Compare Hijri calendar results with ground truth from Iran

[roozbehp]

I have gathered a few volunteers to gather historical data for the Hijri calendar as used in Iran. This is what the country actually observed and based its public holidays and celebrations on.

As of right now, the data presently covers ~2002-2026 CE, but we are planning on expanding it pretty fast: https://github.com/roozbehp/qamari

I thought it may be useful to compare the data with what ICU4X generates, specifically the observational/astronomical code modified to a location based in Iran.

CC @Manishearth

[Manishearth]

@roozbehp do you happen to know what the observational point is for this calendar? Iran is big.

And do you know if the observation is affected by weather (I assume so, but I hear in Egypt at least these days they use a helicopter, though I'm not sure if that still leads to 100% concordance with the space math)

I would love to use this data, thank you for collecting it! I'm not sure how much time I will have to do that before 1.0, but it's a good thing to prioritize as we figure out how we handle calendars.

[roozbehp]

do you happen to know what the observational point is for this calendar? Iran is big.

My informed belief is they use all the country these days. If the crescent is computed to be visible from anywhere in the country, it's the new month. (But I don't have a formal confirmation from the calendar authorities and they have been historically very hard to contact.)

And do you know if the observation is affected by weather (I assume so, but I hear in Egypt at least these days they use a helicopter, though I'm not sure if that still leads to 100% concordance with the space math)

The religious authorities do actual observations for the 9th and 10th months (Muharram and Shawwal), which is theoretically affected by the weather. They have sometimes used planes in the past for those two months and they may continue to do so in the future. For the other ten months, the calendar authorities calculate the observability of the crescent about a year in advance and publish it as the formal calendar. For those ten months, even if nobody can observe the crescent at the predicted day or if someone observes the crescent a day early, they ignore it and keep to the published calendar.

We are marking with an asterisk the days where the actual starting day of the month had differed from the published calendar, in cases where we can confirm it. Of course these can only happen during the 9th or the 10th month, which are religiously very important.

[Manishearth]

My informed belief is they use all the country these days. If the crescent is computed to be visible from anywhere in the country, it's the new month.

That's interesting, from two sides: one is them using a range of observation points (which is not something our algorithms support, but could potentially be tweaked for), and the other is them using computations.

9th and 10th months (Muharram and Shawwal)

I know Muharram to be the first month, both from reading and from living in India: is the month cycle different in Iran?

(The Hebrew calendar has a similar mess where the old calendar started at Nisan but the civil one starts at Tishrei)

For those ten months, even if nobody can observe the crescent at the predicted day or if someone observes the crescent a day early, they ignore it and keep to the published calendar.

Fascinating.

[roozbehp]

9th and 10th months (Muharram and Shawwal)

I know Muharram to be the first month, both from reading and from living in India: is the month cycle different in Iran?

Sorry, a mental typo from my side! I meant Ramadan and Shawwal. (Still the 9th and 10th months.)

[Manishearth]

Oh, yes, that makes more sense.