A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Steven Wepster
Date: 2003 Aug 26, 17:27 +0200
George, I was tought the use of the device commonly known under the names 'nonius' and 'vernier' first under the former name. Whether is is called nonius or vernier might be time and place dependent. Concerning the micrometer, my first reaction was like yours: they probably meant the reading-lens. But I'm afraid that our thinking is governed too much by ignorance. Hopefully someone else is able to reflect on the correctness of your interpretation. The accuracy of the scale (and the reqading of it) as described is consistent with the scale and reading of my old sextant of ca.1945 which still has a vernier. The divisions on the vernier correspond to a reading of 0.2' or 12", and you can guess to half of that or 6". Any finer subdivision is not practicable. For reading the vernier, adequate illumination and the use of the reading glass is of much help. But on the other hand, various forms of micrometers have been fitted to telescopes by astronomers since Gascoigne (1640) and Auzout (1666), to measure distances between various objects in the field of view of the telescope. I have never heard of a micrometer fitted to the telescope of a _sextant_, and I do not see any practical use for it. It would introduce additional reading error (reading of the scale and of the micrometer) and it would also introduce a large probability of blunder (add or subtract the two readings?) while nothing is gained. George, may I take the opportunity to congratulate you with you being distinguished with a Fellowship of the Royal Institute of Navigation, for your "status as an acknowledged authority on historical aspects of navigation, in particular [your] critical contribution towards the Institute's involvement in the BBC TV programme 'The Ship'". The happy event happened almost a year ago. Well deserved, George, and I hope that you will share your sharp thoughts on maritime-historical matters with us for many years to come. Steven.