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Hello, when writing the Principia Mathematica, Isaac Newton declared his hand on most of the big questions in physics.

He outlined the nature of space, explained the motions of the planets, and conceived the operation of gravity.

He also laid down the law on time, declaring, absolute, true and mathematical time of itself and from its own nature flows equably without relation to anything external.

For Newton, time was absolute and set apart from the universe, but with the theories of Albert Einstein, time became more complicated.

It could be squeezed and distorted and was different in different places.

Time is integral to our experience of things, but we find it very difficult to think about.

It may not even exist, and yet seems written into the existence of absolutely everything.

We'd mean to discuss time on Monica Grady, Professor of Planetary and Space Sciences at the Open University.

He ensured, Professor of Mathematics at the University of Warwick, and Jim Alcalili, Professor of Theoretical Physics, and Chair in the Public Engagement Science at the University of Surrey.

Jim Alcalili, let's start with a clock on the wall with his hour, minutes and seconds. What sort of time is that marking?

Well, it's very interesting. A lot of people confuse time with clocks, and so, you know, without clocks, would there be any time?

Clocks are just counters. They're just our human invention, our way of recording how we perceive time to go by.

So a clock is divided up into hours, minutes and seconds, but we could have divided it up into any other units.

Our day is how long it takes for the Earth to spin once around its axis, and dividing a day into 24 hours, and an hour into 60 minutes and so on, is a human invention, goes back to ancient Babylonian times.

So clocks mark our own local time, and we'll get on later on to talk about times being different.

You know, as far as Newton was concerned, if there were clocks spread out across the universe, they could all, in principle, be synchronized and will all tick by at the same rate.

You say it went back to the Babylonian. Why did the Babylonians choose 60?

Since this has got the hexagesimal system, they chose 60, and then the Indians brought in the decimal system units of 10, which is much more convenient since we have 10 digits.

But it's something that goes back to the second millennium BC, and it's mostly lost.

We still keep hours, divided into minutes and seconds, in units of 60, and angles.

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