Calendar


A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A date is the designation of a single and specific day within such a system. A calendar is also a physical record of such a system. A calendar can also mean a list of planned events, such as a court calendar, or a partly or fully chronological list of documents, such as a calendar of wills.
Periods in a calendar are usually, though not necessarily, synchronized with the cycle of the sun or the moon. The most common type of pre-modern calendar was the lunisolar calendar, a lunar calendar that occasionally adds one intercalary month to remain synchronized with the solar year over the long term.

Etymology

The term calendar is taken from calends, the term for the first day of the month in the Roman calendar, related to the verb calare 'to call out', referring to the "calling" of the new moon when it was first seen. Latin calendarium meant 'account book, register'. The Latin term was adopted in Old French as calendrier and from there in Middle English as calender by the 13th century.

History

The course of the Sun and the Moon are the most salient regularly recurring natural events useful for timekeeping, and in pre-modern societies around the world lunation and the year were most commonly used as time units. Nevertheless, the Roman calendar contained remnants of a very ancient pre-Etruscan 10-month solar year.
The first recorded physical calendars, dependent on the development of writing in the Ancient Near East, are the Bronze Age Egyptian and Sumerian calendars.
File:Close-up of calendar in Spanish - Calendario en español.jpg|thumb|Calendar in Spanish language, usually from Spain
During the Vedic period India developed a sophisticated timekeeping methodology and calendars for Vedic rituals. According to Yukio Ohashi, the Vedanga calendar in ancient India was based on astronomical studies during the Vedic Period and was not derived from other cultures.
A large number of calendar systems in the Ancient Near East were based on the Babylonian calendar dating from the Iron Age, among them the calendar system of the Persian Empire, which in turn gave rise to the Zoroastrian calendar and the Hebrew calendar.
A great number of Hellenic calendars were developed in Classical Greece, and during the Hellenistic period they gave rise to the ancient Roman calendar and to various Hindu calendars.
Calendars in antiquity were lunisolar, depending on the introduction of intercalary months to align the solar and the lunar years. This was mostly based on observation, but there may have been early attempts to model the pattern of intercalation algorithmically, as evidenced in the fragmentary 2nd-century Coligny calendar.
The Roman calendar was reformed by Julius Caesar in 46 BC. His "Julian" calendar was no longer dependent on the observation of the new moon, but followed an algorithm of introducing a leap day every four years. This created a dissociation of the calendar month from lunation. The Gregorian calendar, introduced in 1582, corrected most of the remaining difference between the Julian calendar and the solar year.
The Islamic calendar is based on the prohibition of intercalation by Muhammad, in Islamic tradition dated to a sermon given on 9 Dhu al-Hijjah AH 10. This resulted in an observation-based lunar calendar that shifts relative to the seasons of the solar year.
There have been several modern proposals for reform of the modern calendar, such as the World Calendar, the International Fixed Calendar, the Holocene calendar, and the Hanke–Henry Permanent Calendar. Such ideas are promoted from time to time, but have failed to gain traction because of the loss of continuity and the massive upheaval that implementing them would involve, as well as their effect on cycles of religious activity.

Systems

A full calendar system has a different calendar date for every day. Thus the week cycle is by itself not a full calendar system; neither is a system to name the days within a year without a system for identifying the years.
The simplest calendar system just counts time periods from a reference date, or epoch. This applies for the Julian day or Unix Time. Virtually the only possible variation is using a different reference date, in particular, one less distant in the past to make the numbers smaller. Computations in these systems are just a matter of addition and subtraction.
Other calendars have one larger units of time.
Calendars that contain one level of cycles:
Calendars with two levels of cycles:
  • year, month, and day – most systems, including the Gregorian calendar, the Islamic calendar, the Solar Hijri calendar and the Hebrew calendar
  • year, week, and weekday – e.g., the ISO week date
Cycles can be synchronized with periodic phenomena:
File:Sun and Moon Nuremberg chronicle.jpg|thumbnail|Sun and Moon, Schedel's Nuremberg Chronicle, 1493
  • Lunar calendars are synchronized to the motion of the Moon ; an example is the Islamic calendar.
  • Solar calendars are based on perceived seasonal changes synchronized to the apparent motion of the Sun; an example is the Persian calendar.
  • Lunisolar calendars are based on a combination of both solar and lunar reckonings; examples include the traditional calendar of China, the Hindu calendar in India and Nepal, and the Hebrew calendar.
  • The week cycle is an example of one that is not synchronized to any external phenomenon.
Very commonly a calendar includes more than one type of cycle or has both cyclic and non-cyclic elements.
Most calendars incorporate more complex cycles. For example, the vast majority of them track years, months, weeks and days. The seven-day week is practically universal, though its use varies. It has run uninterrupted for millennia.

Solar

Solar calendars assign a date to each solar day, which is based on the apparent motion of the Sun. A day may consist of the period between sunrise and sunset, with a following period of night, or it may be a period between successive events such as two sunsets. The length of the interval between two such successive events may vary slightly during the year, or it may be averaged into a mean solar day. Other types of calendar may also use a solar day.
The Egyptians appear to have been the first to develop a solar calendar, using as a fixed point the annual sunrise reappearance of the Dog Star—Sirius, or Sothis—in the eastern sky, which coincided with the annual flooding of the Nile River. They built a calendar with 365 days, divided into 12 months of 30 days each, with 5 extra days at the end of the year. However, they did not include the extra bit of time in each year, and this caused their calendar to slowly become inaccurate.

Lunar

Not all calendars use the solar year as a unit. A lunar calendar is one in which days are numbered within each lunar phase cycle. Because the length of the lunar month is not an even fraction of the length of the tropical year, a purely lunar calendar quickly drifts against the seasons, which do not vary much near the equator. It does, however, stay constant with respect to other phenomena, notably tides. An example is the Islamic calendar.
Alexander Marshack, in a controversial reading, believed that marks on a bone baton represented a lunar calendar. Other marked bones may also represent lunar calendars. Similarly, Michael Rappenglueck believes that marks on a 15,000-year-old cave painting represent a lunar calendar.

Lunisolar

A lunisolar calendar is a lunar calendar that compensates by adding an extra month as needed to realign the months with the seasons. Prominent examples of lunisolar calendar are Hindu calendar and Buddhist calendar that are popular in South Asia and Southeast Asia. Another example is the Hebrew calendar, which uses a 19-year cycle.

Subdivisions

Nearly all calendar systems group consecutive days into "months" and also into "years". In a solar calendar a year approximates Earth's tropical year, traditionally used to facilitate the planning of agricultural activities. In a lunar calendar, the month approximates the cycle of the moon phase. Consecutive days may be grouped into other periods such as the week.
Because the number of days in the tropical year is not a whole number, a solar calendar must have a different number of days in different years. This may be handled, for example, by adding an extra day in leap years. The same applies to months in a lunar calendar and also the number of months in a year in a lunisolar calendar. This is generally known as intercalation. Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length.
Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years. Many cultures use different baselines for their calendars' starting years. Historically, several countries have based their calendars on regnal years, a calendar based on the reign of their current sovereign. For example, the year 2006 in Japan is year 18 Heisei, with Heisei being the era name of Emperor Akihito.

Other types

Arithmetical and astronomical

An astronomical calendar is based on ongoing observation; examples are the religious Islamic calendar and the old religious Jewish calendar in the time of the Second Temple. Such a calendar is also referred to as an observation-based calendar. The advantage of such a calendar is that it is perfectly and perpetually accurate. The disadvantage is that working out when a particular date would occur is difficult.
An arithmetic calendar is one that is based on a strict set of rules; an example is the current Jewish calendar. Such a calendar is also referred to as a rule-based calendar. The advantage of such a calendar is the ease of calculating when a particular date occurs. The disadvantage is imperfect accuracy. Furthermore, even if the calendar is very accurate, its accuracy diminishes slowly over time, owing to changes in Earth's rotation. This limits the lifetime of an accurate arithmetic calendar to a few thousand years. After then, the rules would need to be modified from observations made since the invention of the calendar.