I am slow these days to answer, sorry for that; I'm getting over the
exams now. I read the mailings for the last few days about the calendar.
It's nice to see new and knowledgeable friends like Hooman Mehr and
Ordak D. Coward taking part here. There were things new for me and mixed
up a bit. Let me brief out what I understand out of the mess about the
solar calendar. Please correct me wherever I'm wrong.

The calendar Hejrie Shamsi comes in types:

- The early solar calendar (Hejrie Shamsie Borji), an observational
solar calendar with tropical years. The months are synchronized
observationally with the duration sun stays in each of the 12 zodiacal
constellations, which vary between 29 to 32 days for each month with an
accumulation of 365 or sometimes 366 days a year.

- The old Jalali calendar, a true solar calendar with twelve 30-day
months followed by 5 or 6 additional days at the end to fill a complete
solar year. It starts with 'Norooze Jalali' [*].

- The modern Jalali calendar in use in Iran (Iranian calendar), reworked
on the old Jalali calendar and uses the same leap structure; consists of
six 31-day plus six 30-day months followed by a month of 29 days or 30
during a leap year. It starts with 'Norooze Jalali'.

- The afghan Jalali calendar. It has the same month lengths as the
Iranian calendar but the leap years are synchronized with the concurrent
Gregorian years. Afghans celebrate Norooze Jalali but the first day of
the year might start with an offset of 1 day from the Iranian calendar.

[*] Norooze Jalali: The first day of a Jalali year. It is defined by the
'Tahvil' moment, the exact tick that the center of sun passes the point
of vernal equinox in the northern hemisphere of the earth. If Tahvil
happens before noon of the meridian of Tehran, then Norooz is the same
day otherwise Norooz is the next day. There are different methods to
estimate the precise moment of VE. The effort is to find the one that is
as much as possible close to the real occurrence of the phenomenon.


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