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Hello, in the 17th century it was thought that the world began in 4,04 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-Oec 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 sway 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 ageing the earth, I Hazel Reimer, senior lecturer in the Department of

Earth Sciences at the Open University.

Henry G. Senior Editor at Nature and author of Deep Time, and Richard Corfield, author of

Architects of Eternity and Research Associate in the Department of Earth Sciences at Oxford University.

Richard Corfield, can you give us an overview of the four Eans,

what they encompass and what they stand for in the broadest sense beginning four and a half billion years ago.

Four and a half billion years ago the earth was formed out of the primordial matter

swirling through the solar system.

It had created out of a disk of materials and this was a time

when that primitive disk and the globe which resulted from it was bombarded continuously by a reign of meteorites.

It's called the heavy bombardment period.

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