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Calendars

By:
Sajjad Nikfahm
Source:
The Oxford Encyclopedia of Philosophy, Science, and Technology in Islam What is This? Includes complete coverage of Islamic philosophy, sciences, and technologies from the classical through contemporary periods.

Calendars

The word “calendar” refers to different systems of organizing time. In older Islamic sources, the word tārīkh was used for “calendar,” but later it was called taqwīm. The calendar is thought to have originated from ancient agricultural patterns. The repeating motions of the heavenly bodies led human beings to establish a template for recording time on the basis of these motions.

The description of different calendars, their important occasions, converting one type of calendar to another, and other related calculations were the subject of the most important genre of mathematical astronomy in the Islamic period, zīj. In almost all zīj the first chapter is devoted to calendars.

The different types of calendar systems formed throughout history can be classified according to their dependence on lunar or solar motion as either solar (totally independent of lunar motion) or lunar (although not purely lunar because they use the “solar day”). There is also a lunisolar system, which is basically lunar but uses an intercalation system to align the seasons with the calendar.

Lunar Calendars.

From the astronomical point of view, there exist different types of “lunar month.” The one that has been used in calendars is the synodic month, which is defined as the time between two consecutive conjunctions of the moon with the sun—that is, the period between two successive new moons. This type of month is longer than others, a bit more than 29.5 days on average. In practice, the observation of the new moon in the western sky at the moment of sunset indicates the beginning of the lunar month, but in calculation, the precise moment of conjunction is considered as the beginning of the lunar month. By contrast, the beginning of a solar year requires accurate calculations, because a solar month is a mathematically defined amount (almost one-twelfth of a solar year) and its beginning has no visible manifestation. Perhaps the visibility of the new moon was humankind’s motivation for using a lunar calendar. The complications involved in the visibility of the new moon were one of the major concerns of timekeepers and astronomers in the medieval Islamic period.

A pure lunar calendar is not synchronized with the natural seasons. This gave rise to some complications in ancient times, and led to some unusual solutions, such as what is called nasī͗ (lunar intercalations) in the Qurʾān.

The Islamic lunar calendar, which is the official calendar in many Islamic countries, is one of the most famous lunar calendars. The beginning of this calendar is the first day of the month of Muḥarram of the year of Prophet Muḥammad’s migration from Mecca to Medina (18 July 622). Each lunar year contains twelve months of twenty-nine or thirty days, or in other words, 354 or 355 days. The Hebrew calendar is another example of a lunar calendar, one that has been harmonized with the sun’s revolution. To make this calendar follow the seasons and have its religious occasions fall at the same time of year, seven leap months are inserted during the course of nineteen years. In this calendar, five of the months always have thirty days, five others always have twenty-nine days, and two of them vary between twenty-nine and thirty days. Therefore the length of a regular calendar year varies between 353 and 355 days, and the length of a leap year varies between 383 and 385 days. The Chinese-Uighur calendar, another example of a lunisolar calendar, was in use in East Asia and for some centuries in Persia under Mongol rule. Other examples of lunar calendars have been in use in East Asia, India, and other parts of the world.

Solar Calendars.

Solar calendars are based on the revolution of the Earth around the sun. A solar year can be defined in different ways, but the tropical year, which is the base of a solar calendar, is the time the Earth takes to pass one of the equinoctial points twice, and is 365.2422 days on average.

Different calendars have been designed at different times on the basis of the sun’s apparent motion in the sky. The most important difference among these calendars is in the determination of the length of the year. Since determining the actual length of the year requires accurate astronomical calculations, the determination of intercalation periods for the adjustment of the solar calendar is quite complicated. Valuable efforts were made in this regard in the Islamic period.

In some solar calendars, such as the Egyptian (Coptic) and the Yazdgerdi, the length of the solar year was taken as 365 complete solar days. These calendars contained twelve months of thirty days plus five extra days. These extra five days were usually added to the end of the year, but sometimes they were added to the end of the eighth month. Since the length of the year in these calendars was less than the real time of the apparent motion of the sun in the sky, these calendars lagged one day behind the real motion of the sun every four years. In some other solar calendars, such as the Julian, the length of the year was determined to be 365.25 days. Although such calendars followed the real motion of the sun better, they still needed adjustment.

Iranian calendars, in which the moment of the vernal equinox is considered the beginning of the year, manifest even greater problems. The kharājī calendar can be counted as one of the solutions to these problems. This calendar was formed during the reign of Yazdgird III (632–651/52) in order to harmonize the official calendar with the harvest times and therefore to ease the tax-collecting process.

The Gregorian calendar, which is used in many parts of the world, has been created to correct the intercalation of the Julian calendar. The same reason led to the development of one of the most famous calendars during the Islamic period, under the reign of the Seljuk ruler Jalāl al-Dīn Malikshāh (d. 1092), whence it is called the Malikī or Jalālī calendar. This calendar is the basis of the solar hijrī calendar, which is now the official calendar of Iran and Afghanistan.

Bibliography

  • Akrami, Musa. “The Development of Iranian Calendar: Historical and Astronomical Foundations.” arxiv.org/abs/1111.4926
  • Kennedy, E. S. “Al-Khwārizmī on Jewish Calendar.” Scripta Mathematica 27 (1964).
  • van Dalen, Benno. “The Chinese-Uighur Calendar in Ṭūsī’s Zīj-i Īlkhānī.” Zeitschrift für Geschichte der arabisch-islamischen Wissenschaften 11 (1997).
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