The Spring equinox occurs at the time when the sun is exactly overhead at the equator but this can be before or after midday at the Iranian Standard Time longitude 52 o 30' E. So years may vary in length by a day more or less in the twelfth month.
The first six months of the year have 31 days each, the next five, 30 days each and the last month 29 or 30 days. His average year length works out to The actual solar tropical year is The Iranian calendars or sometimes called Persian calendars are a succession of calendars invented or used for over two millennia in Greater Iran.
One of the longest chronological records in human history, the Iranian calendar has been modified time and again during its history to suit administrative, climatic, and religious purposes. It begins on the vernal equinox as determined by astronomical calculations for the Iran Standard Time meridian This determination of starting moment is more accurate than the Gregorian calendar as far as predicting the date of the vernal equinox is concerned because it uses astronomical calculation rather than mathematical rules.
Such is the accuracy that more than 13, years elapse before a single day discrepancy between the calendar's average reckoning of the start of months and the mean time of the new Moon. Alignment with the solar year is better than the Julian calendar, but inferior to the Gregorian. The average length of a year is The Islamic calendar is purely lunar and consists of twelve alternating months of 30 and 29 days, with the final 29 day month extended to 30 days during leap years.
Leap years follow a 30 year cycle and occur in years 1, 5, 7, 10, 13, 16, 18, 21, 24, 26, and Days are considered to begin at sunset. The calendar begins on Friday, July 16th, C. The names for the days are just their numbers: Sunday is the first day and Saturday the seventh; the week is considered to begin on Saturday. Since the calendar is fixed to the Moon, not the solar year, the months shift with respect to the seasons, with each month beginning about 11 days earlier in each successive solar year.
The calendar presented here is the most commonly used civil calendar in the Islamic world; for religious purposes months are defined to start with the first observation of the crescent of the new Moon. The modern Persian calendar was adopted in , supplanting while retaining the month names of a traditional calendar dating from the eleventh century. The calendar consists of 12 months, the first six of which are 31 days, the next five 30 days, and the final month 29 days in a normal year and 30 days in a leap year.
Days begin at midnight in the standard time zone. There is no leap year rule; day years do not recur in a regular pattern but instead occur whenever that number of days elapse between equinoxes at the reference meridian.
The calendar therefore stays perfectly aligned with the seasons. No attempt is made to synchronise months with the phases of the Moon. There is some controversy about the reference meridian at which the equinox is determined in this calendar. Various sources cite Tehran, Esfahan, and the central meridian of Iran Standard Time as that where the equinox is determined; in this implementation, the Iran Standard Time longitude is used, as it appears that this is the criterion used in Iran today.
As this calendar is proleptic for all years prior to C. Ahmad Birashk proposed an alternative means of determining leap years for the Persian calendar. His technique avoids the need to determine the moment of the astronomical equinox, replacing it with a very complex leap year structure.
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 years composed of cycles of 29, 33, 33, and 33 years or years, containing cycles of of 29, 33, 33, and 37 years.
A great grand cycle is composed of 21 consecutive year grand cycles and a final grand cycle, for a total of years. The pattern of normal and leap years which began in will not repeat until the year ! This is not the calendar in use in Iran! It is presented here solely because there are many computer implementations of the Persian calendar which use it with which users may wish to compare results , and because its baroque complexity enthralls programmers like myself.
Each year great grand cycle contains normal years of days and leap years of days, with the average year length over the great grand cycle of So close is this to the actual solar tropical year of As a purely solar calendar, months are not synchronised with the phases of the Moon.
Haab: Tzolkin: The Mayans employed three calendars, all organised as hierarchies of cycles of days of various lengths. The Long Count was the principal calendar for historical purposes, the Haab was used as the civil calendar, while the Tzolkin was the religious calendar. All of the Mayan calendars are based on serial counting of days without means for synchronising the calendar to the Sun or Moon, although the Long Count and Haab calendars contain cycles of and days, respectively, which are roughly comparable to the solar year.
Based purely on counting days, the Long Count more closely resembles the Julian Day system and contemporary computer representations of date and time than other calendars devised in antiquity. Also distinctly modern in appearance is that days and cycles count from zero, not one as in most other calendars, which simplifies the computation of dates, and that numbers as opposed to names were used for all of the cycles.
The Long Count calendar is organised into the hierarchy of cycles shown at the right. Each of the cycles is composed of 20 of the next shorter cycle with the exception of the tun , which consists of 18 uinal of 20 days each.
This results in a tun of days, which maintains approximate alignment with the solar year over modest intervals—the calendar comes undone from the Sun 5 days every tun. The Mayans believed at at the conclusion of each pictun cycle of about 7, years the universe is destroyed and re-created. Those with apocalyptic inclinations will be relieved to observe that the present cycle will not end until Columbus Day, October 12, in the Gregorian calendar. Speaking of apocalyptic events, it's amusing to observe that the longest of the cycles in the Mayan calendar, alautun , about 63 million years, is comparable to the 65 million years since the impact which brought down the curtain on the dinosaurs—an impact which occurred near the Yucatan peninsula where, almost an alautun later, the Mayan civilisation flourished.
If the universe is going to be destroyed and the end of the current pictun , there's no point in writing dates using the longer cycles, so we dispense with them here. For civil purposes the Mayans used the Haab calendar in which the year was divided into 18 named periods of 20 days each, followed by five Uayeb days not considered part of any period. Dates in this calendar are written as a day number 0 to 19 for regular periods and 0 to 4 for the days of Uayeb followed by the name of the period.
This calendar has no concept of year numbers; it simply repeats at the end of the complete day cycle. Consequently, it is not possible, given a date in the Haab calendar, to determine the Long Count or year in other calendars. The day cycle provides better alignment with the solar year than the day tun of the Long Count but, lacking a leap year mechanism, the Haab calendar shifted one day with respect to the seasons about every four years. The Mayan religion employed the Tzolkin calendar, composed of 20 named periods of 13 days.
Unlike the Haab calendar, in which the day numbers increment until the end of the period, at which time the next period name is used and the day count reset to 0, the names and numbers in the Tzolkin calendar advance in parallel. On each successive day, the day number is incremented by 1, being reset to 0 upon reaching 13, and the next in the cycle of twenty names is affixed to it.
Since 13 does not evenly divide 20, there are thus a total of day number and period names before the calendar repeats. As with the Haab calendar, cycles are not counted and one cannot, therefore, convert a Tzolkin date into a unique date in other calendars. The day cycle formed the basis for Mayan religious events and has no relation to the solar year or lunar month.
The Mayans frequently specified dates using both the Haab and Tzolkin calendars; dates of this form repeat only every 52 solar years. A bewildering variety of calendars have been and continue to be used in the Indian subcontinent. In the Indian government's Calendar Reform Committee adopted the National Calendar of India for civil purposes and, in addition, defined guidelines to standardise computation of the religious calendar, which is based on astronomical observations. The civil calendar is used throughout India today for administrative purposes, but a variety of religious calendars remain in use.
We present the civil calendar here.
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