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Thursday, February 26, 2009


Blogging the Starry Messenger - The Telescope

A I mentioned before, Galileo's Sidereus Nuncius (The Starry Messenger) doesn't actually have chapters. After the dedication it is all one solid block of text, but it has logical subsections, so I'm blogging those.

After the introduction, Galileo goes on to describe how he built his telescope. First he describes briefly how he hear of telescopes that had been built elsewhere. Many people are under the impression that Galileo invented the telescope. He didn't, and he says so, but he produced far better telescopes than the ones that were circulating at the time.

Galileo's description of his telescope is frustratingly short. He takes a tube of lead and grinds two two lenses, sticks them in and "presto!", a telescope. Details of how long the tube was, how big the lenses were, what depth they were ground to etc. are all missing. Later on, when he says that to follow up his observations, you have to make an excellent telescope without flaws one wonders how you are supposed to do that. 17th century savants were a bit more "hands on" than modern day philosophers, but lens grinding was not a widespread skill amongst them (there were spectacle makers around, they could have asked them though). Even Kepler had to borrow a telescope to confirm Gailileo's observations, rather than make his own.

The lenses he describes will also be unfamiliar to those of us schooled in cartoon versions of the telescope. Most of ys think of telescopes as having an adjustable tube, and two convex lenses, like in this illustration. Galileo's lenses were flat on one side. One was spherically convex and one was concave, as shown in this illustration.

Also, by modern standards his telescope was way underpowered, his first scope was only 9x, about as effective as a pair of 10x50 binoculars, his best telescope was only 30x, around the power of many modern finderscopes. For most of his observations of the Moon he use a scope of around 15x magnification. The higest power magnification was impractical because of the small field of view.

Consier that not only did Galileo have a telescope that was low powered by todays standards, but there was no fancy andt-glare coatings, correction for abberation, a decent mounting or anything thing else. The quality and consistency of the glass available at the time was not as high as today. Yet Galileo made a telescope that could uncover the heavens.

What he does sepend a lot of time on (in his wondrfully convoluted fashion) is his mechanism for determining distances between stars in the telescope (a series of thin metal plates with different sized holes in them that could be placed in front of the lens). This is where Galileo stands out. He wasn't the first to make a telescope, he wasn't the first to turn the telescope to the sky. But he was the first to make a seroius instrument capable of aming reasobably accurate measurements of objects. This will turn out to be important in the discussion of Medicaian staleelites later on, and the very conduct of science.

Next week - The Moon!

Blogging the Starry Messenger - Introduction

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Very interesting blog! I enjoyed it very much. I am considering using large aperture (over 70mm) binocs for star gazing. What do you think about this?
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