Back in 2005 we celebrated this happy event with a Cafe Scientifque - we had a cake & everything. Of course, we also talked about the science involved in determining, as well as we can, the actual age of our planet. So this time round I thought I'd present some of that information here. (You can find the original version - written by Penny Cooke - in our Cafe Scientifique archives.)
Attempts such as Ussher's were based on human generation times - maybe 25 years between one generation & the next. The Old Testment provides lists of genealogies, so it was fairly straightforward to count up the 'begats' and multiply by 25 to get the number of years since Adam & Eve. Ussher used a selection of biblical chronologies, other people's attempts at the calculation, & astronomical cycles to work out that the Earth was created in the evening of October 22, 4004 BC.
He wasn't alone - other biblical chronologists also felt it important to determine the Earth's age. (So do modern geologists & biologists.) It's Ussher's attempt that's remembered by most - often with a smile (how could he get it so wrong?). But that's not really fair: the historical calculations need to be viewed in the context of their times, & certainly they were taken seriously by the church. However, even in the 1600s scholars were questioning just how the biblical accounts fitted in with their geological observations. For example, Nicolaus Steno recognised that mountain building must have taken a long time. The question was: how long??
The industrial revolution gave people the chance to measure the cooling rate of molten iron, and also the increase in temperature that occurs as you go deeper & deeper into a mine. In 1862 Lord Kelvin calculated that if the Earth formed from a white-hot molten globe of iron, then it would take 98 million years to cool to its current temperature. Once radioactivity had been discovered, scientists began to realise that the Earth might not just be cooling from a molten state, but might also be being heated from within by radioactive decay. This threw Lord Kelvin's calculations out of kilter.
Other methods were also proposed. Edmund Halley (of the eponymous comet) suggested working out how long it took the oceans to reach their present degree of saltiness, assuming that they were fresh to start with. (He was just a tad annoyed that the ancients hadn't been considerate enough to measure the oceans' salt content, to give him a starting figure. And he made this measurement himself, so that others would have something to go on.) This idea was used by John Joly (1857-1933) to come up with a figure of 150 million annum (Ma) for the age of the Earth.
Or you could use sedimentation rates: you simply measured the amount of sediment accumulating in your local river or estuary, & how long it took to do so. Then you measured the thickness of known sedimentary rocks and applied the sedimentation rates. This gave a range of ages between 55 and 600 Ma. All were minimum values, because it wasn't possible to measure what had been removed from the land surface by erosion but not deposited in a given layer of sediment.
By the 19th century scientists were attempting to explain how the Earth formed: an answer to this question would also allow them to calculate how long it would have taken. Charles Darwin's son George (whom many view as the father of modern geophysics) proposed that the formation of the Moon & the Earth were linked - the Moon was flung out from the original ball of dust & gas that eventually formed Earth. Based on astronomy & on tidal friction slowing the Earth's rotation, George calculated that the Earth was at least 50 million years old. (We now know that the Earth is a lot older than 50 million years, & the Moon probably formed as a result of a collision between the early Earth & a large asteroid.)
Our own Ernest Rutherford suggested using radioactivity to obtain the Earth's age. In 1906 he came up with a minimum age of 497 Ma. This was quickly pushed out to 1-2 billion years (Ga or giga annums), & by 1927 Arthur Holmes calculated an age of over 3 Ga.
The oldest rocks on Earth are in Canada - the Acasta Gneisses, dated in 1999 at 4.03 Ga. Some slightly younger Australian sedimentary rocks contain older crystals of zircon, dated to 4.2-4.3 Ga, which indicates that they formed from still older rocks. Moon rocks have been dated to between 4.4 and 4.5 Ga, while various meteorites (formed at the beginning of our solar system, have been dated to 4.5-4.6 Ga. So the age of the Earth is now thought to be as much as ~4.54 Ga (4540 million years).
Happy Birthday, Earth!
Some useful references:
Cutler, A. (2003) The seashell on the mountain top, Heinemann: a biography of Nicolaus Steno, who lived in the later part of the 17th century & is regarded as the first person to explain the presence of seashells in mountain rocks.
Hadean times - were they really hell on Earth? New Scientist 14 May 2005, issue 2499