NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Latitude by Spica/the NAV L list
From: Kieran Kelly
Date: 2004 Jan 15, 09:51 +1100
From: Kieran Kelly
Date: 2004 Jan 15, 09:51 +1100
Mr Frank Reed wrote recently: "Land-based surveying had its own needs and preferences. I am under the impression that "Navigation-L" is mostly about navigation at sea. Not that land surveying is irrelevant (it isn't) but it's a different animal." This is an interesting point and raises questions about what types of issues are worthy of discussion on this list. When I was encouraged to join this list several years ago I was told that it was for people interested in celestial navigation - an analogy is the American Foundation for the Promotion of Navigation of which I am a member. Celestial navigation techniques cover four areas *Maritime navigation *Terrestrial navigation *Marine hydrography (which is a combination of the above) and *Surveying All of these topics have been discussed at some point in this forum, thus the contention that the Nav L list is mostly about navigation at sea is a bit adrift, witness the extensive and on-going interest among list members about the work of Lewis and Clark. Unless I am very much mistaken neither spent much productive time at sea. Mr Reed's assertion that land surveying is a different animal confuses exploration and surveying. They are different. A captain on the quarterdeck with a sextant in his hand confronts the same problem as an explorer with a sextant in his hand surrounded by 600 miles of uncharted desert. I have used a sextant extensively on exploring trips in inland Australia and also on H M Barque Endeavour and the techniques and principles, as far as I can see, are identical i.e. you use a sextant to bring a celestial body down to a horizon. The terrestrial sight uses a double elevation shot but after you take off IE and divide by 2 the rest is identical - same reduction techniques, same clock, same tables. It is all based on the solution of the celestial triangle all the rest is detail. The small practical differences between the two areas of navigation were the subject of an article I wrote for the Navigation Foundation newsletter - A "Lubber on the quarterdeck" Issue 78 Winter 2002-2003. Mr Reed also remarks: "... there is no reason why star sights for latitude should be more accurate than sun sights (except that you have more star transits available in principle and you could average them, but that doesn't work at sea)." Quiet true. However the ability to average the sights is critical and probably what Gregory was driving at. Also I have done numerous noon sights in the field as well as star sights and in my case I have always got far better results from the stars. The reason why, I know not but I could guess: 1) Bringing a star down onto its reflection in the mercury pan gives the observer a tiny target to aim at. I think superimposing the small point of light substantially reduces the margin of error 2) When bringing limbs of the sun into coincidence in the pan it is hard to get an exact "kiss" unless the sextant has absolutely zero side error. If there is any side error the point of contact or coincidence can be become confused by guesswork. 3) Looking into the noon-day sun in places like Northern Australia for example is not for the faint hearted. The face quickly begins to fry and I always end up squinting particularly through my left eye and the right eye is often left with mesmerising haloes from accidental sun flashes. Sweat running down your forehead into your eyes is also a problem. I would back a star sight for accuracy anytime. Mr Reed wrote; "MANY readers in the year 2004 make the huge mistake of believing that Norie, Bowditch, etc. describe the *practice* of navigation. They do not. Although Bowditch and Norie and others recommended shooting stars, navigators at sea rarely did so. Equal time in a navigation manual does NOT imply equal time at sea! Now of course, I am describing the evidence from the logbook collections that I personally have seen. These are logbooks from American commercial vessels. Do you have evidence from other vessels? Not just suppositions based on recommendations in Norie... but real evidence. That would be fascinating. But 19th century land-based astronomy is a very different subject," Deriving a view of 19th Century navigation based solely on the evidence of American commercial vessels ignores that great navigating and technically innovative body the Royal Navy. If we pick two of its navigators Lt Matthew Flinders and Captain Philip Parker King we see two men who tried to shoot lunars from shore where-ever possible using artificial horizons. Both were superb maritime celestial navigators, accomplished hydrographers and experienced lunarians. They combined all four of the points I raised above as encompassing celestial navigation. The art of navigation like this list is possibly more multi faceted than Mr Reed believes. Kieran Kelly Sydney Australia