The sequence of events

The introduction of writing is intimately linked with the keeping of records. Important among these, in many societies, are records of events - moments of great significance for the community or its leaders, of the kind to be inscribed on monuments. But commemorating the event itself is not sufficient. There is a desire to tell posterity when it happened. This involves naming a year, and placing it within a recognizable sequence of other years.

Among ancient civilizations several methods evolve for achieving this - most of them extremely misleading and unreliable.

Egyptian dynasties: from 3100 BC

Since great events tend to be recorded by civil servants working for the ruler, the natural way to date them is in relation to the present reign - in the fifth year, or at the time of the fifth harvest, of king so-and-so.

The chronology of Egyptian history, as a succession of dynasties of pharaohs, is compiled with hindsight from such records. The earliest surviving list is put together in the 3rd century BC by Manetho, a priest in Alexandria. He is commissioned by Egypt's new ruler, Ptolemy I, to provide a coherent account of the country's past. Inevitably, in such a bold and difficult task, there are inaccuracies which modern research tends to point up. But Manetho's dynasties have remained the basis of Egyptian chronology.

Jewish AM (Anno Mundi): 3761 BC

Recorded lists of many different kinds can provide chronological information, of varying degrees of unreliability. The authors of the Hebrew Bible like to give the often great age of patriarchs in genesis; later books in the Bible assign periods to Historical events, such as the time of slavery in Egypt. From these hints Hebrew scholars in ancient times devote much energy to trying to work out exactly when the world was created (the moment at which the Bible begins).

Eventually a consensus emerges which is still used in Jewish chronology. The creation is said to take place in a year corresponding to 3761 BC. The years subsequent to that date are given the prefix AM (Anno Mundi, Year of the World). AM 5760 (often written also as 5760 AM) is the same as AD 2000.

Roman AUC (Ab Urbe Condita): 753 BC

Similar efforts are made for Greek history, from sources such as the list of victorious athletes in the Olympic games (making an Olympiad a basic unit in Greek chronology) or of annually elected officials in Athens. The results prove unreliable - posing a major problem for the world's first systematic historians, Herodotus and Thucydides.

In Rome attempts are made to work backwards through lists of city magistrates and through genealogies of patrician families. Eventually, in the 1st century BC, a date is agreed for the founding of the city - the equivalent of 753 BC. Dates based on this starting point are given as AUC (ab urbe condita, from the founding of the city).

Mayan Calendar Round

The most elaborate and yet precise of chronologies is devised by the Maya and is adopted, along with the Mayan calendar, by subsequent central American civilizations.

This system, known as the Calendar Round, is based on each day being described according to its place in four sequences - one of day names, one of day numbers, one of month names, one of month numbers. The effect of this scheme is that no day has the same four attributes as any previous one until exactly fifty-two years have passed. A chronology composed of these 52-year cycles is kept with great accuracy over many centuries by the Mayas and their successors.

Christian AD (Anno Domini): AD 1

The start of the Christian era, like the Founding of rome, is not agreed until many centuries after the event. Such post facto chronologies are of little use for previous history. But once they are in place, a firm framework exists for recording subsequent events.

Christian chronology takes the birth of Jesus christ as its first year - AD 1 (Anno Domini, in the year of the Lord). This system will eventually provide a worldwide frame of reference. But when first formulated, in the 6th century AD, even its starting point is inaccurate.

An Italian monk, Dionysius Exiguus, is commissioned by the pope in AD 525 to work out a Christian chronology. Using earlier tables, he concludes that Jesus christ was born in the Roman year 753 Auc. He therefore proposes that the Christian era begins at the start of the following year. 1 January 754 Auc becomes 1 January AD 1.

From other sources of evidence it later becomes clear that Jesus must have been born before 4 BC - the date of the death of Herod, from whose Massacre the infant Jesus is supposed to have escaped. But this does not make the new chronology any less useful for dating subsequent events, once the chronology is widely used (a process which takes some time to achieve).

Muslim AH (Anno Hegirae): AD 622

The Muslim chronology is the only one to be established at the same period as the history which it sets out to record. In622 Muhammad moves from Mecca to Medina, an event taken as the beginning of Islam. It is known as the Hegira - a word traditionally translated in western texts as 'flight' but usually now described as 'emigration'.

Within twenty years of the event, by the time of the Caliphate of Omar, the Hegira is being officially used as the starting date of the new Muslim era. By analogy with(Anno Domini), dates are given as AH (Anno Hegirae). Being based on a Lunar calendar, the years of Muslim chronology do not tally with the Christian solar years.

The Muslim calendar is the only one in widespread use to be based uncompromisingly on lunar months, with no adjustments to bring the years into balance with the solar cycle.

The twelve months are alternately 29 and 30 days long (the lunar cycle is approximately 29.5 days), giving a year of 354 days. There are two significant results. Muslim months bear no relation to the seasons, and Muslim years do not coincide with those of other chronologies. There are about 103 lunar years in a solar century. By the millennium there will have been 1421 lunar years but only 1378 solar years from the start of Muslim chronology in Ah 1 or622. The year Ah 1421 will be2000.