The Jalali calendar is a

solar calendar A solar calendar is a calendar whose dates indicate the season or almost equivalently the apparent position of the Sun relative to the stars. The Gregorian calendar, widely accepted as a standard in the world, is an example of a solar calendar. T ...

, was compiled during the reign of Jalaluddin

Malik-Shah I Jalāl al-Dawla Mu'izz al-Dunyā Wa'l-Din Abu'l-Fatḥ ibn Alp Arslān (8 August 1055 – 19 November 1092, full name: fa, ), better known by his regnal name of Malik-Shah I ( fa, ), was the third sultan of the Great Seljuk Empire from 1072 to ...

Seljuk Seljuk or Saljuq (سلجوق) may refer to: * Seljuk Empire (1051–1153), a medieval empire in the Middle East and central Asia * Seljuk dynasty (c. 950–1307), the ruling dynasty of the Seljuk Empire and subsequent polities * Seljuk (warlord) (di ...

by the order of

Nizam al-Mulk Abu Ali Hasan ibn Ali Tusi (April 10, 1018 – October 14, 1092), better known by his honorific title of Nizam al-Mulk ( fa, , , Order of the Realm) was a Persian scholar, jurist, political philosopher and Vizier of the Seljuk Empire. Rising fro ...

and the place of observation were the cities of

Isfahan Isfahan ( fa, اصفهان, Esfahân ), from its Achaemenid empire, ancient designation ''Aspadana'' and, later, ''Spahan'' in Sassanian Empire, middle Persian, rendered in English as ''Ispahan'', is a major city in the Greater Isfahan Regio ...

(the capital of the Seljuks), Rey, and

Nishapur Nishapur or officially Romanized as Neyshabur ( fa, ;Or also "نیشاپور" which is closer to its original and historic meaning though it is less commonly used by modern native Persian speakers. In Persian poetry, the name of this city is wr ...

. Variants of the Jalali calendar are still in use today in

Iran Iran, officially the Islamic Republic of Iran, and also called Persia, is a country located in Western Asia. It is bordered by Iraq and Turkey to the west, by Azerbaijan and Armenia to the northwest, by the Caspian Sea and Turkmeni ...

and

Afghanistan Afghanistan, officially the Islamic Emirate of Afghanistan,; prs, امارت اسلامی افغانستان is a landlocked country located at the crossroads of Central Asia and South Asia. Referred to as the Heart of Asia, it is bordere ...

. In Iran, the Persian names of the

zodiac The zodiac is a belt-shaped region of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the Sun path, apparent path of the Sun across the celestial sphere over the course of the year. ...

are used while in

the original Arabic names are used. It gains approximately 1 day on the

Julian calendar The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandr ...

every 128 years. The tropical Jalali calendar ( fa, گاه‌شماری جلالی or تقویم جلالی), which inherited some aspects from the Yazdgerdi calendar, was adopted on 15 March 1079 by the Seljuk

Sultan Sultan (; ar, سلطان ', ) is a position with several historical meanings. Originally, it was an Arabic abstract noun meaning "strength", "authority", "rulership", derived from the verbal noun ', meaning "authority" or "power". Later, it ...

Jalal al-Din Malik Shah I (for whom it was named), based on the recommendations of a committee of astronomers, including

Omar Khayyam Ghiyāth al-Dīn Abū al-Fatḥ ʿUmar ibn Ibrāhīm Nīsābūrī (18 May 1048 – 4 December 1131), commonly known as Omar Khayyam ( fa, عمر خیّام), was a polymath, known for his contributions to mathematics, astronomy, philosophy, an ...

, at the imperial observatory in his capital city of

. Month computations were based on solar transits through the zodiac. It remained in use for eight centuries. It arose out of dissatisfaction with the seasonal drift in the Islamic calendar which is due to that calendar being lunar instead of solar; a lunar year of 354 days, while acceptable to a desert nomad people, proved to be unworkable for settled, agricultural peoples, and the Iranian calendar is one of several non-lunar calendars adopted by settled Muslims for agricultural purposes (others include the

Coptic calendar The Coptic calendar, also called the Alexandrian calendar, is a liturgical calendar used by the Coptic Orthodox Church and also used by the farming populace in Egypt. It was used for fiscal purposes in Egypt until the adoption of the Gregoria ...

, the

, and the Semitic calendars of the Near East). Sultan Jalal commissioned the task in 1073. Its work was completed well before the Sultan's death in 1092, after which the observatory would be abandoned. The year was computed from the

March equinox The March equinox or northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. The March equinox is known as the verna ...

(

Nowruz Nowruz ( fa, نوروز, ; ), zh, 诺鲁孜节, ug, نەۋروز, ka, ნოვრუზ, ku, Newroz, he, נורוז, kk, Наурыз, ky, Нооруз, mn, Наурыз, ur, نوروز, tg, Наврӯз, tr, Nevruz, tk, Nowruz, ...

), and each month was determined by the transit of the sun into the corresponding zodiac region, a system that incorporated improvements on the fourth-century-CE Indian system of the

Surya Siddhanta The ''Surya Siddhanta'' (; ) is a Sanskrit treatise in Indian astronomy dated to 505 CE,Menso Folkerts, Craig G. Fraser, Jeremy John Gray, John L. Berggren, Wilbur R. Knorr (2017)Mathematics Encyclopaedia Britannica, Quote: "(...) its Hindu inven ...

(''Surya''=solar, ''Siddhanta''=analysis), also the basis of most

Hindu calendar The Hindu calendar, Panchanga () or Panjika is one of various lunisolar calendars that are traditionally used in the Indian subcontinent and Southeast Asia, with further regional variations for social and Hindu religious purposes. They adopt a s ...

s. Since the solar transit times can have 24-hour variations, the length of the months vary slightly in different years (each month can be between 29 and 32 days). For example, the months in the two last years of the Jalali calendar had: * 1303 AP: 30, 31, 32, 31, 32, 30, 31, 30, 29, 30, 29, and 30 days, * 1302 AP: 30, 31, 32, 31, 31, 31, 31, 29, 30, 29, 30, and 30 days. Because months were computed based on precise times of solar transit between zodiacal regions, seasonal drift never exceeded one day, and also there was no need for a leap year in the Jalali calendar. However, this calendar was very difficult to compute; it required full

ephemeris In astronomy and celestial navigation, an ephemeris (pl. ephemerides; ) is a book with tables that gives the trajectory of naturally occurring astronomical objects as well as artificial satellites in the sky, i.e., the position (and possibly vel ...

computations and actual observations to determine the apparent movement of the Sun. Some claim that simplifications introduced in the intervening years may have introduced a system with eight leap days in every cycle of 33 years. (Different rules, such as the 2820-year cycle, have also been accredited to Khayyam). However, the original Jalali calendar based on observations (or predictions) of solar transit would not have needed either leap years or seasonal adjustments. Owing to the variations in month lengths, and also the difficulty in computing the calendar itself, the Iranian calendar was modified to simplify these aspects in 1925 (1304 AP), resulting in the

Solar Hijri calendar The Solar calendar ( fa, گاه‌شماری هجری خورشیدی, Gâhšomâri-ye Xoršidi; ps, لمريز لېږدیز کلیز, lamrez legdez kalhandara; ku, ڕۆژژمێری کۆچیی ھەتاوی, Salnameya Koçberiyê) is a solar ...

References

See also