WHAT,S AT THE CENTRE OF THE EARTH ? 

We live on the surface of a Dence , rockey ball, but science has allowed us to peer within its core.

When science fiction writer jules verne wrote journey To 

The Centre of the Earth in 1864.

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In that he probably knew that his plot was pure fantasy. Verne's characters Otto, alex and their guide hans, only made it a few miles down, but the idea that anyone could even contemplate travelling to the Earth,s core had been dismissed before Victorian times.
The idea of the Earth having a meaningful centre goes hand- in - hand  with the planet being shaped  like a ball, and we've known that we don't on the disc for a long time. 
It's a myth that medieval folk thought the Earth  was flat -  this actually  came from a mix of victorian anti-religious propaganda, and a  misinterpretation of the stylised map of the period. it was over 2,200 year ago that greek polymath Eratosthenes first measured the distance around the  Earth's sphere , and it's been clear ever since that it must have a centre.

This doesn't mean , throgh, that early philosophers thought of Earth as we do today. Ancient Greek physics said that the world consisted of a series of concentric spheres of four fundamental elements :- earth, water, airand, finally, fire.  In this idea, the centre of the planet had to be solid, as air couldn,t be inside the sphere of the earth.  Clearly, the sphere of the of the earth wasn't, completely surrounded by water or there or there would be no dry land , so there would be no dry land , so there was thought to be a bit of the earth sticking out , meaning there could only be one continent. As a result , the discovery of the americas was a  significant step on the way to disposing of Ancient Greek science. 
The  idea of the earth being entirely hollow , or with vast caverns reaching to centre as in verne's book, has been popular since ancient times. But it's not clear that any scientist apart from the astronomer Edmond halley, who proposed a hollow Earth to Explain some unusual compass reading in 1692 has ever taken this idea seriously. and in 1798 an english scientist and eccentric put the final nail in the coffin of 'hollow Earth' hypothesis. This was when henry Cavendish ' weighed' the planet.  
Canvendish was an odd man , who only communicated with his servants via notes to avoid meeting them face-to-face. despite his aristocratic background , Cavendish dedicated his life to science , working in both chemistry and physics, and most famously devised an experiment to calculate the density of the earth .using a simple torsion balance, which measured the amount of twisting force caused  by the gravitational pull of the two large balls on a smaller pair , Cavendish was able to calculate the faint gravitational  attraction between the two pairs of balls . By comparing this with the earth's own gravitational pull,he could work out the planet's density (and , as the Earth's size was already known,its mass, too). 

But the density figure showed that our planet must be mostly solid , unless there were extremely dense unknown materials somewhere in the depths. Today , we splits the innards of the earth into three segments :- the crust (5-75km thick), the mantle which extends to a depth of around 2,900km, and the core - the bit we,re interested in here- extending around 3,500km out from the Earth's centre, with two distinct segments. 
At the core's heart is an extremely hot but still solid nickel-iron sphere with a radius of around 1,200km. At approximately 5,400 degree C. this inner core is similar in temperature to the surface of the sun . The remainder is the liquid outer core , also most;y nickel-iron ,with similar temperatures, getting hotter towards the centre. But how can we possibly know such detail about a location  that is so inaccessible ?  through analysing earthquakes.   
         

After a quake , seismic wave travel through the Earth , changing their form and direction depending on the materials they pass through. Geophysicists have used at the Earth,s core . Their seismometers,  device to measure such waves , are the equivalent of telescope for exploring the Earth's interior.


Discovery: Earth has a solid inner core

On 17 june 1929, at around 10:17am local times a 7.3 - magnitude earthquake struck he south Island of new Zealand . Wave from the Earth  quake were recorded on seismometers around the world , notably in Frankfurt, Copenhagen, Baku, Sverdlovsk and Iskutsk.
these device consisted of a heavy  weight suspended from a frame . When the Earth and the frame vibrated the inertia of the weight prevented it from moving with them  , creating a difference in motion that could be captured by a pen on a rolling sheet of paper.
The first accurate seismometers responded to up and down movements in a horizontal arm but, shortly before the New Zealand earthquake , a new kind of seismometer using a vertically suspended weight came into play , and these proved crucial in the discovery.

Danish seismologist Inge Lehmann had been working for a couple of  years comparing the output of seismic stations. Initially working with  puplished data, and then going to the original records as", lehmann discovered were not always satisfactory", Lehmann discovered oddities in the wave patterns. She realised that seismic wave arriving between around 104 degree and 140 degree from the epicentre had interacted with a solid  inner core, disproving the previously accepted belief that the Earth 's core was entirely liquid.

In first lehmann investigated seismomter recordings of  an earthquake in 1929, and found that some of the waves must have interacted with a solid core.

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