You’re How Old?

It might be rude to ask someone’s age, but they don’t have to tell you. Should they keep schtum, you can always estimate and you’d probably be correct plus or minus up to 10 years.

Planet Earth has similarly tantalised scientists by refusing to directly reveal its age – but the history of our estimates of its age until recently produced figures that just kept on falling way, way short.

Earliest ideas about the age of the Earth were rooted in religious belief, and depending on your religion, could date the Earth from a few million years old to as young as a few thousand.

Geologists later tinkered with figures based on the number of layers of rock in a formation and an estimate of how long each layer took to be formed. These suggested a more ancient Earth than suggested by creation stories, but exactly just how ancient, they couldn’t say.

Then in 1862 the famous Lord Kelvin (a.k.a. William Thomson) weighed in, using ideas about heat transfer to estimate an age for Earth. He assumed it started off as a molten blob of rock and cooled steadily over the ages to its present temperature. He dated Earth to between 20 and 400 million years- an old Earth for sure, but an age that displeased biologists as it wasn’t a long enough timespan for natural selection to work its evolutionary magic. Like the biologists, geologists also reckoned this too young, but couldn’t say why or prove conclusively a more aged planet.

Another scientist (John Joly) came up with an estimate based on how salty the oceans were and how quickly erosion from the land dumped salt into rivers. Other scientists used astronomical techniques that also suggested an age like Joly’s of around 100 million years, so Kelvin’s figures looked reasonable in that light. But maybe they were too keen to side with the great man…

The discovery of radioactivity in 1896 killed off the idea of a ‘youthful’ 100 million year old Earth. This was a heat generating process that trashed assumptions made in Kelvin’s calculationsand wound the clock back to the presently accepted age of around 4.6 billion years …

For more info on how the Earth’s age is determined nowadays check out this site.