NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: How Worsley Navigated [Was Navigation and Whaling]
From: Brad Morris
Date: 2009 Feb 23, 19:31 -0800
From: Brad Morris
Date: 2009 Feb 23, 19:31 -0800
Hi George The type of sextant (Heath Hezzanith) and its use for the journey cannot be disputed. The exact sextant used by Worsley tours with the James Caird and can be seen (but not handled)by visitors. Who originally used it or owned that sextant prior to the journey, while technically correct, is IMHO kind of irrelevant. It is the sextant that Worsley used on that journey, for his reasons. I can see why Worsley chose this sextant, it has every feature you could ever dream of. So I went the additional step. I took the photographic evidence of the sextant and tracked it down. I found the same make and model sextant with all of the options as shown in the image. It clearly is a high end sextant as can be seen by all of the features and the arc accuracy presented herein. Fantastically, all of those lines in the telescope tubes are present, just as Bowditch calls for and they are unbroken. There are eyepiece shades in addition to the normal shades, as well as every other bit of kit (including the key and screw driver). Paid an outrageous price for it and giggled all the way home. It was mine! Spent the next few months getting the mirrors re-silvered and then adjusting all of the optics per Bowditch's 1849 description. The index and horizon mirrors are perpendicular to the arc and parallel to each other when the nonius is very close to zero. The rising telescope feature is present and the telescopes are parallel to the arc, using Bowditch's method. I am thrilled, to say the least. There is a sight tube, two inverting scopes (4 & 11 powers) and two erect scopes (3.5 and 4 powers), one of which is a large aperture star scope. It also has the binoculars which attach to the rising telescope feature. I have shot lunars with it (you don't realize how heavy it is until you hold it at those weird angles for minutes at a time!) as well as the standard altitude shots. The patent "greatest angle clamp" is there as well as the patented box clip that holds the sextant in place. The arc is divided to 150 degrees, yet the useful range is only to 125 degrees, as it is a vernier type. The final divisions yield measurements to to 10". At this juncture, I would like to have the arc calibrated but as we all know, those services just don't exist anymore. I keep hoping Frank will determine the economic feasibility of his calibration methodology alluded to in earlier posts. But until then, I am forced to use the National Physical Laboratory record of 1921 of my sextant, which shows ZERO minutes and ZERO seconds for every reading from 15 degrees to 120 degrees. Naturally, the device should have worn over the years. Maybe Bill Morris can comment on the evenness of wear over the years. Uneven wear would contribute to an eccentricity and may throw that unbelievable calibration certificate out. The National Physical Laboratory certificate claims it is a "CLASS A", does anyone know the classifications at the NPL? At this point, you have to ask yourself if Worsley chose poorly. Given the feature set and the fact that the expedition would have obtained the greatest accuracy possible (zero point zero error) then this sextant is an obvious choice. Best Regards Brad --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---