During the 19th century geologists realized that earth was quite older than previously believed, however this discovery posed an even greater question: what about the universe? Did earth (like some fundamental creationists believed and still believe) predate the cosmos, were earth and the cosmos created at the same time or came earth later?
Early geologists had to use indirect methods to determine the age of earth, like rate of sedimentation or erosion, thickness of layers or the rate of faunal changes over time. Astronomers had even less to work on.
The great physicist Lord Kelvin estimated the age of the solar system by calculating the rate that sun, an incandescent and molten sphere of matter as he supposed, needed to cool and solidify. Kelvin believed that the solar system and earth was between 20 and 100 million years old. Naturalist Sir Isaac Newton used a similar thought experiment to calculate the age of earth and, as it was a quite smaller sphere, earth needed just 50.000 years to cool down and become habitable. The German cosmologist Hermann von Helmholtz (1821-1894) published in 1854 his research. Helmholtz realized that a cooling molten mass or burning fuel would provide sun just with 5.000-10.000 years of energy, so he developed an alternative mechanism, where a slowly collapsing star transforms gravitational energy into heat and light. With this premise he calculated an age for the sun of 21 million years. In 1899, based in part on Helmholtz's theory and considering the variability of matter-density in the sun, T.J.J. See calculated a new age of 32 million years.
It was also tempting to use the apparent regular motions of stars and planets as cosmic calendar. Astronomer and mathematician Harold Jeffreys (1891-1989), using the periodical variations of planetary orbits, estimated an age from 1.000 to 10.000 million years to explain the modern position of the planets in our solar system.
James Croll (1821-1890), Scottish self-taught mathematician, used the orbital parameters of earth to explain the episodic occurrence of ice ages. According to Croll's calculations the last glacial period ended 80.000 years ago. By multiplying the identified glacial deposits with the time intervals deduced from the motion of earth, it was therefore possible to estimate an age for earth. At the time only four glacial periods were recognized, but Croll and other mathematicians, based on their calculations, proposed that in fact there were many other episodic cooling events to be found in earth's history.
Fig.1. Outcrop of the Trubi-Formation (Capo Spartivento, Italy). Small variations in earth's orbital parameters during the last ice age influenced the deposition of marls with a low and high content of organic matter, forming here a succession of darker stripes. It is now possible to compare the pattern of the stripes with the calculated curve and so infer the age of these sediments.
Another mathematical approach, using statistics, was adopted by American astronomer Harlow Shapley (1885-1972), who argued that the apparent even distribution of stars in the universe could only be explained by long periods of time. The stars, forming together in a nebular cloud, slowly drifted into space and assuming enough time would disperse in an uniform pattern and fill the void. In 1920 it was discovered that also galaxies move away from each other and based on the supposed average distance and a supposed expansion rate an age of 1.800 to 3.400 million years for the observable universe was calculated.
However as most geologist already worked with a 3.000 million years old earth (first ages using radiometric dating techniques on rocks were published at the time), the younger ages given by astronomers seemed to suggest that earth predates space and time itself!
A curious solution for this apparent contradiction was found by geologist Thomas C. Chamberlin (1843-1928) and astrophysicist F.R. Moultoun in America and cosmologist James Jeans (1877-1946) and geophysicist Harold Jeffreys (1891-1989) in Britain. They proposed that stars were almost eternal (Jeans proposed an age of 10^13 years) and only planets were formed over time (so geologists could use any age without getting into a debate with astronomers).
There had to be a better way to determine the age of earth and extraterrestrial matter... (to be continued).
BRUSH, S.G. (2001): Is the Earth too old? The impact of geochronology on cosmology, 1929-1952. In LEWIS, C L. E. & KNELL, S. J. (eds.) "The Age of the Earth: from 4004 BC to AD 2002", Geological Society, London, Special Publications, 190: 157-175
CAJORI, F. (1908): The Age of the Sun and the Earth. Scientific American Supplement No.1706: 174-175
HEDMAN, M. (2007): The Age of Everything - How Science Explores the Past. the University of Chicago Press, Chicago: 249
HILGEN, F.J. (2010): Astronomical dating in the 19th century. Earth-Science Reviews 98: 65-80