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Newton's Principia

The Jodcast visited the Royal Society in London to learn about one of the most influential scientists in history and his famous publication. Sir Isaac Newton's Principia was first published on 5th July 1687 and contains many ground breaking discoveries. Within the pages you can find Newton's famous laws of motion, his theories on gravity and his work on the motion of the planets. Professor Martin Rees (President of the Royal Society) tells us about Newton's work and we peer into the pages of the Principia.


Emily (): Hello and welcome to the Jodcast. Today we've been invited back to the Royal Society to learn about the most influential scientist in history and his famous publication.

Colin (): No other books have dramatically improved our understanding of the world. Sir Isaac Newton's Principia changed the way we look at the Universe forever.

Emily (): First published on the 5th July 1687, Newton's masterpiece contained many groundbreaking discoveries. Within these 300 year old pages we can find his famous laws of motion, his theories on gravity, as well as his work on the motions of the planets. Colin was fortunate enough to get the chance to speak with Professor Martin Rees - Astronomer Royal - who told him a bit more about Newton and his work.

Colin (): So was Newton the first person to come up with these ideas in the first place?

Prof Rees (): Well the ideas were in the air but what was important about Newton was that he codified these mathematically. He showed us how we can calculate the orbits of the planets and how the same laws that made the apple fall were those that held the planets and the Moon in their orbits. This was an important unification and gave us a picture of the so-called clockwork Universe. Apart from being important in its own right, this was very important in the culture at the time as being the first real evidence that mathematical laws did govern the behaviour of the Universe.

Colin (): That was the main stride that Newton made; that kind of universality?

Prof Rees (): That's right. He showed that the same laws that we see on the Earth apply in the heavens as well and he showed that you could make calculations and you could make predictions of where the planets would be at a particular time and understand why they would be there. Of course, he was lucky actually because even though we now understand a good deal more about nature there are still many things we can't predict. We can't predict the weather although we can predict the orbits of the planets so he was lucky that he seized upon a phenomenon of nature that we can both understand and predict. He became an important figure in the cultrue of his time.

Colin (): So how did he then go on? Tell us a bit more about how he combined his theories of gravity with Kepler.

Prof Rees (): It was already known that the planets moved in ellipses, it was known that they had slower orbits if they were further from the Sun and the one thing which Newton showed was why they had elliptical orbits. The one thing that held him up actually during his work was proving that a spherical planet would exert the same gravitational force as if its mass was concentrated in a point at the centre. That allowed him to simplify the calculations and not worry how big the planet was.

Colin (): Was that to do with how gravity pulls... its a kind of radial force?

Prof Rees (): Well it turns out to be true for the inverse square law but not for other laws of force. What Newton did was show that the inverse square law of gravity applies everywhere and of course this was the standard picture and was only refined 200 years later by Einstein who gave us a deeper understanding into the nature of gravity and gave us laws which applied when the gravity was stronger and the speeds are higher than in regions where Newton is a good approximation. But of course even now, Newton's laws are used by those who program the orbits of spacecraft to the Moon and planets.

Prof Rees (): We've got, in front of us here, Newton's manuscripts for the Principia. It was of course written in Latin although an English version appeared some decades later and became widely circulated. There was even a simple digest called Newtonianism for Ladies which appeared in the 18th century because his work had a very wide impact. We've also got here a replica of Newton's telescope which was a novel design; a reflector which he pioneered which of course is the basis for many much larger telescopes now. Many are said to be Newtonian telescopes and that just means they use the basic optics which Newton pioneered in this little telescope here.

Prof Rees (): Here at the Royal Society we have lots of Newton memorabilia. We have his manuscripts, we have his telescope here, we have many portraits of him. He was a very vain man; he had about 50 portraits painted altogether. We also even have a lock of his hair and he even had a death mask taken as well so he is very well commemorated - there's lots of material about him.

Colin (): It's amazing we have so many artifacts left to have such an insight into his life.

Prof Rees (): That's right. We have to remember that he was not a very pleasant man. It's good to have these relics but what we know about him suggests he was rather solitary and difficult when young and rather vain and vindictive in his old age. He lived into his 80s and was a fairly powerful and famous figure but he was not a pleasant man unlike certain other scientists like Darwin - who we're celebrating next year - who is someone you would have enjoyed meeting.

Colin (): But to have such insights and be such a pleasant man I'm not sure that is something you can necessarily balance together.

Prof Rees (): Well Newton was asked how he succeeded so well in his work. He said "by thinking on the problems continually". What was remarkable from what we read about him was his immense powers of concentration to think through a problem. Although his work appeared in this one big book it was the fruit of literally decades of effort.

Colin (): So he had to kind of make himself a recluse. To shun what you'd almost imagine to be normal life.

Prof Rees (): He seemed to have done that through his most productive years.

Emily (): So the Principia came to be regarded as the rule book of the Universe providing us with rules that always seemed to be obeyed. Newton believed that we lived in a clockwork universe with each tick as steady and reliable as the last. To him, the motions of the planets were just as predictable as a dropped stone falling towards the Earth.

Colin (): For over 200 years the theories contained within this amazing book remained relatively unchallenged as our best grasp of the intracacies of the world around us. Until a certain German born Swiss patent clerk - Albert Einstein - revolutionised physics. He would go on to shatter the clockwork universe forever. But that's another story.

Emily (): Well that's all from the Jodcast here at the Royal Society. Thanks for listening.

Show Credits

Presenters:Colin Stuart & Emily Fair
Writers:Jessica Bland, Colin Stuart & Emily Fair
Interviewee:Professor Martin Rees, Lord Rees of Ludlow OM Kt PRS
Camera:Nicholas Rattenbury
Editor:Emily Fair
Sound Recording:Stuart Lowe
Opening Sequence:Paul Carr & Colin Stuart
Music:Susan M. Lockwood
Executive Producers:Nicholas Rattenbury & Stuart Lowe
Filmed on location at:The Royal Society, London
Special Thanks to:Keith Moore, The Royal Society and Professor Martin Rees (PRS)
Cover Art:A page of Newton's Principia
Website:Stuart Lowe

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