NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Star-star distances for arc error
From: George Huxtable
Date: 2009 Jun 25, 10:07 +0100
From: George Huxtable
Date: 2009 Jun 25, 10:07 +0100
Henry raises an interesting question, about handling the weight of early instruments for lunar observations. The first sextant, made by Bird in 1758, was so big (over 18 inches radius, compared with a more modern instruments 7-inches or less) and therefore so heavy, it required to be supported on a pole, with a ball-joint, as can be seen in the photo from the National Maritime Museum in Greenwich. That must have made it fiendishly difficult to operate on the deck of a ship at sea. Details from their website at http://www.nmm.ac.uk/collections/explore/object.cfm?ID=NAV1177 It would have been interesting if its weight were quoted, but it isn't. It had been preceded by another instrument made by Bird, a working version in brass of a wooden mockup, supplied by the German astronomer Tobias Mayer, of his navigator's circle, which I attach. This was an engraving made for Rees' Cyclopedia in about 1817, from the actual instrument, of which no trace presently survives as far as I know. The support pole fitted into a socket in a belt worn by the navigator. The instrument as a whole was dismissed by Campbell, to whom it was submitted for sea trials, as impractical to use at sea. I must admit to having great difficuty in holding up even a standard brass-framed 7-inch sextant of the early 20th century for a useful length of time. That's one of the reasons why I still prefer an Ebbco plastic job as more practical for me, in measuring altitudes from a small craft at sea, in spite of all the prejudices against it.. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "hch"To: Sent: Thursday, June 25, 2009 5:06 AM Subject: [NavList 8787] Re: Star-star distances for arc error I have followed this discussion with a great degree of interest and am inclined to agree basically with George’s moderate and measured approach. To criticize by assumptions not in evidence is not, in my opinion, the way to go. In the pursuit of practical navigation, the need for measurement of stellar distances seldom, if ever, arises and, although familiar with the principles involved, I have never seriously sought to perfect the technique. The matter of a support or stand for the sextant has, however, been a matter of interest to me: the hydrographical surveyor has long used a rather sophisticated stand ashore in conjunction with the artificial horizon and altitudes measured thereby, however, to the best of my knowledge, no such device has ever been in general use at sea, or for the measurement of non-vertical angles at sea or ashore. For what it’s worth, I throw out the following quotation from Andrew MacKay’s “ The Theory and Practice of finding the Longitude at Sea and on Land” c. 1809. .....”If the sextant is provided with a ball and socket, and a staff, one of whose ends is attached thereto, and the other rests in a belt fitted round the waist of the observer, the greater part of the weight of the instrument may by this means be supported by his body.” .... Assuming the friction between the ball and socket can be regulated, there just might be something worth trying here. Any comments. Regards, Henry --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---