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The history of calendars spans several thousand years. In many early civilizations, calendar systems were developed. For example, in Sumer, the birthplace of the modern sexagesimal system, there were 12 months of 29 or 30 days apiece, much like the modern Gregorian calendar. Roman cultures also developed their own intricate calendars; the ancient Maya had two separate years—the 260-day Sacred Round, and the 365-day Vague Year. Classical Greek and Roman cultures also developed calendars; the ancient Athenians, for one, had a lunisolar calendar that lasted 364 days, with an intercalary month added every other year. The Romans used two different year lengths; the older one had 304 days divided into 10 months; the newer 365 days divided into 12 months; very much like the modern calendar. They counted years from the founding of Rome, or, sometimes, from the reign of the current consul.
The ancient Sumerian calendar divided a year into 12 lunar months of 29 or 30 days. Each month began with the sighting of a new moon. Sumerian months had no uniform name throughout Sumer because of the religious diversity. This resulted in scribes and scholars referring to them as "the first month", "the fifth month" etc. To keep the lunar year of 354 days in step with the solar year of 365.25 days an extra month was added periodically, much like a Gregorian leap year. Also, every six years the Sumerian calendar included an extra month of 62 days.
There were no weeks in the Sumerian calendar. Holy days and time off from work were usually celebrated on the first, seventh and fifteenth of each month. In addition to these holy days, there were also feast days which varied from city to city. A day was divided into twelve hours, six daylight hours, each lasting one-sixth of the day, and six nighttime hours, each lasting one-sixth of the night. This meant the length of hours varied from season to season, daylight hours being shorter in the winter and longer in the summer and vice versa.
Of all the ancient calendar systems, the Maya and other Mesoamerican systems are the most complex. The Mayan calendar had 2 years, the 260-day Sacred Round, or tzolkin, and the 365-day Vague Year, or haab.
The Sacred Round of 260 days is composed of two smaller cycles: the numbers 1 through 13, coupled with 20 different day names: Imix, Ik, Akbal, Kan, Chicchan, Cimi, Manik, Lamat, Muluc, Oc, Chuen, Eb, Ben, Ix, Men, Cib, Caban, Eiznab, Cauac, and Ahau. The Sacred Round was used to determine important activities related to the gods and humans: name individuals, predict the future, decide on auspicious dates for battles, marriages, and so on.
The two cycles of 13 and 20 intermesh and are repeated without interruption: the cycle would begin with 1 Imix, then 2 Ik, then 3 Akbal and so on until the number 13 was reached, at which point the number cycle was restarted so 13 Ben would be followed by 1 Ix, 2 Men and so on. This time Imix would be numbered 8. The cycle ended after 260 days, with the last day being 13 Ahau.
The Vague Year of 365 days is similar to our modern calendar, consisting of 18 months of 20 days each, with an unlucky five-day period at the end. The Vague Year had to do primarily with the seasons and agriculture, and was based on the solar cycle. The 18 Maya months are known, in order, as: Pop, Uo, Zip, Zotz, Tzec, Xuc, Yaxkin, Mol, Chen, Yax, Zac, Ceh, Mac, Kankin, Maun, Pax, Kayab and Cumku. The unlucky five-day period was known as Uayeb, and was considered a time which could hold danger, death and bad luck.
The Vague Year began with the month of Pop. The Maya 20-day month always begins with the seating of the month, followed by days numbered 1 to 19, then the seating of the following month, and so on. This ties in with the Maya notion that each month influences the next. The Maya new year would start with 1 Pop, followed by 2 Pop, all the way through to 19 Pop, followed by the seating of the month of Uo, written as 0 Uo, then 1 Uo, 2 Uo and so on. These two cycles coincided every 52 years. The 52-year period of time was called a "bundle" and was similar to a modern day century.
The ancient Athenian calendar was a lunisolar calendar with 354 day years, consisting of twelve months of alternating length of 29 or 30 days. To keep the calendar in line with the solar year of 365.25 days, an extra, intercalary month was added in every other year. The Athenian months were called Hekatombion, Metageitnion, Boedromion, Pyanepsion, Maimakterion, Poseidon, Gamelion, Anthesterion, Elaphebolion, Munychion, Thargelion, and Skirophorion. The intercalary month usually came after Poseidon, and was called second Poseidon.
In addition to their regular, "festival" calendar, the Athenians maintained a second, political calendar . This "conciliar" calendar divided the year into "prytanies", one for each of the "phylai", the subdivisions of Athenian citizens. The number of phylai, and hence the number of prytanies, varied over time. Until 307 BC, there were 10 phylai. After that the number varies between 11 and 13 (usually 12). Even more confusing, while the conciliar and festival years were about the same length in the 4th century BC, such was not regularly the case earlier or later. documents dated by prytany are frequently very difficult to assign to a particular equivalent in the Gregorian calendar.
The table of Greek Olympiads, following the four-year cycles between the Olympic Games from 1 July 776 BC, continued until the end of the 4th century AD. The Babylonian Era of Nabonassar, beginning on 26 February 747 BC, was used by the Greeks of Alexandria. It was later known in the Middle Ages from the works of Ptolemy. Additionally there was the Macedonian Era of the Seleucids, which began with the conquest of Babylon by Seleucus Nicator in 312 BC. It became widely used in the Levant. The Jews knew it as the "era of contracts", and used it in Europe until the 15th century.
Roman dates were sometimes calculated "from the founding of the city" of Rome, or ab urbe condita (AUC). This was originally assumed to be 750 BC, although calculations by Marcus Terentius Varro in the 1st century BC determined 753 BC to be the founding date. An alternative system had become more common even by Varro's time, whereby the Romans referred to the names of the consuls rather than the date of the year. References to the year of consulship were used in both conversation and official records. Romans from the same family often had the same praenomen, which sometimes makes it difficult to distinguish them, and there were two consuls at any one time, each of whom might sometimes hold the appointment more than once, meaning that it was (and is) necessary to be well educated in history to understand the references. The two systems were compatible; so that the consulship of Quintus Fufius Calenus and Publius Vatinius could be determined as 707 AUC (or 47 BC), the third consulship of Caius Julius Caesar, with Marcus Aemilius Lepidus, as 708 AUC (or 46 BC), and the fourth consulship of Gaius Julius Caesar as 709 AUC (or 45 BC).
The Romans had an eight-day week, with the market-day falling every eight days. It was called a nundinum or 'nine-day', because it was the ancient practice to count both the first and the last day (cf. the practice of calling Easter Sunday the 'third day' after Good Friday)
The old Roman year had 304 days divided into 10 months, beginning on XI Kal. Maias, or 21 April. The extra months Ianuarius and Februarius had been invented as stop-gaps. Julius Caesar realized that the system had become inoperable, so he effected drastic changes in the year of his third consulship. The New Year in 709 AUC began on 1 January and ran over 365 days until 31 December. Further adjustments were made under Augustus, who introduced the concept of the "leap year" in 737 AUC (AD 4). The resultant Julian calendar remained in almost universal use in Europe until 1582.
For the first six centuries since the birth of Jesus Christ, European countries used various local systems to count years, most usually regnal years, modeled on the Old Testament. In some cases, Creation dating was also used. In the 6th century, the Christian monk Dionysius Exiguus devised the Anno Domini system, dating from the Incarnation of Jesus. In the 8th century, the Anglo-Saxon historian Bede the Venerable used another Latin term, "ante uero incarnationis dominicae tempus" ("the time before the Lord's true incarnation", equivalent to the English "before Christ"), to identify years before the first year of this era.
According to the Catholic Encyclopedia, even Popes continued to date documents according to regnal years, and usage of AD only gradually became common in Europe from the 11th to the 14th centuries. In 1422, Portugal became the last Western European country to adopt the Anno Domini system.
In medieval Scandinavia, there were two seasons: summer and winter. There were twelve lunar months in the Old Icelandic lunar calendar: Harpa ("Harpa-month"), Stekktíd ("Lamb-fold-time"), Sólmánuður ("Sun-month"), Miðsumar ("Midsummer"), Heyannir ("Hay-month"), Haustmábuður ("Harvest-month"), Gormánuður ("Slaughtering-month"), Ýlir ("Ylir"), Hrútmánaður ("Ram-month"), Þorri ("Thorri-month"), Góa ("Goa-month") and Einmánuður ("Last month of Winter"). The early days of summer (sumarmál) began in the modern month of April, and summer lasted until October. The first month of summer, Harpa, roughly corresponds to April-May. The local "Spring Assembly", or vorþing, took place at the end of the fourth week of summer; lasting four to seven days between 7 and 27 May. The Althing, or national parliament, was held when eight weeks of summer had passed. Lastly, the Autumn Meeting took place no later than ten weeks before the end of summer.
"Winter" lasted from October to April. The two-day period when winter began, the Winter Nights or veturnætur, occurred around the middle of October. It was a particularly holy time of the year, when sacrifices were made to the local guardian spirits and other social events such as games meetings and weddings took place. The first month of winter, Gormánuður, was also the time when animals were slaughtered so that their meat could be stored for the winter.
In 1267, the medieval scientist Roger Bacon stated the times of full moons as a number of hours, minutes, seconds, thirds, and fourths (horae, minuta, secunda, tertia, and quarta) after noon on specified calendar dates. Although a third for 1⁄60 of a second remains in some languages, for example Polish tercja and Arabic ثالثة, the modern second is further divided decimally.
Rival calendar eras to Anno Domini remained in use in Christian Europe. In Spain, the "Era of the Caesars" was dated from Octavian's conquest of Iberia in 39 BC. It was adopted by the Visigoths and remained in use in Catalonia until 1180, Castille until 1382 and Portugal until 1415.
For chronological purposes, the flaw of the Annon Domini system was that dates have to be reckoned backwards or forwards according as they are BC or AD. According to the Catholic Encyclopedia, "in an ideally perfect system all events would be reckoned in one sequence. The difficulty was to find a starting point whence to reckon, for the beginnings of history in which this should naturally be placed are those of which chronologically we know least." For both Christians and Jews, the prime historical date was the Year of Creation, or Annus Mundi. The Byzantine Church fixed the date of Creation at 5509 BC. This remained the basis of the ecclesiastical calendar in the Greek and Russian Orthodox world until modern times. The Coptic Church fixed on 5500 BC. Later, the Church of England, under Archbishop Ussher in 1650, would pick 4004 BC. Jewish scholars preferred 3761 BC as the date of creation, which forms the basis of the modern Jewish calendar. However, "any attempt thus to determine the age of the world has been long since abandoned."