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 (it is used officially in all but eight countries in the world, sometimes alongside other calendars). 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 1 BCE, which Christians call 1 BC (Before Christ). There was no "Year Zero" and 1 BCE was immediately followed by the year 1 CE, 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 BCE, not in 10,000 BCE. 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 -0054. 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).

Calendars

A problem for chronologists has always been that the various calendars in use throughout the world do not comply with the solstices and, until comparatively recently, did not even measure astronomical time accurately. The time taken by any astronomical object to complete an orbit around its parent body is termed one year and so another term for a year is "orbital period". Earth orbits the Sun in 365.2425 days. The orbit is anti-clockwise as defined conventionally by a view from the North Pole, and a day on Earth is the time taken by the planet to fully rotate on its axis, which is again an anti-clockwise motion. The length of the day is defined as 86,400 seconds, which is 1,440 minutes which in turn is 24 hours. As a result of the anti-clockwise rotation, an observer on Earth sees the Sun cross the sky each day from east to west, the amount of daylight on each day being dependent on the time of year and the location on Earth.

On Earth, the Equator is an imaginary line, some 24,900 miles long, equidistant from the North and South Poles. Conceptually, it divides the planet into the northern and southern hemispheres as its plane passes through the centre of the globe. The Equator is an important factor in the planet's seasonal cycles as are the two tropics which surround it. The Equator is, by definition, the line of zero degrees latitude. It intersects with all lines of longitude which run from pole to pole including the Prime Meridian, the line of zero degrees longitude upon which Greenwich Mean Time is established. Intersection of the Equator with the Prime Meridian occurs in the Gulf of Guinea about 380 miles south of Ghana and 670 miles west of Gabon. All places on Earth have geographic co-ordinates based on longitude and latitude.

The tropics are two lines of latitude drawn at 23.43695 degrees North (the Tropic of Cancer) and 23.43695 degrees South (the Tropic of Capricorn). The essential point about the tropics is that they are the most northerly (Cancer) and southerly (Capricorn) lines of latitude at which the Sun can be directly overhead, an annual event occurring in either hemisphere at the time of its summer solstice. Any place within the limits of the two tropics is termed "tropical". There is a loose definition of "sub-tropical" for anywhere else within latitudes 40 degrees north and south. Antarctica and Europe are the only continents which are neither tropical nor sub-tropical. Most of Europe is in what is called a "temperate zone" as are the northern U.S., most of Canada, northern China and most of Japan. In the southern hemisphere, New Zealand and the southern halves of Argentina and Chile are temperate. Antarctica is in the southern "frigid zone"; the northern frigid zone is mostly the Arctic Ocean and includes Greenland with the northern extremities of Alaska, Canada, Scandinavia and Russia. The frigid zones are defined by latitudes 66.33 degrees South (Antarctic Circle) and 66.33 degrees North (Arctic Circle).

In temperate climates, the year is divided into four seasons which are generally known as Spring, Summer, Autumn and Winter. These are determined largely by available daylight and to some extent by climate or weather. The availability of daylight to any region is dependent on the Earth's axial tilt as it orbits the Sun. The axial tilt from both poles is about 23.4 degrees. It is constant and its effect is to expose each hemisphere to greater sunlight in one half of the year than in the other half.

It is on or about 21 June each year when Earth reaches one extremity of its orbit in that the North Pole is at its fullest extent of tilt towards the Sun. At a certain time on this day, the summer solstice occurs in the northern hemisphere and the winter solstice in the southern hemisphere. The sun is at this time directly overhead on the Tropic of Cancer. The word "solstice" is derived from Latin and, broadly speaking, means "sun standing still". A solstice is the point at which the poles reach their fullest extent of tilt, either towards the Sun or towards outer space, and the extent of tilt now begins to reduce as orbit continues. Six months later, on or about 21 December, the tilt reaches fullest extent in the other direction with the winter solstice in the northern hemisphere and the summer solstice in the southern hemisphere when the Sun is directly overhead on the Tropic of Capricorn. Equinox (twelve hours of daylight and twelve hours of darkness) occurs on about 21 March and again on about 21 September. These two dates are the vernal equinox (Spring) and the autumn equinox respectively (again, equal and opposite in the southern hemisphere). The word "equinox" is from the Latin for "equal" and "night". Just as the winter solstice marks the supposed beginning of winter and the summer solstice that of summer, the vernal equinox is held to be the first day of spring and the autumnal equinox the first day of autumn (calendrically; meteorologists divide differently to suit the weather patterns in particular countries).

As the Earth completes a full orbit by reaching the second solstice, the annual cycle begins again but the new orbit is not the start of the calendar year because its date is not constant. The problem with calendars is age old. It begins with the Egyptian calendar. This was based on astronomical observations and began on a day when the star Sirius (called "Sothis" by the Egyptians), which had not been visible for some time, was seen to rise simultaneously with the Sun. That happened on 19 July 2781 BCE but it is not certain that the Egyptians began their calendar then. The calendar was flawed in that it assumed exactly 365 days in the year. The resultant Sothic Error amounted to one year in every 1,460 years and this period was called the Sothic Cycle. It was thus in 1321 BCE and again in 139 CE (as recorded by the Roman writer Censorinus) that the Egyptian calendar and the celestial calendar were realigned.

In 46 BCE, Julius Caesar authorised an improvement suggested by Sosigenes and other Greek astronomers in Alexandria. They added an extra day (now 29 February) every four years and so created the Julian Calendar, named after Caesar. The extra day did not completely correct the Sothic Error because the exact length of the solar year is 365 days, 5 hr, 48 min, and 45.5 sec. The Julian Calendar in an average year was 11 minutes and 14 seconds longer than the solar year. This discrepancy accumulated until by 1582 the vernal equinox occurred ten days early and holidays did not occur in the appropriate seasons. In 1582, Pope Gregory XIII introduced the Gregorian calendar, devised by the Jesuit Father Christopher Clavius, and, from the start, recommended 1 January as the date of the new year. The Gregorian calendar almost exactly corrects the Sothic Error by omitting Caesar's extra day (29 February) in century years that are not divisible by 400 (hence 29 February 2000 is a valid date but 29 February 1900 is not).

In the old Roman calendar and its Julian successor, New Year's Day was 1 January. The Roman Senate fixed the date by edict from 1 January 45 BCE when Caesar's calendar took effect. Sometime after 1100 in the Middle Ages, 25 March became the first day of the year in much of Europe, including the British Isles. That date (which is not the Ides of March as often thought; that is 15 March) was, and presumably remains, important in the Roman Catholic religion as the Feast of the Annunciation but there is no astronomical reason for its choice as New Year's Day, although it is close to the Spring Equinox in the northern hemisphere.

Adoption of the new calendar was slow as Gregory's edict had no civil remit and, in any case, many countries had by then become Protestant and would not recognise a Catholic reform, even though this one did correct computation of the calendar. The most curious case of adoption occurred in the British Isles. An Act of the Scottish Parliament on 17 December 1599 moved the first day of the year from 25 March and back to 1 January. It was effective from 1 January 1600 and remained so, in Scotland only, through the Stuart accession in 1603 and the Act of Union in 1707. England and, thereby, Ireland and Wales did not follow suit. As it happened, much of continental Europe had re-adopted 1 January before Gregory's reform and this included many of the anti-Catholic Protestant nations such as Denmark, Norway, Sweden and the north German states. The next significant adoption after Scotland was by Russia in 1700 and then, in 1752, amidst widespread unrest which sometimes culminated in riots, England finally decided to adopt it too and it became standard throughout the British Empire including North America, so it was already in use when the USA was constituted 35 years later.

England made the change by following Wednesday, 2 September 1752 (Julian date) with Thursday, 14 September (Gregorian date) and so 3 to 13 September 1752 are non-dates in English history. The next new year was on 1 January 1753 so, as the new year in 1752 was on 25 March, the year 1752 throughout the British Empire (except Scotland which was already Gregorian) had only 271 days and was, therefore, the shortest-ever year.

The sole issue, as such, with 1 January is that it is a date of convenience because it is not the start of the year in astronomical terms. Having said that, the date on which the astronomical year commences is not constant so having a date of convenience is the wiser option for a supposedly civilised society. The astronomical year begins at the precise time of the winter solstice and that is a problem in itself because there are two winter solstices as described above, one in each hemisphere. The date of the northern winter solstice differs between 2018 and 2019 in time zones around Greenwich. In 2018, it occurred at 22:23 GMT on Friday, 21 December and, in 2019, it will be at 04:19 GMT on Sunday, 22 December.