Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 Celebrating the age of the Earth SIMON J. KNELL 1 & CHERRY L. E. LEWIS 2 1 Department of Museum Studies, 105 Princess Road East, Leicester LE1 7LG, UK (email: sjk8 @leicester.ac.uk) 2History of Geology Group, 21 Fowler Street, Macclesfield, Cheshire SKIO 2AN, UK (email: [email protected]) Abstract: The age of the Earth has been a subject of intellectual interest for many centuries, even millennia. Of the early estimates, Archbishop Ussher's famous calculation of 4004 BC for the date of Creation represents one of the shortest time periods ever assigned to the Earth's age, but by the seventeenth century many naturalists were sceptical of such chronologies. In the eighteenth century it was Nature that provided the record for Hutton and others. But not all observers of geology enquired about time. Many, like William Smith, simply earned a living from their practical knowledge of it, although his nephew, John Phillips, was one of the first geologists to attempt a numerical age for the Earth from the depositional rates of sediments. For more than fifty years variations of that method prevailed as geology's main tool for dating the Earth, while the physicists constrained requirements for a long timescale with ever more rigorous, and declining, estimates of a cooling Sun and Earth. In 1896 the advent of radioactivity provided the means by which the Earth's age would at last be accurately documented, although it took another sixty years. Since that time ever more sophisticated chronological techniques have contributed to a search for the oldest rocks, the start of life, and human evolution. In the attempt to identify those landmarks, and others, we have greatly progressed our understanding about the processes that shape our planet and the Universe, although in doing so we discover that the now-accepted age of the Earth is but a 'geochemical accident' which remains a contentious issue. Establishing the Earth's age and chronology has combined with the wide adoption of uniformi- been something of an intellectual baton passed tarianism, gave the sacience new and seemingly from one academic creed to another as each rational ways to think about time. But the new seemed to offer new hope of solution or under- geologists were often reticent about discussing standing. In the seventeenth century the Earth's time in terms of years. By the 1860s, however, the age was subject to the strictures of religious and subject added the great evolutionary debate to its social orthodoxy. The product of Creation, intellectual baggage as T. H. Huxley defended theological study offered one logical path to its Darwin's billion-year estimate for natural selec- understanding, but also to its containment. This tion. Now both biologists and geologists joined pattern would repeat itself, as forces for an forces to demand a longer timescale which per- expanded timescale repeatedly did battle with mitted the playing out of those evolutionary and those who thought the Earth relatively young. uniformitarian processes necessary to explain the This was never simply a conflict between 'pro- condition of the Earth. gressive' science and a resistant society, but However, the age of the Earth had acquired a rather a reflection of the path travelled by an new interest group. Applying theoretical ideas idea through a complexity of beliefs, dogmas, developed in the arly years of the century, a theories, measurements and methodologies. number of respected physicists began to offer In the eighteenth century, Theories of the hope of at last calculating the Earth's age in Earth marshalled existing evidence, scientific years. But if their calculations were correct, then practice and analogy, to construct models for those great underpinning theories of the natural the origin and development of the planet. At the sciences could not be. With the discovery of beginning of the nineteenth century, modern radioactivity in 1896, the future of the Earth's geology emerged as a rigorous and empirical age was firmly in the court of that emerging inter- field of study then centred on stratigraphy with disciplinary subgroup, the geophysicists. In the its implied notions of relative age. This, when twentieth century these extended the timescale From: LEwis, C. L. E. & KNELL, S. J. (eds). The Age of the Earth: from 4004BC to aB 2002. Geolog- ical Society, London, Special Publications, 190, 1-14. 0305-8719/01/$15.00 © The Geological Society of London 2001. Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 2 S. J. KNELL & C. L. E. LEWIS remarkably. With Einstein's general relativity also from arithmetic, geometry, and trigono- field equations, the topic was also returning to metry, both plain and spherical; and like- cosmology, to explanations of the Earth's age in wise from a studious and laboured application the context of the age of the universe, to the realm of various sources of information, supplied of astronomers. by the observation of eclipses, by the testi- What unifies these different approaches and monies of credible authors, and by ancient their practitioners is their extrapolation of time medals, coins, monuments and inscriptions from the study of process, whether that process (Anon. 1819). was observable or modelled by theory. Early chronologers rigorously applied contemporary Chronologies were constructed as an analytical historiographic methods to chart the 'progress' tool so as to distinguish 'cause' and realize a of history. Later geologists would extrapolate 'chain of events' (Anon. 1837, p. 131). To achieve the time necessary for shaping the surface of the this, their content was selective and thematic. Earth or for the deposition or erosion of observ- To discern the age of the Earth, the only 'cred- able strata. Biologists too envisaged a time- ible authors' were those relating creation stories. dependent process of evolution. Physicists made The Bible, or rather the texts from which it was predictions and measurements from the physics written, then became the primary historical of heat and radioactive decay, and astronomers texts, key sources for the interpretation of deduced process from planetary construction time. Historians generally point to Theophilus and arrangement. In the relative calm of the of Antioch, of the second century AD, as perhaps early twenty-first century, in a field which has the first to use the Biblical record as a source seen centuries of deep controversy, we should for the construction of a chronology. The art of perhaps reflect upon the confidence our pre- chronology, however, extended back long before decessors had in their age for the Earth. They the Christian era, often deriving 'fabulous ages' too thought they had 'got it right'. of hundreds of thousands of years for particular civilizations (Anon. 1819, 1833, 1849). In the sixth century BC Zoroaster, a religious teacher Chronologies and theories who lived in Persia (now Iran), believed that the world had been in existence for over 12000 In one of his popular articles, Stephen Jay years; the Roman writer Cicero relates that the Gould (1993, pp. 181-193) responded to James venerable priesthood of Chaldea in ancient Barr's (1985) definitive study of a great histor- Babylonia held the belief that the Earth emerged ical chronologer with a diatribe against previous from chaos two million years ago, while the old 'Whiggish' interpretations of this early form of Brahmins of India regarded Time and the Earth scholarship. The historiography of science had as eternal (Holmes 1913). long rejected the notion of 'progress', of the past By the sixteenth century the Bible formed a having been a succession of errors, but Gould core resource for the writing of histories in was here talking to scientists who necessarily Europe. Of particular significance was the remained wedded to, and driven by, this con- interpretation of the Second Letter of Simon cept of progress towards 'truth'. Though not Peter, which states 'One day is with the Lord as the first to do so, Barr saw chronologers not a thousand years, and a thousand years as one as religious dogmatists but as rigorous scholars day' (2 Peter 3:8) (see also Dean 1981a; Oldroyd working within the social and intellectual 1996a). It draws on Psalm 90 of the Old constraints of their time. They were partici- Testament, 'The human condition': 'Lord ... pating in the earliest of historiographic prac- To you, a thousand years are a single day'. tices- the construction of a linear timeline of Chronologers read this literally, and made events - which, in reconceived form, would later each of the six days of creation equivalent to underpin the great stratigraphic enterprise of the 1000 years. The Biblical motivation for discuss- nineteenth century. ing the Earth's limited age was to encourage the Rees's early-nineteenth-century Cyclopaedia living of 'saintly lives'. For here was not simply separated 'sacred chronology' from other histor- the birth but also the death of the Earth, the ical chronologies. Here chronology was 'one of Judgement Day, which will 'come like a thief, the eyes of history' and not simply the study and then with a roar the sky will vanish, the of ancient theological texts. Its potential sources elements catch fire and fall apart, and the Earth were many: and all it contains will be burnt up' (2 Peter As its use is extensive, the difficulty of acquir- 3:10). If this were not enough to encourage ing it is not inconsiderable. It derives necessary piety, Psalm 90 also pondered human mortality, assistance from astronomy and geography, 'over in a trice, and then we are gone.' Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 AGES OF THE EARTH 3 became very familiar to early-nineteenth-century British audiences, but he had predecessors and contemporaries across Europe (Anon.