Ancient cultures used the phases of the moon to record the passing of time and track the seasons, but this method was not reliable. A month was calculated by ancient people as the time between two full moons - the number of days required for the moon to circle the earth. This measurement of twenty-nine and a half days is called a synodic or lunar month and resulted in a lunar year of 354 days.
The week is derived from the Judeo-Christian tradition which required rest from labour every seventh day. It is not based on a natural phenomenon such as the rotation of the earth and the moon. However, the Romans named the days of the week in honour of the sun, moon and various planets.
The ancient Babylonians used a calendar of twelve lunar months of thirty days each, adding extra months when necessary to keep the calendar in line with the seasons of the year. This is known as a lunisolar calendar - periodic adjustments being made to keep the lunar and solar cycles reconciled.
It was the ancient Egyptians, some 6,000 years ago, who first developed an accurate calendar by measuring the ebbs and flows of the Nile River with a notched stick in the riverbank. The Nile floods with an annual regularity that Egyptians had long understood. By timing the high water marks, they realised that the year was 365 days long. They established a calendar of twelve months, each of thirty days, with an extra five days at the end before the cycle repeated.
Egyptian astronomers had noticed that the flood occurred when Sirius, the Dog Star, rose at dawn and aligned with the rising sun. As this happened only once a year, by observing its annual appearance, they understood that the year was 365 and a quarter days long. Unfortunately, as in so many other cultues, time was regarded as sacred and controlled by priests who stubbornly refused to add the extra quarter day, or insert a leap lear. This had to wait until a time when Rome had conquered Egypt, some 4,000 years later.
The first Roman calendar was introduced in the 7th century BC. It had ten months adding up to 304 days in a year beginning in March. Two more months, January and February, were soon added but even then, because the months were only twenty-nine or thirty days long, an extra month had to be inserted approximately every second year.
The calculation of the length of a month was an awkward task which involved counting backwards from three dates - the calends or first day of the month, the ides or middle day of the month (the thirteenth of some months and the fifteenth of others), and the nones or ninth day before the ides. The calculation of the days and months became hopelessly confused when the responsible officials abused their authority to prolong their term of office or to hasten or delay elections.
It was the Roman Emperor, Julius Caesar, who, in 46 BC, unified time across the Roman Empire and gave the world the basis of its modern calendar. Ironically, it was Caesar's time in Egypt with Cleopatra that influenced him to reform the calendar in line with the solar year.
Caesar extended the number of days in a Roman year from 355 to 365, included a 'leap year' to balance the annual extra quarter of a day and rearranged the number of days in the months to, alternately, thirty and thirty-one. The leap year was so named because the extra day in February causes any day after February in a leap year to 'leap' over one day in the week and to occur two days later in the week than it did in the previous year.
After he was assinated, the fifth month was renamed 'Julius' in his honour. Unfortunately, when Augustus Caesar succeeded Julius, the Senate renamed the sixth month in Augustus' honour, and 'borrowed' a day from February to ensure that Augustus' month was the same length as that named after Julius. To avoid having three thirty-one day months in a row, they switched the alternating number of days in the last four months.
Despite the Senate's meddling, the new calendar was a great improvement. However, it was still flawed by a matter of minutes and scholars and astronomers quickly realised that time was once again slipping away. A solar year - the average time taken for one revolution of the earth around the sun - is 365 days, 5 hours, 48 minutes and 45.5 seconds. The Julian year was thus 11 minutes and 14.5 seconds longer than the solar year.
In the late 7th century, an English monk, the Venerable Bede, realised that the vernal equinox (when day and night are equal in spring) was occurring three days earlier than its declared calendar date. In the middle of the 13th century, British friar Roger Bacon made impassioned pleas to the church to correct the calendar, as religious dates had become hopelessly out of kilter.
Adding to all this confusion was the fact that time had no formal beginning. Events were dated according to the reigns of kings and emperors in each region. In the 6th century, a Scythian monk named Dionysius Exiguus (Denis the Short) first proposed dating things from the Birth of Christ, but it was Bede who first dated and significantly documented history in this way. The BC (before Christ) concept did not appear until the 17th century.
As the concept of zero had not been invented in either Denis the Short or Bede's time, they both began with the year 1AD (Anno Domini - latin for 'in the year of our Lord'). When time prior to that was later denoted 1BC, the absence of the year zero became the cause of eternal arguments over when a new century and a new millennium actually begins.
By the 16th century, it was widely apparent that the minutes overlooked by Julius Caesar had accumulated into a 10-day error in the calendar. Pope Gregory XIII finally reformed the calendar on 4th October, 1582 when he decided that the vernal equinox should next occur on March 21, as it had in 325 AD - the year of the Council of Nicaea. To achieve this, he issued a decreed in which ten days were dropped, skipping the calendar forward to again match the solar year.
To prevent displacement in the future, he decreed that century years divisible evenly by 400 should remain as leap years, but all other century years should be common years. Thus 1600 and 2000 were leap years, while 1700, 1800 and 1900 were not.
This highly accurate calendar, named the Gregorian calendar, was adopted in staunchly Catholic countries throughout Europe, while less devout Protestant countries did not follow suit for nearly 200 years. By the time Great Britain adopted the Gregorian calendar in 1752, yet another day had slipped away. An eleven day adjustment was made where the day after September 2, 1752 became September 14. Many Eastern European countries did not change until the outbreak of World War I, and Russia waited until 1918.
Today, the Gregorian calendar is used throughout most of the world. Minor changes are still made by the British Royal Observatory and National Physical Laboratory, in Greenwich near London, where milli-seconds and occasionally seconds are introduced to keep the system completely accurate.
Derived from an ancient Hebrew calendar, the Jewish calendar has remained unchanged since about 900AD. The starting point of Hebrew chronology is the year 3761 BC, the estimated date of the creation of the world described in the Old Testament. The Jewish calendar is lunisolar, based on alternating lunar months of twenty-nine and thirty days, with an extra month intercalculated every three years, within a ninteen year cycle. Dates of the Jewish calendar are designated AM (Anno Mundi - latin for 'the year of the world') and BCE (before the Christian era).
In the middle east, the Islamic calendar is used extensively. It has a starting date of 16 July, 622AD, the day after the Hegira. It also, is a lunisolar calendar, with twelve lunar months of either twenty-nine or thirty days, with intercalculated days based on a thirty-year cycle.
Months of the Julian and Gregorian Calendars.
The months of the calendar are featured as privy marks on each of the 12 coins in the 2000 Australian Kookaburra Millennium Calendar Privy Mark Collection