1. History and Establishment of the Hijri Calendar
The Islamic Hijri calendar (Al-Taqwim Al-Hijri) was officially established in 638 CE by Caliph Umar ibn al-Khattab. To eliminate administrative ambiguity in governance and document dating, Caliph Umar consulted the Prophet’s companions and chose the Migration (Hijrah) of Prophet Muhammad from Mecca to Medina in 622 CE as the epochal starting point of the Islamic calendar (Year 1 AH).
2. Astronomical Lunar Mechanics vs Solar Calendars
Unlike the Gregorian calendar which tracks the Earth's orbit around the Sun (365.2425 days), the Hijri calendar is strictly lunar, tracking the phases of the Moon around the Earth: • Synodic Lunar Month: Averages 29.53059 days (alternating between 29 and 30 days). • Lunar Year: 12 synodic months total 354 or 355 days. • Annual Seasonal Shift: Because a lunar year is 10 to 11 days shorter than a solar year, Islamic dates (such as Ramadan and Hajj) move earlier through the Gregorian calendar seasons by approximately 1 month every 3 years.
3. The 12 Months of the Islamic Year & Sacred Months
The 12 Islamic months in chronological order are: 1. Muharram (Sacred) 2. Safar 3. Rabi al-Awwal 4. Rabi al-Thani 5. Jumada al-Awwal 6. Jumada al-Thani 7. Rajab (Sacred) 8. Sha'ban 9. Ramadan (Month of Fasting) 10. Shawwal (Month of Eid al-Fitr) 11. Dhu al-Qi'dah (Sacred) 12. Dhu al-Hijjah (Sacred - Month of Hajj & Eid al-Adha)
4. Physical Moon Sighting (Hilal) vs Astronomical Tabular Calculations
Religious observances (such as starting Ramadan or celebrating Eid) traditionally require the physical sighting of the new crescent moon (Hilal) with the naked eye or optical telescope. However, global civil administration, banking, and scheduling rely on astronomical tabular calculations (such as the Umm al-Qura calendar of Saudi Arabia) which predict lunar conjunction precisely.
5. Mathematical Conversion Formulas Between AH and CE
A simplified mathematical formula for converting Hijri year (H) to Gregorian year (G): • G ≈ H - (H / 33) + 622 • H ≈ (G - 622) × (33 / 32)
Online conversion software uses Julian Day Numbers (JDN) to map specific days with accuracy, adjusting for leap years in both calendars.
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