Talk:Iranian calendar
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Let's add some URL for 'Persian date today' or simply converter. --Ilya 12:02, 16 Dec 2003 (UTC)
- Ilya, I have an online Online calendar converter (http://www.blibbleblobble.co.uk/Calendar/?view=7&d=5&m=7&y=2004) of unknown accuracy -- OJW
midnight at what longitude? --juuitchan
I'm guessing midnight in Persia (Iran), which according to Iran has time zone UTC +3.30. -- DavidCary 19:30, 12 May 2004 (UTC)
"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)."
OK, I'm confused. What is the difference ? Should we add a link to the Year article which should (hopefully) explain the difference ? -- DavidCary 19:30, 12 May 2004 (UTC)
Hi David,
It is confusing. I didn't write it, but I think I understand somewhat. The "Mean Tropical year" is 365.24219 days. It defines the average period for the average seasonal event to repeat. However the earth's orbit is an ellipse, so its motion is speeding up and slowing down over the year. In addition that year defined by the ellipse period (Anomalistic years) 365.2596 days is different, so when you go exactly 1 mean tropical year forward, the earth will be moving at a slightly different orientation in the ellipse and a different speed. Thus a given seasonal event will happen at a slightly different time. This difference is cumulative so the exact lengths of the various seasons will oscillate over time. Thus, if you measure a year defined between March Equinoxes will not be the same length as defined between September Equinoxes!
The quote "the mean interval between vernal equinoxes (365.2424 days, approximated here with a near 33-year cycle)." implies that the CURRENT period between VE is close to 365.2424 days rather than the mean 365.2422 days. So the Iranian calendar actually keeps the VE closer to a given date than would using the mean year which would work better over the long term.
See
- Error in Statement of Tropical Year (http://www.hermetic.ch/cal_stud/cassidy/err_trop.htm)
I'm scare to rewrite anything here, too busy to do a good job without adding more confusion. I had a watch on this page because I added the pretty graphic showing the 33-year rule running over 500 years against the mean tropical year. My purpose was to contrast the 33 year rule with the Gregorian 400 year rules which diverge much more greatly with the tropical mean dates.
--Tomruen 21:52, 12 May 2004 (UTC)
OK, now that I've thought about what you've written (and read Year and Tropical year), I think I understand.
When I first read that
the vernal equinox year is 365.24237404 days the autumn equinox year is 365.24201767 days
I got the impression that it was saying that those 2 days, currently half a year apart, would drift closer and closer together until they occured on the same day, which is clearly impossible.
Now I think I understand:
Earth's axis precesses, pointing at different stars in a small circle that takes around 20 000 years (?). This makes the mean Tropical year slightly different than the sidereal year. If Earth traveled in a circle around the sun, then the vernal equinox year would be the same as the autumn equinox year, and all 4 equinox/solstice points would be exactly the same amount of time apart.
Earth travels in an ellipse. This makes the sun appear to move through the stars in its great circle faster at some times, slower at others. The sun appears to move the fastest when Earth is at perihelion (currently around 2 January, making the winter season the shortest season). The sun appears to move the slowest when Earth is at apohelion (currently around _ July, making the summer season the longest season). If Earth's axis did not precess, then the the vernal equinox year would be the same as the autumn equinox year, each season would last exactly the same amount of time from one year to the next, but the seasons would be 4 different lengths.
The combination of the ellipse and the precession causes the longest season to gradually change: from winter to spring to summer to fall to winter again (over the course of 20 000 years (?) ).
At the present millenium, we've just switched from fall being the longest month to winter being the longest month. (We've just switched from spring being the shortest month to summer being the shortest month). This means that during this and the previous millenium, spring is expanding, summer is shrinking, fall is shrinking, winter is expanding. So the winter (southern) solstice is coming earlier each year, and northern (summer) solstice is coming later every year.
This makes the southern solstice year a bit shorter than the nothern solstice year.
(Except my conclusion is exactly the *opposite* of what Tropical year says. I've got a sign bit flipped somewhere.)
EditHint: Perhaps we could combine your comment, my comment, polish it bit, and stuck into the Tropical Year article.
-- DavidCary 13:04, 13 May 2004 (UTC)
The seasons need to be reversed: in the thirteenth century summer overtook spring for the longest season. Winter is currently shortest, therefore. So now autumn is expanding relatively rapidly, summer is expanding at a slower rate, spring is shrinking rapidly, and winter is shrinking at a slower rate. This is because perihelon, and thus the shortest seasons, is on January 4 and shifting forward in the calendar.
-- 04:16, 21 Mar 2005
the "Khayyam cycle"
Tropical year mentions the "Khayyam cycle"; should we mention that in this article as well ? Perhaps by simply extending the last sentence "(365.2424 days, approximated here with a near 33-year cycle)" to "(365.2424 days, approximated here with a 33-year cycle called the Khayyam cycle)" ?
-- DavidCary 13:04, 13 May 2004 (UTC)
Weeks
Do we have any information on weekday names for this calendar, and the start of the week? I've found the following names, but I'm no expert. "Yekshanbeh, Doshanbeh, Seshhanbeh, Chaharshanbeh, Panjshanbeh, Jomeh, Shanbeh"
-- OJW 01:04, 30 June 2004 (UTC)
- Hello OJW. I wrote most of this article. The reason I did not include the weekdays, is because technically the week is neither a natural cycle of any sort, nor technically a part of this calendar systems. This is also the case for the Gregorian and many other calendars. --K1 04:21, 30 Jun 2004 (UTC)
Hello K1. At the risk of this becoming a "calendar-programming help", it's pretty difficult to produce or understand a calendar of any sort without weekdays. (try finding a gregorian calendar in the shops without any mention of weeks...) - why do we not consider it part of the calendar system? -- OJW (4 Jul 2004, not logged-in)
- I did not design this calendar system, so it is not me who doesn't consider the week as part of this system, the designers of this calendar system did not include a concept called "week" in their design. As I mentioned to you before, the Gregorian calendar also does not have a concept called "week" inherent in its design. I suggest that you take a look at the official Calendar FAQ (http://www.tondering.dk/claus/cal/calendar26.html) which has been maintained by Claus Tøndering for the past several years. There is a whole section about the "week" in that FAQ, infact immediately following the section about the Persian Calendar. --K1 19:30, 5 Jul 2004 (UTC)
Some more detail on the Calendar FAQ (http://www.tondering.dk/claus/cal/node7.html) section on weeks: it starts with the phrase "The Christian, the Hebrew, the Islamic, and the Persian calendars all have a 7-day week".
- Yes, but it is not generally a good idea to read just the first sentence of an entire section and draw conclusions. If you read the first paragraph, try to also read the last paragraph. Anyway, the concept of "week" is more of religious or administrative nature, rather than an inherent part of this calendar system. The day, the month and the solar year all are natural phenomena, the week is not. --K1 20:19, 5 Jul 2004 (UTC)
If you suggest this article needs to be merged with "Solar Islamic Calendar" perhaps you should first explain what exactly "Solar Islamic Calendar" is. I have spent a good part of my life working on calendars and have never heard of such a calendar. On the other hand, the Persian Calendar is a very well-known one among people who know a few things about calendars. Best Regards, Sergio C.
- The merge message here is a double merge message, and I had not originally put it there. These two are clearly not about different calendar systems. If you think one of the articles should be deleted, you should raise that on the Wikipedia:Votes for Deletion. roozbeh 11:42, Jul 25, 2004 (UTC)
- Thank you for the link. I created a Vote for Deletion. Please vote as you like. Best Regards, Sergio.
- Merging simply means "Combining the best features of both articles". This article here is much more informative than the other, but frankly most the information it offers it deeply buried inside it, historical references mixed with info about the function of the calendar itself. Will try to make some changes for purposes of readability and hopefully you'll approve. Aris Katsaris 22:56, 29 Jul 2004 (UTC)
Missing important details
I think one important detail is missing in the description of the Persian calendar system and that is the "great grand cylces". The modern Iranian calendar has a much higher accuracy than stated in the current version of this page. To read the missing detail (and the code), you may take a look at John Walker's page: [1] (http://www.fourmilab.ch/documents/calendar) As one of the few calendars designed in the era of accurate positional astronomy, the Persian calendar uses a very complex leap year structure which makes it the most accurate solar calendar in use today. Years are grouped into cycles which begin with four normal years after which every fourth subsequent year in the cycle is a leap year. Cycles are grouped into grand cycles of either 128 years (composed of cycles of 29, 33, 33, and 33 years) or 132 years, containing cycles of of 29, 33, 33, and 37 years. A great grand cycle is composed of 21 consecutive 128 year grand cycles and a final 132 grand cycle, for a total of 2820 years. The pattern of normal and leap years which began in 1925 will not repeat until the year 4745! Each 2820 year great grand cycle contains 2137 normal years of 365 days and 683 leap years of 366 days, with the average year length over the great grand cycle of 365.24219852. So close is this to the actual solar tropical year of 365.24219878 days that the Persian calendar accumulates an error of one day only every 3.8 million years. --Salarian 13:11, 23 Mar 2005 (UTC)