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The Giant Telescope Hidden in the Heart of London

Updated: Nov 7, 2018


I was in London last week and took this photo of the Monument to the Great Fire of London from the outside deck of the Sky Garden (a lovely spot, by the way). It shows, just about, the hinged lid of the bowl of fire that sits atop the 62 m column. The lid serves to keep the rain out, I suppose, but also to obscure the wonderful ulterior purpose that the designer Robert Hooke had for the monument.

As it was being built in the 1670s, the issue of whether the earth rotated around the sun, or vice versa, had still not been conclusively resolved. One way of establishing the movement of the earth - and thus supporting the heliocentric model - was to look for parallax amongst the stars. If the earth moved as the year progressed, this should create measurable, though tiny, variations in the arrangement of the stars as we view them from earth.

It was clear that any such movements were very slight, but Hooke believed that with a sufficiently powerful telescope he would be able to observe them. Such a telescope would have to be very long, and it would also have to be almost entirely rigid, in order to take readings about the position of stars and to compare those readings meaningfully from one month to another. Certainly, any possible wooden design would move far too much to be of any use. So he determined to build a telescope to run through the centre of the monument he was designing along with his friend Christopher Wren. The telescope would only ever have been pointed in one direction of course, but that would have been sufficient for its purpose.

Sadly, the plan failed. I'm not sure if the lens technology of the day would have been up to the task, but there was a more prosaic problem. Even the 800 cubic metres of Portland Stone used in the construction were not perfectly rigid, and the vibrations caused by the passing London traffic proved too great to achieve the precise readings required.

The ambition of the scheme, though, is revealed by the fact that it took another 150 years for Bissel to observe parallax in the stars. And that the telescope built into the monument was more than twice as long as the Leviathan of Birr, which wasn't built until 1845 and served as the world's largest telescope until the early 1900s.


The story is very well told here: http://www.bbc.com/future/story/20170810-the-medieval-lab-hidden-inside-a-famous-monument



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