Just before this BBC podcast gets underway, here's something you may not know.

My name's Linda Davies and I Commission Podcasts for BBC Sounds.

As you'd expect, at the BBC we make podcasts of the very highest quality featuring the most knowledgeable experts and genuinely engaging voices.

What you may not know is that the BBC makes podcasts about all kinds of things like pop stars,

poltergeist, cricket, and conspiracy theories and that's just a few examples.

If you'd like to discover something a little bit unexpected, find your next podcast over at BBC Sounds.

Thanks for downloading the In Our Time Podcast.

For more details about In Our Time and for our terms of use, please go to BBC.co. UK

forward slash radio for. I hope you enjoy the program.

Hello in the 17th century it was thought that the world began in

four thousand and four b BC. Lord Kelvin in the 19th century calculated

the cooling temperature over rock the size of our planet and came up with a figure of

20 million years for the age of the earth. Today the history of our planet

is divided into four great eons, the Hedian, the Archaean, the Proto-Oic and the Fanarozoic.

Together they're taken to encompass four and a half billion years.

How can we begin to make sense of such a huge swathe of time?

And can we be sure that we've got the Earth's age right?

Geologists use eras, periods and epochs to further punctuate what's known as deep time.

But can we be sure that the classifications we use don't obscure more than they reveal?

With me, to discuss aging the Earth, I... don't obscure more than they reveal.

With me to discuss aging the earth,

I Hazel Reimer, senior lecturer in the Department of Earth Sciences

...