Islamic Calendar

I have had a lot of questions by Non-Muslims and unfortunatly Muslims as well about what is A.H. so i am going to try and clear up how the Islamic calendar works.

The Islamic calendar (or Hijri calendar) is a purely lunar calendar. It contains 12 months that are based on the motion of the moon, and because 12 synodic months is only 12 x 29.53=354.36 days, the Islamic calendar is consistently shorter than a solar year, and therefore it shifts with respect to the Christian calendar.
The calendar is based on the Qur'an (Sura IX, 36-37), which reads:
[9.36] Surely the number of months with Allah is twelve months in Allah's ordinance since the day when He created the heavens and the earth, of these four being sacred; that is the right reckoning; therefore be not unjust to yourselves regarding them, and fight the polytheists all together as they fight you all together; and know that Allah is with those who guard (against evil).[9.37] Postponing (of the sacred month) is only an addition in unbelief, wherewith those who disbelieve are led astray, violating it one year and keeping it sacred another, that they may agree in the number (of months) that Allah has made sacred, and thus violate what Allah has made sacred; the evil of their doings is made fairseeming to them; and Allah does not guide the unbelieving people.

The names of the 12 months that comprise the Islamic year are:
1. Muharram 2. Safar 3. Rabi' al-awwal (Rabi' I) 4. Rabi' al-thani (Rabi' II)
5. Jumada al-awwal (Jumada I) 6. Jumada al-thani (Jumada II) 7. Rajab 8. Sha'ban
9. Ramadan 10. Shawwal 11. Dhu al-Qi'dah 12. Dhu al-Hijjah
(Due to different transliterations of the Arabic alphabet, other spellings of the months are possible.)
Each month starts when the lunar crescent is first seen (by a human observer's eye) after a new moon.
Although new moons may be calculated quite precisely, the actual visibility of the crescent is much more difficult to predict. It depends on factors such as weather, the optical properties of the atmosphere, and the location of the observer. It is therefore very difficult to give accurate information in advance about when a new month will start.
Furthermore, some Muslims depend on a local sighting of the moon, whereas others depend on a sighting by authorities somewhere in the Muslim world. Both are valid Islamic practices, but they may lead to different starting days for the months.
Years are counted since the Hijra, that is, Mohammed's emigration to Medina from Mecca in AD 622. On 16 July (Julian calendar) of that year, AH 1 started (AH = Anno Hegirae = year of the Hijra). Right now (2006) it is 1427 A.H.
I hope that this has helped make things a little clearer.