Theory Time-keeping: Calendars

Calendars are time-keeping systems. Historically ancient people had at their disposal regularly occurring astronomical events which seemed obvious to them for time-keeping : day, i.e. daylight and nightime alternating due to Earth's rotation; Moon's phases which gave the month; year, resulting of seasons' unfolding and matching one Earth's revolution about Sun. Usually, people were building stony solar observatories to be able to check the solstices and the other important seasonal moments for planting or harvesting. Shrines and tombs were often also designed with solar, or stellar remarkable moments in mind. Calendards proper began to appear since the 4th century B.C. Most calendars parted into those based on the Sun -which are termed "solar calendars", those based on the Moon ("lunar calendars"), and those based on both ("lunisolar calendars")

As far as they are concerned, solar calendars' main worry is to have any year having the same number of days, and matching however that Earth's revolution around Sun is not completed in a integral number of days. Years shift along time, and do not match seasons' cycle anymore. Plain lunar calendars for their part are systematically shifting relative to the Sun as a solar year does not contain an integral number of lunar months. Lunisolar calendars are trying to adjust lunar and solar calendars so, though their months are based on Moon, they do not shift relatively to a solar year

The Standard: the Gregorian Calendar

The first of rational solar calendars, and first to address the latter problem was the Julian calendar. As Rome until then had a lunar calendar adapted to the solar year, it was deviced at Julius Caesar time, about 45 BC. To avoid years' shift relative to Sun and seasons, it was inserting a leap day to year each four years. Year this day was inserted was called a leap, or bissextile, year. Julian calendar was too arranging March, 21st to be the day of vernal (spring) equinox. But even this intercalation proved short. About the 16th century AD, i.e. about 1,600 years after the calendar was devised, calendar had shifted by about 12 days and spring occurred early March. It was to Pope Gregory XIII to improve the system: leap years were to be only years which were exactly divisible by four, except for those exactly divisible by 100, which were not to be, and centurial years exactly divisible by 400, which were to (year 2000 is leap, 1900 is not). Implementation of Gregorian calendar abruptly shifted the date, and time jumped from October 4th, 1582 to October 15th. It is this Gregorian calendar which nowadays is the international standard civil calendar and it is the same leap system which is still in use. Protestant German countries passed to Gregorian calendar in 1700, England (hence USA) in 1752 (when September, 2nd was followed by September, 14th) and Sweden in 1753 only. Orthodox countries like Russia kept following the old Julian calendar until about 1918

As far as terminology is concerned, English language is using the prefix "AD" and the suffix "BC" to distinguish between years before the year 0 and years after. AD stands for "Anno Domini" (Latin for "the year of our Lord"), BC for "Before Christ". Strictly AD must be used like a prefix before the date, like: AD 1850. "BCE" and "CE" -which stand for "Before Common Era" and "Common Era" respectively- may be used instead. They are the secular equivalents to BC and AD. As far as historical researches are concerned, the Julian calendar does not include the year 0. The year 1 BC is followed by the year AD 1. This dates back to the 6th century, as the concept of zero was still unknown. Most astronomical softwares work with the astronomical years instead, which include a year 0. Their system however is transparent for the user who, for his part, is working with the years BC and AD. The year 1 BC (BCE) is the astronomical year 0; the year 101 BC (BCE) is the astronomical year -100

Contemporary Lunar and Lunisolar Calendars

The Islamic calendar is a lunar calendar. As based on lunar months, it is shifting relative to the Gregorian calendar. A complete shift takes about 33 years. The lunar aspect of the Islamic calendar is seen when Muslims clerics look for the very first crescent after the new moon to determine the fast period in the Islamic countries. Israel's civil calendar, India's and China's religious calendars are of the lunisolar type. They are trying to adjust lunar and solar rythms in the way that, although they are based on lunar months, they do not shift relative to the solar year. As far as the Chinese calendar is concerned, its present working was devised by Johann Adam Schall von Bell (1599-1666), a Jesuit missionary in China as the Jesuits had been asked to reform the computing rules of the annual ephemerides as such rules, over time, had brought errors of prediction, which the state of the astronomy understanding amongst Chinese did not allow them to understand and correct. In ancient China, the annual ephemeris was one of the most important state affair, with its astrological implications. The Chinese New Year falls each year on the day of the second new moon of winter