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Persians have always been keen on the idea and importance of
calendar system for as long as their documented history has been
recorded. They are among the first cultures to use solar calendar
systems and have always favoured the solar calendar. In general,
the sun has always, even to this day, had a special meaning and
great symbolic significance in the Iranian culture.
Today Iran uses a solar calendar with a leap day system which is
older but more scientifically accurate than the Greqorian
calendar. The present Iranian calendar's leap day system was
devised in the 11th century by a panel of scientists including
Omar Khayyam, who was one of the foremost leading mathematicians
and astronomers of his time, but today is well-known in the West
for his poetry.
The Iranian solar calendar year begins with the midnight closest
to the instant of vernal equinox, when the sun enters the northern
hemisphere. The first day of this calendar year is also the day of
the greatest festival of the year in Iran called Norouz (a single
word made up of two parts, no and rouz, meaning "new
day"). The calendar consists of 12 months which have Persian
names. The first six months are 31 days each, the next five 30
days, and the last month has 29 days but 30 days in leap years.
The reason the first 6 months have 31 days and the rest 30, is not
a random decision -- it has to do with the fact that the sun moves
slightly more slowly along ecliptic in the northern spring and
summer than in the northern autumn and winter.
The Persian new year is determined by noon-time observation of the
Northern spring equinox. If between two consecutive noons the
sun's altitude rises through its equinoctial altitude then the
first noon is on the last day of one calendar year and the second
noon is on the first day (Norouz)of the next calendar year.
Typically leap years are devised and used by various solar
calendar systems, usually every four years. Four-year leap years
add 0.25 day to each year in the period. But that is a slight
overcompensation compared to the actual behaviour of the sun.
Remedying this overcompensation, after about every seven four-year
leap years, the Persian solar calendar produces a five-year leap
year, thus following a thirty-three year cycle for many centuries
before interruptions by single twenty-nine year subcycles.
This general picture of the Persian calendar's leap-year behaviour
contrasts with other ill-informed predictive algorithms which are
based on confusion between the astronomers average tropical year
(365.2422 days, approximated with mistaken near 128-year cycles)
and the mean interval between vernal equinoxes (365.2424 days,
approximated here with a near 33-year cycle).
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