Relative dating provided a history of life on Earth, a history that clearly showed changes in geology, climate, and life. Today radiometric dating places absolute dates on the relative time scale. There were many early attempts at establishing an absolute time scale before the use of radiometric dating. Some of these efforts were biblically based while others represented non-religious estimates. We will survey the work of specific individuals to explore a variety of strategies.

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Biblical Chronology
James Ussher (1581-1656) Archbishop of Armagh (Ireland) developed a chronology, entitled Annalium pars postierior in 1654, based upon the life spans of people in the Bible and other ancient documents. Ussher calculated the date of creation to have been nightfall preceding October 23, 4004 BC. The 9:00 a.m. time of creation often attributed to Ussher was actually from previous work by John Lightfoot (1602-1675) a distinguished biblical and Greek scholar (Dalrymple, 1991, p. 21).

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Decline in Sea Level
Benoit de Maillet (1656-1738) was a French anthropologist and diplomat. de Maillet estimated the Earth was 2 billion years old. de Maillet's estimate was based upon declining sea levels. Fossils of ocean organisms on land and in the mountains seemed to support that the Earth was once covered by one large ocean. As this ocean evaporated, water levels declined. de Maillet used measured sea level changes to estimate when the Earth was completely covered with water. This theory was based on Descartes theory on the birth and death of the sun and planets. de Maillet's estimate was presented in a fictional conversation between a French missionary and an Indian philosopher Telliamed (de Maillet spelled backwards). de Maillet never published his work for fear of repercussions. Telliamed was published by Abbé J.B. le Mascrier in 1748. le Mascrier did not publish under his real name and changed the estimate from millions to billions. Telliamed was not published in its original format until 1968. Although de Maillet’s formulations were made without knowledge of geological uplift they were important because they represented logical extrapolations from measurements taken from nature. Furthermore, de Maillet’s method excluded human life spans as a measurement (Dalrymple 1991, p.27). The dynamic nature of Earth’s surface was unknown to Maillet, but even during his time it was known that sea levels had dropped in some areas and were rising in others. Sea level change was quickly abandoned as an Earth dating method.

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Cooling of Earth
Georges-Louis Lecterc, Comte de Buffon (1707-1788) was a French naturalist. Buffon calculated the world to be much older than Ussher. In his 1778 publication Epochs of Nature Buffon calculated the age of the Earth to be 75,000 years. Buffon arrived at this age by heating small iron spheres and scaling their cooling rates to an earth-sized mass. Sir Isaac Newton was the first to suggest using this strategy to estimate the cooling rate of Earth. The Catholic Church in France condemned Buffon for his calculation and burned his books. Lord Kelvin would later greatly increase the precision of Buffon’s crude methods (Burchfield 1990, p. 34). Kelvin’s methods were mathematically elegant and deductively sound; however, without the knowledge of energy created from radioactive decay his premises and thus his conclusions were wrong. Cooling of the Earth has been abandoned as a method to determine Earth’s age.

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Cooling of Sun
Hermann Ludwig Ferdinand von Helmholtz (1821-1894) was a German physician and physicist. Helmholtz used the cooling of the sun to estimate Earth's age. In 1856 Helmoltz calculated that it would take the Sun 22 million years to condense down to its current diameter and temperature from a nebula of dust and gas. He assumed the heat of the sun was generated from gravitational contraction. Lord Kelvin’s version of the Helmholtz model included heat generated by meteoric impacts. The gravitational potential energy of the meteor would be converted to kinetic energy and upon collision into heat energy. Although meteoric impact was not supported by the observations made by astronomers Kelvin could see no alternative to gravitational potential energy (Bachall, 2000). Kelvin’s calculations indicated that even the most vigorous chemical reactions could not account for the known age of human civilization, thus chemical reactions could not be a heat source for the sun. With the discovery of nuclear reactions the model developed by Helmholtz and Kelvin became obsolete.

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Weathering & Erosion
Charles Darwin speculated on the weathering and erosion of the Weald Chalk formations in his book The Origin of Species by Means of Natural Selection. Chalk formations in North and South Downs at one time extended over the sandstone and clay layers of the High and Low Weald forming a dome. This chalk layer was eroded and now only exits at the edges. Darwin sets up his scientific intuition from his personal geologic field observations. Darwin asks the reader to imagine the immense time needed to weather, erode, and deposit rock. Darwin turns attention to the chalk formations of the Weald and, with what he argues are reasonable rates of weathering and erosion, concludes the denudation of the Weald required 300,000,000 million years (Darwin 1859/2004, pp. 230-232). Although not an estimate for the age of Earth, Darwin’s reasoning represents an interesting attempt at using denudation (exposing of rock strata through erosion) to date a rock formation. Kelvin was critical of Darwin’s estimate when writing on the age of the Sun though cooling. Kelvin believed that the Sun could not have existed for the time required by Darwin’s theory; he was working without knowledge of nuclear energy. Darwin was so shaken by Kelvin’s criticism that he removed his estimate in later editions of the Origin of Species (Bachall 2000). Kelvin’s impressive mathematical models based on physics also influenced the science of geology in both positive and negative ways. The limits Kelvin’s quantitative methods placed on Earth’s age had the unfortunate effect of making uniformatarianism less tenable. Due to Kelvin’s work the science of geology became more quantitative and geologists and physicists interacted with each other (Burchfield 1990, pp. 10-12). It was clear to both sides that scientific hypotheses and theories must be internally coherent and externally consistent with findings in other fields of science.

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Sediment Accumulation
John Phillips (1800-1874) was an English geologist. Phillips came under the charge of his uncle William Smith (the father of modern geology) when his parents died. In 1860 Phillips utilized sediment accumulation to estimate the Earth's age at 38 to 96 million years. As a method of determining the age of the Earth, sediment accumulation involved comparing measured rates of continental erosion with the aggregate thickness of sedimentary rock layers from successive time divisions. Charles D. Walcott (1850-1927) an American geologist and paleontologist is probably best known for his discovery and subsequent work on fossils of the Burgess Shale formation of British Columbia, Canada. Walcott made the most detailed model of sediment accumulation (Dalrymple 1991, pp. 59-66). Walcott’s estimate was between 35 and 80 million years. Many uncertainties plagued this method of age determination. These uncertainties included the relationship between areas of erosion and areas of deposition, the rates of sedimentation, and the missing time represented by unconformities, etc. (Burchfield 1990, pp. 16-18). Furthermore, this method could only attempt to reach back to the Cambrium period. These rock layers did not reveal the majority of Earth’s history (Dalrymple 1991, pp 68-69). Sediment accumulation was eventually abandoned as a method to determine the age of the Earth.

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Tidal Effects
William Thompson, Lord Kelvin (1824-1907) was an Irish mathematician, physicist and engineer. Between the years of 1862 and 1897 Kelvin used multiple methods to calculate the age of the Earth. These methods included cooling of the Earth, Sun, and Earth tidal effects. Immanuael Kant (1724-1804) a German philosopher was the first to recognize the breaking effect of the tides on Earth’s rotation. Kelvin was the first to Estimating Earth's age using tidal effects (Dalrymple, 1991, p. 48). As the tides rise and fall gravitational friction causes a reduction in the Earth’s rotation, Moons orbital velocity and an increase in the Moon’s distance from Earth. Kelvin was the first to show that a transfer of angular momentum from the Earth to the Moon caused the Moon to recede from the Earth. Kelvin's estimates using these different methods ranged from 10 million to less than 1 billion years. The 1 billion year figure was arrived at using tidal effects. George H. Darwin (1845-1912) a mathematical astronomer was the son of Charles Darwin. G.H. Darwin refined the method of determining Earth’s age using tidal effects to a high degree (Dalrymple 1991, pp 47-52). Darwin’s minimum age for the Earth using this method was 56 million years. Darwin believed the Earth was most likely much older. Using the Moon’s recession rate from the Earth due to tidal effects did not become an accurate method for determining Earth’s age until the movement of the continents described by Plate Tectonic theory became part of the mathematical model during the 1960’s. The current model is supported by paleontological evidence in the form of tidal rhythmites or tidally laminated sediments (Thompson, 1999). The current age of the Earth using this model is in agreement with radiometric dating.

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Salt Accumulation in Oceans
Edmund Halley (1656-1742) was an English Astronomer. In 1715 Halley hypothesized that the age of the Earth could be calculated by determining the rate at which salt is added to the ocean through erosion. Halley recommended that salt concentrations be measured on a regular basis to establish the rate at which they were added so that future scientists could use his method. Thomas Mellard Reade (1832-1909) an English geologist was the first to apply Halley’s suggestion of using the oceans as a sort of “salt clock” in 1876. Instead of using changes in salt concentrations over time as suggested by Halley, Reade used estimates for the amount of salts added by erosion. Reade knew the estimated amounts of chloride and sulfate salts added to the oceans from the major rivers of the world. He used this data to determine how long it would take the ocean to reach its present salt concentrations. His estimate was 25 million years for sulfates and 200 million years for chlorides (Dalyrumple 1991, p. 53). Reade called his salt clock model “chemical denundation”. John Joly (1857-1933) an English geologist would refine Reade’s model. Jolly used sodium concentrations as his salt clock because he believed that sodium is added, but not withdrawn from the oceans. Jolly’s estimate for Earth’s age was 90 to 100 million years. Today we know that the salts are not only added but also removed from the ocean. Salt accumulation in the oceans was abandoned as a way to determine Earth’s age.

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Eccentricity of Mercury’s Orbit
In 1918 Harold Jeffreys (1891-1989), British Astronomer and Geophysicist, used the changing eccentricity of Mercury's orbit over time to estimate the age of our solar system to be 3 billion years (Dalrymple, 1991, p. 17). All of the above methods would become obsolete with the discovery of radioactivity.