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.
A new month begins when the crescent moon is sighted after sunset on the 29th day, or automatically after the 30th day.
Calculation versus moon sighting
Two approaches are both in use, and they are not the same thing.
Traditional moon sighting (Ru'yat al-Hilal). Physically observing the new crescent moon after sunset. This is the classical method, in use 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.
Important
The dates shown on our website are calculations, for planning purposes. For religious obligations such as Ramadan and Eid, please follow official announcements from your local Islamic centre 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. It explains why different communities may start Islamic months on different days.
What it means
If the moon is sighted in one location, that sighting only applies to other locations sharing 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, because the horizons differ.
Unity of Horizon diagram showing how moon sighting applies across similar latitudes
Practical examples
| Likely to share a horizon | Different horizons |
|---|---|
| London and Paris (close latitudes) | London and Cape Town |
| New York and Toronto | Saudi Arabia and Canada |
Religious authority guidance
Different maraji (religious authorities) hold varying positions on moon sighting versus astronomical calculation. The positions of two prominent Shia authorities are set out below.
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.
Naked-eye sighting is required and telescopes are not permitted; calculations alone are insufficient; the 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.
Physical sighting is required and telescopes and binoculars are permitted; calculations alone are insufficient; the local horizon principle applies.
The two positions compared
| Aspect | Ayatullah Sistani | Imam Khamenei |
|---|---|---|
| Calculations alone | Not sufficient | Not sufficient |
| Naked-eye sighting | Required | Acceptable |
| Telescopes and binoculars | Not permitted | Permitted |
Our approach
We use a hybrid of calculation and verification.
- Base calculation — astronomical algorithms give the initial date predictions.
- Administrator adjustments — our team monitors official moon sighting announcements.
- Date offsets — where official sightings differ from calculations, we adjust by ±1–5 days.
- 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.