Origin of the calendar system
Paleolithic
Knowledge of regularly occurring animal migrations was already important for early hunter cultures.
An awareness of seasonally and astronomically repeating events, for corresponding cycles of his environment, man may have had very early. This included the change of day and night and the phases of the moon. Seasonal climate fluctuations played an important role in the agriculture of most regions of the world and could be perceived by man at the latest in the Paleolithic Age. An observation of the changes in the night sky as well as the proper movements of the planets was also possible at this time.
Settlement and Neolithic Age
The Tower of Jericho from the 9th millennium BC already indicates knowledge of the summer solstice, and Neolithic buildings such as Stonehenge bear witness to the efforts of the settled population to be able to precisely determine the natural length of the year and selected cyclically recurring celestial events such as the solstice and the equinox. For agriculture in particular, it was important to be able to determine the time of sowing and harvesting independently of the actual weather conditions. Religious fertility cults were connected with the systematic observation of the sky - based on the hope of a favourable return of fertility conditions. Thus, certain agricultural dates were tied to festivals, which in turn were linked to celestial events.
For the transition from hunter cultures to agriculture in the Neolithic period (New Stone Age), a change in calendrical ideas from the lunar to the solar calendar is assumed. This Stone Age calendar, also called the Neolithic calendar (by Alexander Thom also called the megalithic calendar) probably contains the oldest calendrical ideas of mankind and is the basis of later calendar variants. Analogous to the concept of the Neolithic Revolution (transition to agriculture) is also spoken of the Neolithic Calendar Revolution.
Copper and Bronze Age
The oldest calendars still known today come from the early advanced civilizations of Egypt and Mesopotamia. Here, two basic types of calendars were already apparent, which still characterize most calendar systems today: the lunar calendar oriented to the phases of the moon and the astronomical calendar, which reflects the course of the heavenly bodies.
At the latest by the Babylonians, the seven-day weekly cycle was developed, which today regulates the course of everyday life almost worldwide. In other calendars there were similar cycles, between five and ten days.
The adaptation of weeks and month sequences to the fixed size of the astronomical year was not easy to solve. It came to the formation of different calendar systems.
Observation calendar
Early calendar systems were obtained by observation (astronomical calendars). With the occurrence of a certain defined celestial event (e.g. the new moon or the equinox in spring) a new cycle began. They had to be regularly readjusted.
This method had a decisive disadvantage: In large dominions an event could be perceived at different places possibly at different times, so that also different dates were counted. If, on the other hand, the occurrence of an event was only decisive at a certain place (e.g. the capital or the main temple), then areas far away could often only be informed of it days later. Such problems existed, for example, in the earlier Jewish calendar, where the high priest decided on the first sighting of the crescent moon at new moon. Due to the long information paths, it could happen that a religious festival was celebrated on the "wrong" day in remote areas. Also, shortly before the end of the month it was not possible to predict which date would be, for example, in seven days, because the new moon was not calculated in advance, but determined by daily observation.
More and more cultures therefore began to calculate their calendars. The last serious attempt to establish an observational calendar was made during the French Revolution (French Revolutionary Calendar).
Calendar calculation
The calculation of calendars (arithmetic calendars) requires extensive astronomical and mathematical knowledge. In the development of the early Egyptian astral Sothic calendar, this knowledge was available. The introduction of an Egyptian administrative calendar on a 365-day basis followed at the latest in the third millennium B.C. This, however, could not prevent the passing of the seasons. The Egyptian kings complained about the shifting of the seasons, but it was not until 238 BC that Ptolemy III made an attempt to introduce a leap day. After his death, however, the old Egyptian administrative calendar was again used alongside the new leap day calendar. The Julian calendar, which was introduced by Julius Caesar in 45 BC, was nevertheless based on the calendar form of Ptolemy III.
Switching days
Both lunar and solar calendars must operate with leap days, or different lengths of months, inserted into the normal calendar cycle according to a fixed mathematical rule. A solar calendar normally requires an extra day approximately every four years (in the Gregorian calendar, this is February 29) to adjust the average number of days to the length of the solar year. A lunar calendar must vary the lengths of the months between 29 and 30 days, because the time between two equal lunar phases lasts on average about 29.531 days.
The insertion of an additional day, month, or year into a calendar system is called an embolism (ancient Greek ἐμβάλλειν "to turn on").
