Here is an audio transcription: "Speaker1: This is the BBC. Speaker2: This podcast is supported by advertising outside the UK. Speaker3: Tired of ads barging into your favorite news podcasts? Good news. Ad free listening on Amazon Music is included with your Prime membership. Just head to Amazon.com slash ad free news podcasts to catch up on the latest episodes without the ads. Enjoy thousands of shows ad free for Prime subscribers. Some shows may have ads. Speaker4: This podcast is brought to you by eHarmony. Finding someone who gets you is hard, right? You're not alone. That's because we're human and there's a lot of different humans out there, which is why Eharmony's personality based dating app helps you find someone. You can be your whole self with, someone you can be fully comfortable with. That's what true connection and compatibility are all about. Being seen, heard, understood. When you match based on personality, you're already one step ahead when it comes to getting to know one another. So try eHarmony and get started today for free. Eharmony get who gets you. Speaker5: Thank you for downloading this episode of In Our Time. For more details about In Our Time and for our Terms of use, please go to BBC.co.uk radio four. I hope you enjoyed the programme. Hello. It's said that we know more about the composition of the sun 90 million miles away than we do about the core of our own planet, Earth. It's only the last eight years the scientists have moved towards a view that Earth has a solid inner core of mainly iron, the size of the moon, which is then surrounded by a larger liquid outer core, an ocean of molten metal, again mainly iron. The edge of this core begins 3000km beneath our feet. Our growing understanding of the core has helped explain the creation of the Earth's magnetic field, which shields this planet from the worst of the cosmic rays, so making life as we know it possible. With me to discuss the Earth's core are Stephen Blundell, professor of physics and fellow of Mansfield College at the University of Oxford. Alwyn Doyce, associate professor in seismology at Utrecht University. And Simon Redfern, professor of mineral physics and fellow of Jesus College at the University of Cambridge. Stephen Blundell, let's start with the this scientific journey to the centre of the earth in the 19th century, when people began to be interested in it. What did they know about it then? Speaker6: Well, in 1798, Henry Cavendish had measured the density of Earth. He'd done this by looking at the gravitational force between lead spheres, and this had calibrated the strength of gravity. And he worked out that the density of Earth was about five and a half times the density of water. Now, why that was significant is that the density of rocks is only about two and a half to three times the density of water. And so if the Earth is just made of rocks, there's something has gone wrong with the calculation. And it looks like to compensate that, you need a much more dense core in the Earth. And the density of iron is about eight times the density of water. Iron is a very common material, and so it seemed potentially logical that the Earth could have a much more dense core than the the density of Earth. The density of the surrounding rocks would. Speaker5: Be following the great banner an idea of curiosity. Or was he doing it because there was a purpose he was going to find something worth which could be put to use, for instance. Well, I. Speaker6: Think what he was doing was following Newton's theory of gravity and trying to test it essentially to try and calibrate it. And then also what came from this in France, again, right at the end of the 18th century, just before the 19th century. Simon Pierre-Simon, Simon de Laplace, 1796. What he noticed was all of the planets in the solar system were going around the sun in the same sense, and also the most of the planets, Earth included, and the sun also rotate in"

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