The Weight of the World! 

Image of the Earth.

Credit: NASA/Apollo 17 crew

We now know the planets go round the Sun, but what are they made of? How heavy is the Earth? What is inside it? In the 1700s and 1800s, two North East scientists did some experiments down a mine and on a mountain to find out.

How much do you think Earth weighs? A lot, right?! Up until the 1800s, people were still not sure. Scientists were also curious about what was inside Earth. Some wild theories floated around, like Earth being like a water balloon or even a giant hollow sphere! Knowing the weight of Earth would solve the puzzle, but that was tricky too. Can you imagine trying to put Earth on your bathroom scales?

North East scientists Charles Hutton (1737-1823) and George Biddell Airy (1801-1892) came up with different ways to measure the weight of Earth. This would help prove what was inside. To solve the puzzle, they both used pendulums to compare gravity in different places. This helped them to calculate Earth’s density.

Hutton and Airy both correctly concluded that the Earth was rocky with a metallic core. Hutton used the density of the Earth to calculate, for the first time, the density of the other solar system planets! This told us that Jupiter and Saturn are made of gas and that Mercury, Venus and Mars are rocky.

Did you know... 

Some people believe that Airy’s ghost haunts the Royal Greenwich Observatory in London!

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Airy made his measurements at the Harton pit. The pit was run by the Harton and Hilda Coal Company. Harton started as a small mining village but is now a part of South Shields.

Brick cuboid-shaped building with large windows. Large wheels are located on the top of the building and on the ground, connected by ropes or belts.

St. Hilda Colliery 2003. Credit: TWBPT.

Related artefacts

 ‘A memoir of Charles Hutton’ by John Bruce, 1823

This is a biography of mathematician and map maker Charles Hutton. He was one of the first people to accurately (for the time) measure the density of the Earth. Hutton was born in Percy Street, Newcastle in 1737. He worked in Long Benton Colliery before becoming a teacher in Jesmond. 

Photo of Airy’s pendulum experiment set up at the bottom of Harton pit

George Biddell Airy hung pendulums at the top and bottom of a mine. He measured how much they moved. This let him record the difference in gravity between them. He found that the gravity at the bottom of the mine was a tiny amount more than that at the top.  He worked out the density of the Earth was 6.566 g/cm^3. The actual figure is closer to 5.5153 g/cm^3. This was still an incredible achievement without using computers.

Pencil sketch of an undergrand room containing a large pendulum structure on a triangular wooden frame.

Drawing of the pendulum in Harton Pit used in Airy's experiments

Map of mines in the North East showing Long Benton colliery and Harton pit

Map of South Shields highlighting the location of Harton Pit

A 1951 map of South Shields showing the location of Harton pit (green circle) which is where Airy conducted his pendulum experiment.
Credit: Durham Mining Museum

Map of Newcastle highlighting the location of Long Benton mine

A 1807 map Newcastle and Northumberland showing the location of Long Benton pit (green circle) where Charles Hutton worked in before an arm injury meant he was sent to school instead of working in the mines.

Credit: Durham Mining Museum