lunar-app.com provides Hijri (Islamic) calendar dates and crescent moon visibility predictions based on astronomical calculations — no observations or user data required.
Hijri Date Calculation
The app shows two Hijri dates side by side. Umm al-Qura (fetched from Aladhan API) is the official calendar used in Saudi Arabia. Calculated is computed directly from the actual astronomical conjunction (new moon) using astronomy-engine — the exact moment the moon passes between Earth and the Sun. The Hijri day is then floor((now − conjunction) / 24h) + 1. These two calendars can differ by 1–3 days because the calculated calendar starts the month at conjunction, while Umm al-Qura uses additional visibility criteria.
Crescent Visibility Prediction
The 3D globe and 2D map divide the world into a 2° grid (~16,000 points). For each point, it computes sunset time (via SunCalc), then checks the moon's altitude and its angular distance from the sun (elongation) at that moment. Visibility is classified using the Yallop/Shaukat criteria:
| A | Easily visible with the naked eye | alt > 3°, elong > 6° |
| B | Visible in perfect conditions | alt > 2°, elong > 4° |
| C | Optical aid may help | alt > 1°, elong > 2.5° |
| D | Visible with optical aid only | alt > 0.5°, elong > 1.5° |
| N | Not visible | below thresholds |
The grid computation runs in a Web Worker so it doesn't freeze the page. The same SunCalc engine powers the Horizon View, which draws the sky at your exact coordinates using moon altitude, sun position, and twilight state.
Limitations
- The visibility prediction assumes clear skies and a flat horizon. Real sighting conditions vary with weather, elevation, atmospheric dust, and observer experience.
- The 2° grid covers ~220 km per cell. Coastal and inland areas within the same cell get the same prediction.
- Local times use a longitude-based approximation (
lng ÷ 15 rounded), which ignores DST and political timezone boundaries.
- Umm al-Qura dates depend on the Aladhan API being online. When offline, the calculated date is used as fallback.
Data Sources
Astronomical calculations by astronomy-engine and SunCalc. Umm al-Qura dates via Aladhan API. Coastline data from geo-countries.