Saifullah AhadHow I built it
Krepusko: Crescent GuideLive on Google PlayKrepusko: finding the new moon on the horizon
Crescent Guide, live on Google Play. It tells you exactly where and when to look for the new crescent — from real astronomy.
01
The problem
Every Islamic month begins when the new crescent is seen, and almost nobody knows where to look. It is a hair of light just above where the sun went down, for about fifteen minutes, in a part of the sky that is still bright — so people look up, see nothing, and learn nothing.
02
The plan I wrote
For the exact place you are standing, the app works out where on the horizon to aim, what minute to start, how long the window lasts, and whether the crescent is too thin for eyes alone. It works with no internet.
Then it gathers what people actually found — those who saw it and those who looked and did not — and shows the evening as a whole. When a claimed sighting could not have happened, it says so from the physics and suggests what the person most likely saw instead, such as Venus. It never declares when the month begins: the scholar decides.
03
The build
AI coding agents built it from my written specification. My work was the specification, the tests it had to pass, and the review.
04
The proof
The astronomy follows published crescent-visibility criteria (Yallop and Odeh), and the app refuses sightings that are physically impossible. 23 of 23 automated tests passed.
On a real Infinix phone, the signed release build passed a cold start, all six main screens, a real change of place to London, and a restart that kept everything — with no crashes.
05
The launch
Live on Google Play, with screenshots for phones and tablets.
What this shows about how I work
- Real science on a simple screen: aim here, start at this minute.
- Honest about limits: it shows what was possible, and people decide.
- Tested in the real world: a real change of place, on a real phone.
Where and when to look for the new crescent moon
The new crescent is a thin line of light in a sky that is still bright, so most people miss it. What helps:
- Look just after sunset, low over the place where the sun went down, a little to one side.
- Be quick. The window can be short before the crescent sets too.
- Find a clear, low western horizon with no buildings, trees or haze in the way.
- Know that a very young crescent may be too thin for eyes alone. Binoculars help, but never point them at the sky while the sun is still up.
- Do not confuse it with Venus, a bright point of light in the same part of the sky.
Krepusko works this out for the exact place you are standing, with no internet: where to aim, what minute to start, how long the window lasts, and whether the crescent is too thin for eyes alone. It never declares when the month begins: the scholar decides.
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