Schiehallion, Perth and Kinross
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Scientific fields: Physics, Cartography, Astronomy
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Summary description:
Schiehallion (the name is thought to mean the “fairy mountain of Caledonia”) is a mountain in central Scotland where an early attempt was made to measure Earth’s density, and from that, Earth’s mass and the gravitational constant, G. The experiment was performed in 1774 by the Astronomer Royal of the time, Nevil Maskelyne, based on observing how much the mass of Schiehallion deflected a weight suspended on a plumb-line. As part of the experiment, the volume of Schiehallion – chosen because it was an isolated and somewhat symmetrical mountain – had to be calculated, for which purpose contour lines were invented by Charles Hutton. Visiting the site need involve no more than viewing and possibly climbing Schiehallion. A memorial to the experiment can be viewed at a car park commonly used as a starting point for climbing the mountain.
Location:
Perth and Kinross in central Scotland, about 9 miles (15 km) northwest of Aberfeldy. The memorial to Maskelyne’s experiment can be found at the Brae of Foss car park, at the foot of the northeastern flank of Schiehallion.
Getting there:
To reach the Brae of Foss car park, turn west off the B846 (Aberfeldy to Tummel Bridge B-road) at Tomphubil, by Loch Kinardochy, and continue west for a mile. Aberfeldy is on the A827, which branches off the A9 running northwards from Perth.
Background:
If you are in the vicinity of Perth and looking for an energetic hike, you can place yourself in the footsteps of Maskelyne as he measured the density of the Earth. The experiment, in 1774, was funded by a grant from the Royal Society. It was based on the reasoning that the deflection angle of a weighted plumb line held in the vicinity of Schiehallion would depend on the relative densities and volumes of Earth and the mountain. Since Earth’s volume was known, and the density of the rocks making up Schiehallion, as well as the volume of the mountain, could be fairly accurately estimated, Earth’s density should be directly ascertainable from the deflection angle. Once this was known, it would be relatively straightforward to calculate Earth’s mass and the gravitational constant, as well as the masses of the other planets, their moons, and the Sun.
The amount of deflection of the plumb line in relation to the vertical that had to be measured was of the order of a tiny fraction (hundredths) of a degree. It worked out that such a small deflection could be determined only by observing the plumb line’s orientation in relation to the stars – which is why an astronomer was given the job. Maskelyne's measurement gave an average density of Earth that is about 80% of the accepted value today. Importantly, it made clear that Earth’s average density is much greater than the density of its surface rocks, debunking a common idea of the time that Earth is a hollow sphere. Some 24 years later, a much more accurate value for Earth’s density, only 1% different from the currently accepted value, was made in London by the aristocratic and eccentric scientist Henry Cavendish. Cavendish's determination was based on measuring the attraction between large masses of lead.
Opening hours and charges:
Not applicable.
Website (concerning Schiehallion experiment): http://www.sillittopages.co.uk/schie/schie57.html