Chronology

Chronology is the branch of historical research which involves the study of documented records to establish the dates of past events. A person who studies chronology is called a chronologist. It is not always possible to confirm an exact date and the chronologist must then deduce an approximate one through analysis of the available evidence. Generally, the events are then ordered by date, often in the form of a list or in the form of a narrative written chronologically. Therefore, chronology can roughly be defined as the sequence, measured by time, in which events took place.

Measurement of time

Time itself is not just the everyday cycle of seconds, minutes and hours because chronology is concerned with time across billions of years that is divided into Eras, Periods, Epochs and Ages before we come down to the more familiar concepts of Millennia, Centuries, Decades, Years, Months, Weeks and Days. Measurement of such vast timespans is important and the first thing to do is identify the key units and describe their use.

The two units which measure long periods of time are the mega annum (Ma) and the kilo annum (ka):

• "Ma" = mega annum (one million years); "ka" = kilo annum (one thousand years)

The Ma unit is usually applied to dates earlier than 10,000 BCE because the Holocene Epoch (the current epoch) began c. 9,700 BCE. The meaning is "million years ago" and, therefore, 4,600 Ma (the estimated origin of Earth) means "approximately 4,600,000,000 years ago". While "Ma" is generally the preferred unit, there is an alternative unit "mya" which specifically means "million years ago".

The ka unit is applicable to all dates from the beginning of the Holocene (i.e., 9,700 BCE = 11.7 ka) but, for convenience, is also used for dates in the Pleistocene from 100 ka (0.1 Ma) to 11.7 ka. The meaning of ka is "thousand years ago". As ka is substantially less approximate than Ma, and can even be accurate, a benchmark of 1 January 2001 is used, being the first day of the current millennium. Therefore, 11.7 ka means "approximately 11,700 years before 1 January 2001" while 2 ka means "approximately 2,000 years before 1 January 2001". Dated precisely, 2 ka is 1 January 1 CE and 11.7 ka is 1 January 9,700 BCE.

The BCE/CE question

Creation of a new "Year One" would probably be logical but would moreover be impractical because Christian chronology, which originated with Dionysius Exiguus, is too well-established. Illogical though it may be, pragmatism takes precedence – not so much "real world calling" as "it ain't broke". Exiguus based his system on the belief that Jesus Christ was born in December of the year which Christians call 1 BC (Before Christ). There was no "Year Zero" and 1 BC was immediately followed by the year 1, which Christians call 1 AD. "AD" is the Latin Anno Domini which means "Year of the Lord". Those chronologists who dislike association of chronology with one religion use "CE" (Common Era) instead of AD and "BCE" (Before the Common Era) instead of BC.

Two alternative options are worth a mention. One is to use the Holocene (Human Era) Calendar which begins with 10,000 BCE as its Year One and so simply adds 10k to all CE years but that creates unfamiliar dates for events BCE and the name of this calendar also presents a drawback in that it is generally agreed that the Holocene began c. 9,700 BC, not in 10,000 BC. The second alternative is the ISO 8601 standard but this fails as a standard because it includes the date 0000 as Year Zero which it equates to 1 BCE and thereby throws all BCE dates out of kilter: for example, whereas it is widely known that Julius Caesar first came to Britannia in 55 BCE, the ISO date is -0056. The Common Era system (BCE and CE) retains the familiar BCE dates and excludes Year Zero while distancing itself from Christianity and it is, therefore, the most practical method.

Dates from 100 BCE to 100 CE are the ones mostly subject to the BCE/CE qualifier unless it is clear from the context which applies. The millennia BCE are in "reverse order" and so the first millennium BCE is from 1000 BCE to 1 BCE, the second is from 2000 BCE to 1001 BCE, etc. Similarly, the century from 1000 BCE to 901 BCE is the tenth century BCE and the century commencing 100 BCE is the first century BCE.

Dating the Geological Record

The Geological Record is the strata (layers) of rock in the planet's crust and the science of geology is much concerned with the age and origin of all rocks to determine the history and formation of Earth and to understand the forces that have acted upon it. Geologic Time is the timescale used to calculate dates in the planet's geologic history from its origin (currently estimated to have been some 4,600 million years ago) to the present time. It is generally reckoned that there have been four Geological Eras: Precambrian, Palaeozoic, Mesozoic and Cenozoic (ongoing).

Radiometric Dating measures the steady decay of radioactive elements in an object to determine its age. It is used to calculate dates for the older part of the planet's geological record. The theory is very complicated but, in essence, the radioactive elements within an object decay to form isotopes of each element. Isotopes are atoms of the element that differ in mass but share the same general properties. Geologists are most interested in the decay of isotopes carbon-14 (into nitrogen-14) and potassium-40 (into argon-40).

Carbon-14 aka radiocarbon dating, works for organic materials that are less than about 50,000 years old. For older periods, the potassium-argon dating process is more accurate. Radiocarbon dating is carried out by measuring how much of the carbon-14 and nitrogen-14 isotopes are found in a material. The ratio between the two is used to estimate the material's age. Suitable materials include wood, charcoal, paper, fabrics, fossils and shells. It is assumed that rock exists in layers according to age, with older beds below later ones. This is the basis of stratigraphy.

The ages of more recent layers are calculated primarily by the study of fossils, which are remains of ancient life preserved in the rock. These occur consistently and so a theory is feasible. Most of the boundaries in recent geologic time coincide with extinctions (e.g., the dinosaurs) and with the appearances of new species (e.g., hominids).