Calendar
Since the earliest days of civilisation, calendars have expressed the human need to systematise their experience of time by dividing it into smaller numbered segments thus making it easier to keep a record of it. Virtually all calendar systems have been based on cosmic cycles. The calendar had both religious and practical purposes. It created the framework for the rhythm of festive days on which humans honoured the divine and made it possible to plan ahead and calculate the most favourable times for the sowing and harvesting of crops.
The earliest calendars were probably lunar calendars with a cycle of 29.5 days (which is equivalent to one synodic lunar cycle). According to this method, twelve Moons (months), or the time taken for the Moon to travel through the whole zodiac, is 354 days. This is the length of one lunar year. With the passage of time, the solar cycle proved to be more reliable, firstly because of its greater length, and secondly because it was a more reliable method for calculating the seasons. However, the synchronisation of the solar and lunar cycles proved to be difficult. To prevent the disparity between the two from becoming too great, ancient cultures alternated between using a lunar year of twelve or thirteen months in length.
The oldest evidence of a luni-solar calendar (which incorporates the cycles of both the Moon and the Sun) originates from ancient Babylon and is barely 4000 years old. The Egyptians were the first to use an exclusively solar calendar. The solar calendar made it easier to predict the most important natural phenomenon in ancient Egypt - the annual flooding of the Nile upon which Egyptian agriculture was dependent. One indicator of this was the reappearance of the star Sirius, the brightest fixed star in the Northern hemisphere. Egyptian priests calculated the length of one year to be 365 days, divided into twelve months of thirty days plus five days left over. However, they soon realised that the actual length of the solar year was somewhat longer. They therefore introduced one extra day (leap day) every four years.
Julian Calendar
By the time of the reign of Julius Caesar (100-44 BCE), each culture had developed fairly accurate calendars. Jews, Arabians and Greeks still based their calendars on lunar cycles, a custom that was continued by the Islamic religion which has its roots in Arabic culture. In 46 BCE Caesar finally ordered a thorough reform of the calendar. The main aim would appear to be that Caesar believed that the standardisation of time throughout the Roman Empire was a necessary prerequisite for the centralisation of political power. The so-called Julian Calendar was based on Egyptian calculations, including the use of a leap day. The length of the year was then 365.25 days, and the months alternated between 29 and 30 days in length. This division had many advantages. After 28 years all dates fell on the same weekday, making it easier to calculate both retrospectively and in advance on which day a particular date would fall. The calendar had only one disadvantage, it was a bit - 0.0075 days - longer than the solar cycle.
Gregorian Calendar
Astronomers in the Middle Ages came to recognise that the Julian calendar was slightly longer than the solar cycle. In the year 325, the council of Nicaea defined the start of spring to be the 21st March. Twelve hundred years later, astronomically it had moved to the 31st March. This caused Pope Gregory 13th to order a new calendar reform in 1582. By this time, astronomers were aware that the length of one solar cycle is 365.2415 days. For practical purposes this meant dropping leap days in the years 1700, 1900, 2100 and so on, every 200 years. This allowed for such an accurate correlation between the calendar and the solar cycle that the former would only deviate from the latter by one day every 3000 years. At the time of the calendar reform, the deviation had already reached 11 days. In order to bring the calendar back into alignment with the solar cycle, the Pope decreed that the days between the 4th and the 15th October 1582 be left out of the new calendar on its introduction in the autumn of 1582.
Despite the astronomical accuracy, it was a long time before all other cultures adopted the Gregorian calendar. The catholic church introduced it immediately whereas some of the protestant churches retained the Julian calendar up until the 18th century. The Julian calendar was first introduced into orthodox Russia in the 13th century and was used throughout the reign of Tsars. The Papal reform was first introduced during the Bolshevik Revolution, and by that time 13 days had to be left out in order to bring the calendar into alignment with the solar cycle. It was therefore decreed in 1914 that the 1st February would be followed by the 14th February. Turkey followed suit in 1927 and China finally introduced the Gregorian reform in 1949.
Since then, the Gregorian calendar has become the world's standard, although the starting date varies with different cultures. The Jewish calendar begins in 3761 BCE which marks the beginning of creation according to Jewish teaching. The Chinese calendar begins in 2637 BCE with the epoch of the mythical emperor Huang-ti, and the Buddhist calendar, in 544 BCE, the date of Buddha's enlightenment. The Tibetan calendar begins in 127 BCE, the beginning of their royal lineage. The Moslem calendar begins in 622 CE, the year in which Mohammed fled Mecca for Medina.
See also
Weblinks
- Wikipedia: Calendar
- Wikipedia: Month
- Wikipedia: Lunisolar Calendar
- Calendars through the Ages (WebExhibits, 2008)
- The Moon & Ancient Calendars (Robin Heath, 2004)
- Astronomy & Astrology of the Solar Hero Myth (Heath, 2004)
- Calendars and Calendar Reform ("Starry Messenger", University of Cambridge, 1999)
- The Egyptian Calendar and the Zodiac (Deborah Houlding 1996, 2003)
- 13 Months: The Kodak Calendar Experiment (theinternetsaysitstrue.com; 2022)