# List of calendar systems

The following list of calendar systems is sorted according to what is the basis for determining the length of a calendar year . So there is:

Depending on which astronomical observations are based on the length of the year, one differentiates:

• Solar calendar - according to the sun, more precisely the position of the sun (and thus the seasons , moon phases are not taken into account). The length of the year is the assumed mean distance between the points in time with the same position of the sun. The beginning of the year is either a certain position of the sun, usually the spring equinox , or an arbitrary day determined by arithmetic algorithms.
• Lunar calendar - according to the moon, more precisely the moon phases . The year is defined as twelve months, the beginning of which corresponds to identical moon phases, mostly to the new light . Because such a certain year is shorter than the tropical year , it becomes one
• The lunisolar calendar , a subset of the lunar calendar, re-establishes synchronization with the tropical year by inserting a leap month according to fixed rules.
• Lunistellar calendars - according to the moon and stars

If one of the above calendars has not yet been implemented in any society or religion, one speaks of calendar drafts .

## Hints

### Basis of the calendar system: calculation or observation

The basis of all astronomical calendars is the observation of the heavenly bodies. Arithmetic calendars , on the other hand, are based purely on calculations. The best known is the Gregorian calendar with its arithmetic, cyclic leap day arrangements.

Within a calendar system, the basis of the system can change from observation to calculation. Examples are the Jewish calendar and Zoroastrian forms of the Iranian calendar. The background to such a change was the loss of the authorities who had to declare the astronomical observations to be binding (in the case of the Zoroastrians the fall of the Sasanid Empire), or in the case of the Jews, the functionality of these authorities was endangered.

If leap day regulations are included in the conversion formulas for data from different calendar systems (e.g. when converting between dates from the Islamic calendar and the Gregorian calendar), these are approximate formulas, the result of which is occasionally one day off the "real" (historical) date.

### Link to a specific era

A calendar system is usually associated with a certain era (→ epoch ), after which the year is counted or named. However, both calendar and era must be separated. Some Asian calendar systems, such as B. the Japanese calendar , have their traditional calendar calculation in the 19th and 20th centuries. Century abandoned and - with regard to the beginning of the year, month division and leap year calculation - adopted the Gregorian calendar, but its traditional year counting (after the rule of the Tenno in Japan, "since the establishment of the Republic of China in 1912" in Taiwan etc.) is retained. Conversely, the Julian calendar was next to the Christian era also with the Byzantine era (counting the years since the creation of the world), the Diocletian era , the Varronian era (since the foundation of the city of Rome), the year designation after the Roman consuls, the Seleucid era and used in the Rumi calendar with the Islamic era.

The proleptic use of epochs and calendars, i.e. their application to dates before their reference date (date of origin), is uncommon with the exception of the Christian era. Dates before the birth of Christ are made according to the Gregorian or Julian calendar, dates between the Julian calendar reform 45 BC. BC and the Gregorian reform, which came into force on October 15, 1582 in Rome and several Catholic territories, are determined according to the Julian calendar.

### Common and historical calendar systems

A reliable division into common and historical calendar systems is difficult, as some calendars that have hardly been used for centuries are still used by small groups (e.g. the Julian calendar ) and originally forgotten calendars have found new followers after their rediscovery (e.g. . Mayan calendar and Irish Calendar ).

• So far there is no evidence for the existence of “nature calendars”. They are based on regular natural events. In the case of annual events such as the flood of the Nile , "natural calendars" would be, more precisely, solar calendars, since annual natural events coincide with the seasons and are thus also based on the sun.

## Change year calendar

These calendars have a fixed, constant year length of mostly 365 days. They are based on the solar year with 365 14 days, but are not solar calendars because they do not have any activation (intercalation, leap year regulation ). Over the years, the calendar year shifts to the solar year. The result are so-called change years.

## Lunisolar calendar

A lunisolar calendar is the combination of a lunar and a solar calendar.

## Calendar systems that could not previously be assigned

• Indonesian calendar systems