How to Hide a Giant Telescope...and other stories from the Physics Hub

I found the time to sit in on a Physics Hub last night, which are always interesting and last night was no exception. And I am in awe of David Keenehan's ability to continuously come up with new and stimulating topics for each week.

Last night the topic was 'Stories'. Essentially a guide to 'Who did what, where and when did they do it, and why.' We heard about an early Irish X-ray - taken by a de la Salle priest in Waterford within a few months of the discovery by Rontgen in Germany. And we were reminded of the wonderful IoP resource put together a few years ago by Alison Hackett, placing scientific history in the context of world history. And more....and all such resources are available at the talk physics link.

I also got the chance to recount one of my own favourite stories from the history of physics: the secret telescope hidden in the middle of the London. The summary of the tale is this: Robert Hooke - alongside Christopher Wren - worked on the rebuilding of London after the great fire of 1666. And the job of designing and overseeing the construction of the monument to the great fire fell to him - so he took advantage of the situation to build an enormous telescope....

He was already a renowned scientist at the time. Quite how accomplished he was is a little jaw-dropping: he had built the apparatus with which Boyle demonstrated his famous law, proposed (before Newton) that gravity must follow an inverse square law and published the Micrographia, which became the bestselling book of its time, thus introducing the public for the first time to the world of the very, very small. While also originating the concept of biological cells and speculating on the idea of evolution.

Almost the least interesting things the man did, in fact, was to devise the law with which he is synonymous, relating to elasticity.

As he worked on the design of the monument to the great fire 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 (and he had tried). So he determined to build a telescope to run through the centre of the monument. 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 Bessel 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.

more here:

and here: Lisa_Jardine_2006_Medlicott_Lecture (1).pdf

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