Show HN: Tiny Moon – Swift library to calculate the moon phase

That’s why if a scholar based with intensive research and data, has concluded that X is true. If in the end its really true, they’ll receive two rewards; when its debunked and it was actually false, they receive one reward. The sin is to be ignorant and without any research and data concluded that X is true.

Specifically for Ramadan, the guideline is to use Moon sighting. If you see a new moon, then you begin fasting. If not, then you postpone for a day before starting Ramadan. Usually there’s a committee that does this and they will announce the result. But it doesn’t prevent anyone with the ability to observe to decide when to start Ramadan.

Salah guideline is the sun, e.g the dawn prayer is when there’s thin strike of the sun on the horizon. So to calculate the prayer time you use the sun position relative to your position on earth. If there’s a bug that somehow err the prayer time to ~5 minutes. We can always observe the sun first.

So there’s always this second factor that you can use to validate the first method. Time seems off? Look at the sun. Sun not visible? Estimate with time. Both seems off? Estimate with the variables that you can observe.

“ God does not burden any soul with more than it can bear “ (Al Baqarah 286)

——- So to answer the question, the tolerance depends on your effort on trying to reduce the err.

God knows best

  $ curl wttr.in/Moon

tbh the small error can be very easily fixed. its close to the difference between a sine wave and its linear piecewise approximation from tip to tip, hence the very high accuracy on the quarters

you could go even further and remove the error from path being nearly a precessing ellipse...

in both cases what you will get is nothing that you would detect by eye or even with a telescope.

for the stated purpose this algorithm does fine in almost every use case including astronomical ones.

1. Moon phase (or "age") is trivial to compute using modular arithmetic, given an absolute moment in time as an offset relative to a reference full moon time.

2. Converting datetime formats to a reference time can get complicated, particularly if you want to handle historical dates where calendar systems changed.

3. Converting phase into named bins is a pretty trivial lookup table to quantize the continuous phase value.

4. Converting phase into an approximate percent illumination is pretty trivial geometry, using a simplified model of the moon as a circle with an elliptical boundary for the shaded versus lit part.

5. Converting percent illumination into apparent brightness requires much more work to actually compute the positions of the sun, moon, and earthbound observer.

6. Finding moonrise and moonset times requires even more work to repeatedly find the positions and search for horizon crossings.

Edit: and computing practical brightness is even more complex as you need to fuse together the above with meteorological data to consider what light actually reaches the observer!