Why Search for Hilal in Sabang?
One of my favorite endpoints on mabims.dev isn’t /today or /convert, but /hilal/viz. This endpoint generates a 720×1280 PNG showing a complete evening sky visualization with moon position, MABIMS criteria pass/fail status, and a sky chart with stars.
No other Hijri calendar API (at least that I’ve found) has this feature. So this post discusses how it works, from astronomy to pixel rendering.
Why Sabang?
Section titled “Why Sabang?”This is the question I get most often. The answer is simple: Sabang is Indonesia’s westernmost point.
For moon sighting (rukyah), geographic position matters — the further west, the later sunset occurs. This means if the hilal doesn’t meet criteria elsewhere, its last chance will be in Sabang.
This isn’t an official MABIMS rule about “must use Sabang,” but it’s a practical reference point that makes sense for representing Indonesia-wide hilal visibility in a single coordinate.
How It Works
Section titled “How It Works”A simplified version of the actual implementation:
FUNCTION hilal_viz(month, year): sighting = resolve_sighting_evening(year, month) observation = observe_sighting_evening(sighting.evening_date) alt_ok = observation.moon_alt >= 3.0° elong_ok = observation.elongation >= 6.4° data = build_chart_data(...) img = render_chart(data) return PNG1. Resolve the sighting evening
Section titled “1. Resolve the sighting evening”If you request visibility for Ramadan, this endpoint doesn’t calculate the 1st of Ramadan itself — it goes back to the 29th evening of Sya’ban, because that’s the actual evening observed to determine whether tomorrow marks the start of Ramadan. This is the fundamental logic of rukyah: you observe the hilal at the end of the current month, not at the start of the target month.
2. Calculate that evening’s astronomy, from Sabang
Section titled “2. Calculate that evening’s astronomy, from Sabang”Two categories of data are computed:
- Geocentric criteria —
moon_alt(altitude),moon_az(azimuth),sun_alt, andelongation(angular distance between moon and sun). These are the numbers directly compared against MABIMS thresholds. - Observer-clock times — illumination, local sunset time, and moonset time. This is what’s displayed as additional info on the bottom card.
3. Check Neo MABIMS thresholds
Section titled “3. Check Neo MABIMS thresholds”alt_ok = moon_alt >= 3.0°elong_ok = elongation >= 6.4°Both conditions must be met simultaneously. If either fails, the hilal is considered not to meet visibility criteria, even if the moon is above the horizon.
Example Result
Section titled “Example Result”
The screenshot above shows visibility for 1 Muharram 1448 H (evaluating the evening of 29 Dzulhijjah 1447 H, June 15 2026, from Sabang):
- Moon altitude +5.0° (passes, threshold ≥3.0°)
- Elongation 7.0° (passes, threshold ≥6.4°)
- Illumination 0.2% — very thin, the hilal is extremely young
- Sunset 18:53, moonset 19:11, only 18 minutes apart
Status: VISIBLE, though with not much margin.
Why Bother Building This?
Section titled “Why Bother Building This?”Because raw numbers (moon_alt: 5.2, elongation: 8.7) aren’t intuitive for most people, including myself. But once visualized — see the moon’s position relative to the horizon, see the green/red pill, see the criteria table — it becomes much easier to digest. This endpoint isn’t just for developers who need JSON, but for anyone curious about “how can the hilal be declared visible/not visible” without needing to understand astronomy.
Try it yourself on the playground:
GET https://api.mabims.dev/api/v1/hilal/viz?month=1&year=1448Change month and year to the Hijri month you want to check. Full parameter documentation at mabims.dev/en/endpoints/hilal.