Week , Calendar epoch , Month , Lunisolar calendar , and Computus Equinox seen from the astronomic calendar of Pizzo Vento at Fondachelli Fantina , Sicily The course of the Sun and the Moon are the most evident forms of timekeeping , and the year and lunation were most commonly used in pre-modern societies worldwide as time units.
Nevertheless, the Roman calendar contained very ancient remnants of a pre-Etruscan month solar year. A large number of calendar systems which were based on the Babylonian calendar , and which were found in the Ancient Near East, date from the Iron Age. Amongst such calendar systems was the calendar system of the Persian Empire, which in turn gave rise to the Zoroastrian calendar as well as the Hebrew calendar.
A great number of Hellenic calendars developed in Classical Greece , and with the Hellenistic period also influenced calendars outside of the immediate sphere of Greek influence, giving rise to the various Hindu calendars as well as to the ancient Roman calendar. 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 Julian calendar was no longer dependent on the observation of the new moon but simply followed an algorithm of introducing a leap day every four years. This created a dissociation of the calendar month from the lunation. The Islamic calendar is based on the prohibition of intercalation nasi' by Muhammad , in Islamic tradition dated to a sermon held on 9 Dhu al-Hijjah AH 10 Julian date: This resulted in an observation-based lunar calendar that shifts relative to the seasons of the solar year.
Calendar reform The first calendar reform of the early modern era was the Gregorian calendar , introduced in based on the observation of a long-term shift between the Julian calendar and the solar year.
There have been a number of modern proposals for reform of the calendar, such as the World Calendar , International Fixed Calendar , Holocene calendar , and, recently, the Hanke-Henry Permanent Calendar. Such ideas are mooted from time to time but have failed to gain traction because of the loss of continuity, massive upheaval in implementation, and religious objections. Calendar systems[ edit ] 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. 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 or multiple larger units of time. Calendars that contain one level of cycles: Sun and Moon , Schedel's Nuremberg Chronicle, Lunar calendars are synchronized to the motion of the Moon lunar phases ; 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 the Hebrew calendar. The week cycle is an example of one that is not synchronized to any external phenomenon although it may have been derived from lunar phases, beginning anew every month.
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 calendar Solar calendars assign a date to each solar day.
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 be allowed to 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. Lunar calendar 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 c.
Other marked bones may also represent lunar calendars. Similarly, Michael Rappenglueck believes that marks on a 15,year-old cave painting represent a lunar calendar. An example is the Hebrew calendar which uses a year cycle. Decade , Century , and Millennium Nearly all calendar systems group consecutive days into "months" and also into "years".
In a solar calendar a year approximates Earth's tropical year that is, the time it takes for a complete cycle of seasons , 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. For example, the year in Japan is based on the reign of the current emperor: Other calendar types[ edit ] Arithmetic and astronomical calendars[ edit ] Calendar of the Qahal , 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. Complete and incomplete calendars[ edit ] This section's factual accuracy is disputed.
Relevant discussion may be found on Talk: Please help to ensure that disputed statements are reliably sourced. June Learn how and when to remove this template message Calendars may be either complete or incomplete. Complete calendars provide a way of naming each consecutive day, while incomplete calendars do not.
The early Roman calendar , which had no way of designating the days of the winter months other than to lump them together as "winter", is an example of an incomplete calendar, while the Gregorian calendar is an example of a complete calendar. Calendars in use[ edit ] The primary practical use of a calendar is to identify days: Days may be significant for agricultural, civil, religious or social reasons.
For example, a calendar provides a way to determine when to start planting or harvesting, which days are religious or civil holidays , which days mark the beginning and end of business accounting periods, and which days have legal significance, such as the day taxes are due or a contract expires.
Also a calendar may, by identifying a day, provide other useful information about the day such as its season. Calendars are also used to help people manage their personal schedules, time and activities, particularly when individuals have numerous work, school, and family commitments. People frequently use multiple systems, and may keep both a business and family calendar to help prevent them from overcommitting their time. Calendars are also used as part of a complete timekeeping system: In the modern world, timekeepers can show time, date and weekday.
Gregorian calendar[ edit ] The Gregorian calendar is the de facto international standard, and is used almost everywhere in the world for civil purposes. It is a purely solar calendar, with a cycle of leap days in a year cycle designed to keep the duration of the year aligned with the solar year. Each Gregorian year has either or days the leap day being inserted as 29 February , amounting to an average Gregorian year of It was introduced in as a refinement to the Julian calendar which had been in use throughout the European Middle Ages, amounting to a 0.
During the Early Modern period, however, its adoption was mostly limited to Roman Catholic nations, but by the 19th century, it became widely adopted worldwide for the sake of convenience in international trade.
The last European country to adopt the reform was Greece, in The calendar epoch used by the Gregorian calendar is inherited from the medieval convention established by Dionysius Exiguus and associated with the Julian calendar.
While the Gregorian calendar is itself historically motivated in relation to the calculation of the Easter date , it is now in worldwide secular use as the de facto standard. Alongside the use of the Gregorian calendar for secular matters, there remain a number of calendars in use for religious purposes.
Eastern Christians , including the Orthodox Church , use the Julian calendar. The Islamic calendar or Hijri calendar, is a lunar calendar consisting of 12 lunar months in a year of or days. It is used to date events in most of the Muslim countries concurrently with the Gregorian calendar , and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals.
Its epoch is the Hijra corresponding to AD With an annual drift of 11 or 12 days, the seasonal relation is repeated approximately each 33 Islamic years. Various Hindu calendars remain in use in the Indian subcontinent, including the Nepali calendar , Bengali calendar , Malayalam calendar , Tamil calendar , Vikrama Samvat used in Northern India, and Shalivahana calendar in the Deccan states.
The Buddhist calendar and the traditional lunisolar calendars of Cambodia , Laos , Myanmar , Sri Lanka and Thailand are also based on an older version of the Hindu calendar. The Hebrew calendar is used by Jews worldwide for religious and cultural affairs, also influences civil matters in Israel such as national holidays and can be used there for business dealings such as for the dating of cheques.
National calendars[ edit ] The Chinese , Hebrew , Hindu , and Julian calendars are widely used for religious and social purposes. The Iranian Persian calendar is used in Iran and some parts of Afghanistan.