Hijri Calendar Guide
Understanding the Islamic lunar calendar, moon sighting, and Unity of Horizon principles.
How the Hijri Calendar Works
The Islamic (Hijri) calendar is a lunar calendar consisting of 12 months in a year of 354 or 355 days. Each month begins with the sighting of the new crescent moon (hilal) after sunset.
Because the lunar year is about 11 days shorter than the solar year, Islamic dates shift relative to the Gregorian calendar. This is why Ramadan, for example, occurs about 11 days earlier each year.
Lunar Months
Each month is 29 or 30 days, determined by the moon cycle. A month cannot be 28 or 31 days.
New Month
A new month begins when the crescent moon is sighted after sunset on the 29th day, or automatically after the 30th day.
Calculation vs Moon Sighting
Two Valid Approaches
- Traditional Moon Sighting (Ru'yat al-Hilal): Physically observing the new crescent moon after sunset. This is the classical method since the time of the Prophet.
- Astronomical Calculation (Hisab): Using mathematical models to predict when the moon will be visible. Useful for planning but not universally accepted for religious determination.
Our Calendar Display
The dates shown on our website are calculations for planning purposes. For religious obligations (Ramadan, Eid), please follow official announcements from your local Islamic center or the office of your marja.
Unity of Horizon (Wahdat al-Ufuq)
Unity of Horizon is a principle in Islamic jurisprudence that determines when a moon sighting in one location can apply to other locations. This explains why different communities may start Islamic months on different days.
What It Means
If the moon is sighted in one location, it only applies to other locations that share a similar "horizon" - meaning places where if the moon is visible in one, it would theoretically be visible in the other (weather permitting).
Geographic Considerations
- Cities at similar latitudes and longitudes typically share horizons
- East-west distance matters less than north-south distance
- A sighting in Saudi Arabia may not apply to North America due to different horizons
Visual representation of how Unity of Horizon determines which regions share moon sighting
Practical Examples
Likely Share Horizon
London & Paris (close latitudes)
New York & Toronto
Different Horizons
London & Cape Town
Saudi Arabia & Canada
Religious Authority Guidance
Different maraji (religious authorities) have varying positions on moon sighting versus astronomical calculations. Here are the positions of two prominent Shia authorities:
Grand Ayatullah Sayyid Ali al-Sistani
Position: Physical moon sighting with the naked eye is required. Astronomical calculations alone cannot establish Islamic months, though they may be used to negate false sighting claims.
Key Points: Naked-eye sighting required (telescopes not permitted) | Calculations alone insufficient | Local horizon principle applies
Ayatullah Sayyid Ali al-Husayni al-Khamenei
Position: Physical sighting is required, but telescopes and optical aids are permitted. Astronomical calculations alone cannot establish Islamic months.
Key Points: Physical sighting required | Telescopes and binoculars ARE permitted | Calculations alone insufficient | Local horizon principle applies
| Aspect | Ayatullah Sistani | Imam Khamenei |
|---|---|---|
| Calculations alone | Not sufficient | Not sufficient |
| Naked-eye sighting | Required | Acceptable |
| Telescopes/binoculars | Not permitted | Permitted |
Our Approach
Hybrid Method: Calculation + Verification
- 1.Base Calculation: We use astronomical algorithms for initial date predictions
- 2.Administrator Adjustments: Our team monitors official moon sighting announcements
- 3.Date Offsets: When official sightings differ from calculations, we adjust by ±1-5 days
- 4.Continuous Monitoring: Especially for Ramadan, Muharram, and Dhul Hijjah
Important: For religious obligations (fasting, Eid prayers, Hajj dates), always follow the guidance of your local Islamic authority or marja. Our calendar is provided as a planning tool.
Related Topics
Official Resources: