By Dr. Hossein Bagher Zadeh



AAA wrote:


I've noticed a number of posts here about "Norooz" and they appear to be *so* precise. It also appears that it has something to do with the Spring Equinox; am I correct? My question is, why such precision for a holiday? I mean why get all stressed out over this, it's isn't as though you are calibrating telescopes to hone in on a distance star or planet; or are you? Why not declare it a day (like Christmas) and be done with it?


The following may not be the answer to your question but give you some thoughts. First published in 1992. Reprinted here for the benefit of newcomers to the net.




We have all being used to see the Iranian and Western leap years coincide. However, this year is going to be the last we experience this! Read on ..



The new western} (Gregorian Named after Pope Gregory XIII who introduced the latest changes in the calendar in 1582.) leap year of 1992 has already begun and the Iranian (Jalaali Named after Jalaal-ol-Din Malek-shaah-e Saljuqi by Omar Khayyam who re-worked it in late fifth century)  leap year of 1370 has just ended. This means that the month of February had 29 days, and Esfand (the last month of the the Iranian calendar) 30 days, with a small variation in correspondence between the two calendars for the 20 days in between. And on the 21st of March, the Iranian new year's day, the relation between the two calendars reverted to normal with no discrepancies in the dates.


There is of course nothing new in that. We are all used to both the Jalaali and Gregorian calendars. These are both solar and have the same length of 365 days in normal years and 366 days in leap years. The leap years usually come once every four years. And as long as we can remember, the leap years in the two calendars have come so close to each other that the distance between the two extra (leap) days in the two calendars have been less than three weeks (from the end of February to the end of Esfand = 20th March).


This close correspondence between the two calendars has reduced the problem of converting one calendar to the other to the very minimum. Indeed, apart from the last 20 days of the Jalaali leap years, one could usually use any calendar of any year to find the corresponding dates between the two systems. And for those particular dates in the leap years, the Gregorian calendar is just one day behind the Jalaali one, when compared to other years. (For instance, 8th of March, the International Women's Day, corresponds to 18th of Esfand in the leap years and to the 17th of Esfand in other years.)


However, this cozy relationship between the two calendars is going to end for the time being, i.e., at least for 99 years (unless the calendar system changes)! Why? Because while there will be a three--year gap before the next Gregorian leap year of 1996, we need to wait an extra year to the next Jalaali leap year. The next Jalaali leap year is not going to be in 1374 (as may have been expected) but 1375. And the next time the two leap years correspond as they do now, will be in the Gregorian year of 2096 and Jalaali year of 1474.


The reason is both complicated and fascinating. But before going into that, let us have a look at the Jalaali calendar and appreciate some of its unique features.


The Jalaali calendar is more ``natural'' than the Gregorian calendar. It starts with the natural cycle of the year, the first day of spring (for the northern hemisphere). As such, it has no national, regional or religious significance. The years' count is based on a historical religious event (hejrat). But that's a different story. Gregorian calendar is based on religion for both its base date and start of the year.} It is truly universal. Every season is associated with three full months of the year. So no confusion arises as which day of the year, for example, the beginning of a season is. The first six months all have 31 days and the second six months all have 30 days in leap years, with the last month having 29 days in non-leap years.


Compare these with the Gregorian calendar. There you will find no easy correspondence between months and seasons' starts and endings, nor between the natural and calendar years. The months have arbitrary length of seven 31 days, four 30 days and one 28/29 days interleaved irregularly. It took only a Roman dictator like Augustus to increase the days of the month of August by one day just because July, the previous month (named after his predecessor Julius Caesar), had 31 days in it. Luckily, for Iranians, in spite of being ruled by stream of dictators who occasionally fancied playing with calendars (the last one done by the Shah who tried to add some 1180 years to the base date!), the Jalaali calendar has remained almost intact.


Add to the above features, the fact that the Jalaali year follows closely the movement of the earth (round the sun). A natural solar year is neither 365 days nor 366. It is something like 365 days, 5 hours and 49 minutes. While for westerners, the new year begins at midnight, for us there is an exact time (to the seconds) worked out by astronomy which specifies the beginning of the new year. This is known as ``{tahvil-e saal}''  A solar calendar year begins at the point when the sun appears to cross the equator from the southern hemisphere to the northern hemisphere as viewed from the centre of the earth.. However, for the purposes of the calendar, a day (of 24 hours) can either belong to the past year (say, the last day of Esfand) or the new year (1st of Farvardin). So what we do is this: if the exact moment of {tahvil-e saal} is before midday (Tehran time), we regard the same day as the New Year's Day (1st Farvardin). Otherwise, the New Year begins on the following day. This has made the Jalaali calendar year much more accurate than the Gregorian one (details later).


Now, because the natural year is approximately 365 days and 6 hours, it means that {approximately} every four years we have an extra day in the year - hence the leap years. But the year's length is about 11 minutes shorter than that. So, over the years, these 11 minutes added together affect the cycle of the leap years. For instance, while this year {tahvil-e saal} occurs just after midday (Tehran time), and so the following day will be {Nowruz}, in four years time it will happen before midday, and so the same day will be regarded as {Nowruz} and not the following day. As a result, the leap year is pushed to the next year. This happens every 33 years, when the differences of 11 minutes add up to approximately 360 minutes - equal to 6 hours. That is enough to cause an extra non-leap year every 33 years (with an extra minor adjustment in a 128 years cycle).


And that is how the Jalaali calendar system is organized: In this calendar 8 years out of 33 are leap years. The leap years are those with a remainder (after dividing by 33) of 1, 5, 9, 13, 17, 22, 26, and 30. For instance, 1370, the current year, divided by 33 leaves 17 as the remainder, and so is a leap year. Moreover, the above figures show that there will be a five-year span (instead of the usual four years) between this leap year and the next. The last time the same thing happened was between the leap years of 1337 and 1342 (Gregorian years 1958-59 and 1963-64, respectively).


And how does the Gregorian calendar deal with these anomalies? Here too, they have made things much simpler for themselves and not bothered about the natural year. They have fixed the leap years for every four years except for the years which are divisible by 100 but not by 400! So, the year 2000 will be a leap year (as it is divisible by 400) but not, say, 2100. In this way, they have managed to compensate for the shortfall of 11 minutes per year over a span of 400 years. A clever way but not consistent with the actual size of a natural year. Moreover, the system is not accurate over a longer period of time, but who cares?


The only problem is for us who have to deal with the two systems, and convert the days from one to another. The effect of these variations in the leap years is that, when converting the dates, there may be one or two days difference between the corresponding days from one year to another.


In view of this fact, it may be worthwhile to look back at the years before 1339. If you happen to have a date in the years before 1339 (before March 1960, to be exact) converted into the Gregorian calendar, say your date of birth in the passport, it is quite likely that the conversion is {wrong}. But it may also be the case even for the dates between the 10th to 30th of Esfand in every leap years, anyway. Passport issuing officers, past and present, have no time for such niceties as these calculations require, and usually use a current calendar for converting dates to each other. The question is: does it {really} matter, anyway? Not for the bulk of Iranians who never care(d) about their birthdays. But over here, if you have taken an insurance on your life, the insurance company may be interested to check your {exact} date of birth if the insurance happens to mature on a date on the borderline. Enough to give some sleepless nights to many!




The above article was first posted by Dr. Hossein Bagher Zadeh on  SCI Usenet Newsgroup on March 16, 1994.