Measure for measure

The now or present of Earth Time is not exactly the same as what you and I normally mean by now or the present. After all, when you are some 4.5 billion years old what does a day or so, a week, a month or even a century or two matter? Earth Time is normally measured in years before the present (BP). And, although our Earth Time clocks are getting more and more accurate, they still cannot resolve time much better than a few hundred and sometimes thousand years or so at best. So tomorrow's 'now' is effectively much the same as last year or next year.

Whether standing on the rim of the Grand Canyon or by the River Thames in the middle of the city of London, the common concept of the present time that we live in is based on a western and Christian-dominated cultural chronology rather than the geological one. It is globally accepted (at least for general purposes of travel, commerce and international law) that we are living in the first decade of the twenty-first century and the third millennium AD (anno domini).

Geologically, this is year zero. The present is always zero: the past recedes from now because geologists do not use a fixed traumatic point in the past as our prehistoric 'ground zero' from which all else is measured. This is fine when we are dealing with the remote past, but it creates problems when dealing with the recent past. Consequently, the AD-BC scheme is widely used for Holocene dates and older dates are referred to as Before Present (BP).

The Holocene Epoch began 11,500 years ago, the Pleistocene Epoch, famous for its ice ages, began 1806 million years ago (often abbreviated to ma or mya) and the Jurassic Period, famous for its dinosaurs, began around 205.7 million years ago.

The dating of the geological past is largely extracted from the rock record using a complex technology and it is a topic to which we will return (see p. 399 et seq.). However, it is important to realise that geological dates are essentially calculated estimates that often include margins of error in the order of a few per cent. So a relatively recent date of, say, 3000 BP will have an error margin of around 100 years, that is + or —50 years, and 200 ma might have an error of + or —4 million years. This kind of error margin needs to be borne in mind when any date from the remote past is talked about. Precision of geological dates, in the everyday sense, there is not. Consequently, claims that for instance an extinction event dated in one place is really the same as that in another place, just because the calculated dates seem to be the same, is not necessarily true. But with the accumulation of many dates clustering closely around the same point, the probability of synchroneity increases.

The success of western economic growth and global dominance has ensured that other cultural, religious and historical chronologies have become largely subservient to the international hegemony of a Christian-based chronology. Nevertheless, other chronologies and calendars such as the Buddhist one, which is based on the death of Buddha (544 BC), or the Islamic one, based on the flight of the prophet from Mecca (AD 622), may still be dominant in their home territories. In a similar way, a single geological chronology has developed over the last 200 years and come to dominate the division of deep time and the development of an international geological timescale. There is an importance difference in that this timescale has been achieved through international cooperation. Although for various historical reasons British and European geologists controlled most of the early development of the chronology and subdivision of Earth Time, subsequently finer divisions have been recognised in rock sequences all over the world and named accordingly. Indeed, there has been a lot of competition among geologists from different parts of the world to get some part of their geological home 'patches' officially recognised within the international timescale.

The details of how our present Christian-based Gregorian calendar, a reformed version (in 1582 by Pope Gregory XIII) of the older Roman Julian calendar, came to dominance was extensively discussed and dissected at the turn of the twentieth century by writers such as E. G. Richards (whose fascinating book is entitled Mapping Time: The Calendar and its History, 1998, Oxford), much better qualified than myself. The historical multiplicity of calendars is largely the result of the astronomical fact that an Earth year has the awkward length of 365.242199 solar days at the moment. This is not the easiest of measures to subdivide into neat calendar packages of days, weeks and months that are repeated on an annual basis. Nor is our year length fixed. In Devonian times, over 350 million years ago, the Earth year was some 400 days long; the planet's rotation has slowed due to tidal friction. Counts of diurnal growth rings laid down in lunar cycles and recovered from fossil corals and clams verify these measures.

Apart from calendar-based chronologies, we also divide historical time up in a number of different ways. Traditionally, many cultures have recognised time intervals based on reigns and dynasties. Acts of Parliament in England are still dated in this way, for instance those passed in 2004 will be dated as 51 Elizabeth II because the Queen came to the throne in 1953 - but then England does like to hang on to its traditions. Of particular significance is the Christian Era, a mainstay of the Gregorian calendar, which was first proposed by Dionysius Exiguus, an Abbot from Scythia, now Moldavia, in AD 532. AD 1 was taken as the supposed birth year of Christ, although it is almost certainly incorrect, the more likely date being 4 BC. Even historians have problems with accurately dating important events and have error margins to take into account.

As we have seen, the Dionysian calendar was not widely accepted until it was taken up in 1582 by Pope Gregory XIII - and the rest, as they say, is history. Several more recent reforms have been attempted, of which the most radical was that by French revolutionaries of the late eighteenth century. They tried to introduce a whole swathe of reforms ranging from a io-hour republican day to a new era - the Republican Era (RE), beginning in 1792 - but it did not catch on. Even more recently, members of the international Baha'i religion recognise a Baha'i Era that began in 1844, the year Ali Muhommed was declared Bab.

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