At the equator, it's 6378 kilometres to the centre of the Earth. There's a lot of rock in between, so it's a remarkable thing that we have a reasonably good idea of its structure.
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In the early 1900s, thanks to seismology, a dim understanding was beginning to emerge. It was thought then, that the Earth had a completely molten core.
When Inge Lehmann (born 1888) looked at the data, she realised something was not quite right with that theory. The Earth does have a molten core, but within that, is a solid inner core.
If you imagine the squiggly lines on a seismograph (actually lots of seismographs), this wasn't a trivial insight because it required her to analyse data from seismic stations around the world.
Fortunately she'd attended a progressive high school where girls were encouraged as much as the boys. From there she went to Copenhagen University where she studied mathematics, chemistry and physics.
Later she worked in an actuary office, where she developed her mathematical skills.
The job of an actuary is to analyse the financial consequences of risk, which means she needed a deep understanding of mathematics and statistics. This proved to be a foundation for the rest of her career.
After finishing her degree, she earned a position working in seismology, where she studied with leading experts of the day.
Although she was very shy, it was clear that Inge Lehmann was exceptionally talented, and one of her first jobs was setting up seismological observatories in Denmark and Greenland.
In 1936, she published a paper that analysed discrepancies in the data that revealed the Earth's inner solid core.
P-waves (pressure waves) and S-waves (shear waves) travel at different speeds, and react differently when travelling through solids or liquids. These patterns indicated that the Earth's centre is solid.
Some years later, she was studying the Earth's crust and upper mantle when she discovered another seismic discontinuity at depths between 190km and 250km. In her honour this is named the Lehmann discontinuity.
Fortunately Lehmann was not forgotten, having (among other things) asteroid 5632 named after her.
In 2015, coinciding with the 100th anniversary of women's suffrage in Denmark, the beetle species Globicornis (Hadrotoma) ingelehmannae was named after her.
Inge Lehmann died in 1993, aged 104.
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