hegirae, "in the year of the Hegira". Muharram 1, 1A.H. corresponds to July 16, 622 C.E.
The Hijrah, which chronicles the migration of the Prophet Muhammad (Pbuh) from Makkah to Madinah in September 622 C.E., is the central historical event of early Islam. It led to the foundation of the first Muslim city-state, a turning point in Islamic and world history.
To Muslims, the Hijri calendar is not just a sentimental system of time reckoning and dating important religious 'events, e.g., Siyam (fasting) and Hajj (pilgrimage to Makkah). It has a much deeper religious and historical significance.
Muhammad Ilyas [llyas84] quotes Nadvi who wrote :
"It (the advent of the 15th century) is indeed, a unique occasion to ponder that the Islamic Era did not start with the victories of Islamic wars, nor with the birth or death of the Prophet (Pbuh), nor with the Revelation itself. It starts with Hijra, or the sacrifice for the cause of Truth and for the preservation of the Revelation. It was a divinely inspired selection. God wanted to teach man that struggle between Truth and Evil is eternal. The Islamic year reminds Muslims every year not of the pomp and glory of Islam but of its sacrifice and prepares them to do the same."
From a historical angle, Ilyas quotes Samiullah who writes :
"All the events of Islamic history, especially those which took place during the life of the Holy Prophet and afterwards are quoted in the Hijra calendar era. But our calculations in the Gregorian calendar keep us away from those events and happenings, which are pregnant of admonitory lessons and guiding instructions.
.. .And this chronological study is possible only by adopting the Hijri calendar to indicate the year and the lunar month in line with our cherished traditions."
The Islamic (Hijri) year consists of twelve (purely lunar) months. They are : 1 Muharram; 2 Safar; 3 RabyulAwa1; 4 Rabyus -Sany; 5 Jumada alAwa1; 6. Jumada al-Sany; 7 Rajab; 8 Sha'ban; 9 Ramadhan; 10 Shawal; 11 Jil-qad and 12 Jil Hajj.
The most important dates in the Islamic (Hijri) year are: 1 Muharram (Islamic new year); 27 Rajab (lsra & Miraj); 1 Ramadhan (first day of fasting); 27 Ramadhan (Nuzul AlQur'an); Last 10 days of Ramadhan which include Laylatul Qadar; 1 Shawwal Eid ul-Fitr); 8-10 Jil-Hajj (the Hajj to Makkah); and 10 Jil Hajj Eid ul Azha).
It is considered a Divine command to use a (Hijra) calendar with 12 (purely) lunar months without intercalation [llys84], as evident from the following verses of the Holy Qur'an (Trans: A Yusuf Ali):
"They ask thee the New/Moons Say: /They are but signs /To mark fixed periods of time/In (the affairs of) men/And for Pilgrimage." (11:189)
"The number of months/In the sight of Allah/Is twelve (in a year)/.So ordained by Him/The day He created/The heavens and the earth;/Of them four are sacred;/That is the straight usage/So wrong not yourselves/Therein, and fight the Pagans."
"Verily the transposing /(Of a prohibited month)/Is an addition to Unbelief:/The Unbelievers are led/To wrong thereby: for they make/it lawful one year,/ And forbidden another year,/Of months forbidden by Allah/And make such forbidden ones Lawful. /The evil of their course/Seems pleasing to them./But Allah guideth not/Those who reject Faith." (IX: 37)
Since the Islamic calendar is purely lunar, as opposed to solar or lunisolar, the Muslim (Hijri) year is shorter than the Gregorian year by about 11 days, and months in the Islamic (Hijri) year are not related to seasons, which are fundamentally determined by the solar cycle. This means that important Muslim festivals, which always fall in the same Hijri month, may occur in different seasons. For example, the Hajj and Ramdhan can take place in the summer as well as the winter. It is only over a 33 year cycle that lunar months take a complete turn and fall during the same season.
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 actua1 human) sighting of the crescent moon at a given locale. From the Fiqhi standpoint, one may begin the fast in Ramdhan, for example, based on "local" sighting ('Ikhtilaf al-Matale').
Astronomically, some data are definitive and conclusive (i.e. the time of the Birth of a new moon). However, determining the visibility of the crescent is not as definitive or conclusive; rather it is dependent upon several factors, mostly optical in nature. This makes it difficult to produce (in advance) Islamic calendars that are reliable (in the sense that they are consistent with actual crescent visibility).
Efforts for obtaining an astronomical criterion for predicting the time of first lunar visibility go back the Babylonian era, with significant improvements and work done later by Muslims 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 Ramadhan and the two Eids.
The slight differences in printed Islamic calendars, worldwide, can therefore be traced to two primary 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.
Readers interested in further information should consult Mohammad Ilyas' excellent book "A Modern Guide to Astronomical Calculations of Islamic Calendar, Times & Qibla," Berita Publishing, 1984, (ISBN: 967-969-009-1). The book contains a thorough discussion of the Islamic calendrical system and related historical and scientific developments. It also presents an interesting proposal for a universal Islamic Calendar based on a global visibility criterion and the concept of a Lunar Day (or International Lunar Date Line).