Thanks for downloading the NRTIME podcast. For more details about NRTIME and for our terms of use, please go to bbc.co.uk forward slash radio for.

I hope you enjoy the program.

Hello, the calendar shapes the lives of millions of people.

It's an invention that gives meaning to the passing of time that orders our daily existence.

It links us to the arcane movements of the heavens and the natural rhythms of the earth.

It's supposed deeply practical and profoundly sacred.

But where does this strange and complex creation come from?

Why does the week last seven days, but the year 12 months?

Who named these concepts and through them shaped our lives?

The answers involve Babylonian astronomers and Hebrew theologians, Roman emperors and English scholars.

Isaac Newton, for instance, designed a calendar said to be mathematically flawless, but no one can understand it.

Gregory XIII seems to have got it right.

With me to discuss the calendar, a Robert Poole, read in history at Martin's College Lancaster and author of Times Alteration, calendar reform in early modern England,

Kristen Lippincott, deputy director of the National Maritime Museum in Greenwich,

and Peter Watson, research associate at the McDonald Institute for Archaeological Research at Cambridge University,

and author of a terrible beauty, a history of the people and ideas that shaped the modern mind.

Kristen Lippincott, what were the first calendars based on?

Well, the best way to think about the calendar is to put yourself back in the shoes of ancient man,

and you're sitting there in the middle of some field watching the daily rhythms,

and the first rhythm that you really notice, either than beyond day and night,

is the waxing and waning of the moon.

And you notice at every 28 days you've got a full moon back,

...