CALENDARS OF THE WORLD

Julian and Gregorian Calendar

The Julian calendar, also called the Old Style calendar, is a dating system established by Julius Caesar as a reform of the Roman republican calendar. Caesar, advised by the Alexandrian astronomer Sosigenes, made the new calendar solar, not lunar, and he took the length of the solar year as 365¼ days. The year was divided into 12 months, all of which had either 30 or 31 days except February, which contained 28 days in common (365-day) years and 29 in every fourth year (a leap year, of 366 days). Because of misunderstandings, the calendar was not established in smooth operation until AD 8. Further, Sosigenes had overestimated the length of the year by 11 minutes 14 seconds, and by the mid-1500s, the cumulative effect of this error had shifted the dates of the seasons by about 10 days from Caesar’s time.

This inaccuracy led Pope Gregory XIII to reform the Julian calendar. His Gregorian calendar, also called the New Style calendar, is still in general use. Gregory’s proclamation in 1582 restored the calendar to the seasonal dates of AD 325, an adjustment of 10 days. Although the amount of regression was some 14 days by Pope Gregory’s time, Gregory based his reform on restoration of the vernal equinox, then falling on 11 March, to the date (21 March) it had in AD 325, the time of the Council of Nicaea. Advancing the calendar 10 days after 4 Oct 1582, the day following being reckoned as 15 October, effected the change.

The Gregorian calendar differs from the Julian only in that no century year is a leap year unless it is exactly divisible by 400 (e.g., 1600, 2000). A further refinement, the designation of years evenly divisible by 4,000 as common (not leap) years, will keep the Gregorian calendar accurate to within one day in 20,000 years.

Jewish Calendar

The Jewish calendar is lunisolar—i.e., regulated by the positions of both the Moon and the Sun. It consists usually of 12 alternating lunar months of 29 and 30 days each (except for Heshvan and Kislev, which sometimes have either 29 or 30 days), and totals 353, 354, or 355 days per year. The average lunar year (354 days) is adjusted to the solar year (365¼ days) by the periodic introduction of leap years in order to assure that the major festivals fall in their proper season. The leap year consists of an additional 30-day month called First Adar, which always precedes the month of (Second) Adar. (During leap year, the Adar holidays are postponed to Second Adar.) A leap year consists of either 383, 384, or 385 days and occurs seven times during every 19-year period (the so-called Metonic cycle). Among the consequences of the lunisolar structure are these:

(1) The number of days in a year may vary considerably, from 353 to 385 days.
(2) The first day of a month can fall on any day of the week, that day varying from year to year. Consequently, the days of the week upon which an annual Jewish festival falls vary from year to year despite the festival’s fixed position in the Jewish month.

Muslim Calendar

The Muslim calendar (also called the Islamic calendar, or Hijrah) is a dating system used in the Muslim world that is based on a year of 12 months. Each month begins with the sighting of the crescent of the new moon as it emerges from eclipse. The months of the Muslim calendar are Muharram, Safar, Rabi I, Rabi II, Jumada I, Jumada II, Rajab, ShaAban, Ramadan, Shawwal, Dhu al-QaAdah, and Dhu al-Hijjah.

In the standard Muslim calendar the months are alternately 30 and 29 days long except for the 12th month, Dhu al-Hijjah, the length of which is varied in a 30-year cycle intended to keep the calendar in step with the true phases of the Moon. In 11 years of this cycle, Dhu al-Hijjah has 30 days, and in the other 19 years it has 29. Thus the year has either 354 or 355 days. No months are intercalated, so that the named months do not remain in the same seasons but retrogress through the entire solar, or seasonal, year (of about 365.25 days) every 32.5 solar years.

There are some exceptions to this calendar in the Muslim world. Turkey uses the Gregorian calendar, while the Iranian Muslim calendar is based on a solar year. The Iranian calendar still begins from the same dating point as other Muslim calendars—that is, some 10 years prior to the death of Muhammad in AD 632. Thus, the Gregorian year AD 2010 corresponds to the Hijrah years of AH 1431–32.

Chinese Calendar

The Chinese calendar is a dating system used concurrently with the Gregorian (Western) calendar in China and Taiwan and in neighboring countries (e.g., Japan). The calendar consists of 12 months of alternately 29 and 30 days, equal to 354 or 355 days, or approximately 12 full lunar cycles. Intercalary months have been inserted to keep the calendar year in step with the solar year of about 365 days. Months have no names but are instead referred to by numbers within a year and sometimes also by a series of 12 animal names that from ancient times have been attached to years and to hours of the day.

The calendar also incorporates a meteorologic cycle that contains 24 points, each beginning one of the periods named. The establishment of this cycle required a fair amount of astronomical understanding of the Earth as a celestial body. Modern scholars acknowledge the superiority of pre-Sung Chinese astronomy (at least until about the 13th century AD) over that of other, contemporary nations.

The 24 points within the meteorologic cycle coincide with points 15° apart on the ecliptic (the plane of the Earth’s yearly journey around the Sun or, if it is thought that the Sun turns around the Earth, the apparent journey of the Sun against the stars). It takes about 15.2 days for the Sun to travel from one of these points to another (because the ecliptic is a complete circle of 360°), and the Sun needs 365¼ days to finish its journey in this cycle. Supposedly, each of the 12 months of the year contains two points, but, because a lunar month has only 29½ days and the two points share about 30.4 days, there is always the chance that a lunar month will fail to contain both points, though the distance between any two given points is only 15°. If such an occasion occurs, the intercalation of an extra month takes place. For instance, one may find a year with two “Julys” or with two “Augusts” in the Chinese calendar. In fact, as mentioned above, the exact length of the month in the Chinese calendar is either 30 days or 29 days—a phenomenon that reflects its lunar origin.

Excerpted from "Encyclopedia Britannica 2010 Almanac p.118-120. Adapted and illustrated to be posted by Leopoldo Costa.