NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: first sextant sights
From: Jim Hickey
Date: 2006 May 8, 13:14 -0500
-----Original Message-----
From: Alexandre E Eremenko <eremenko@math.purdue.edu>
To: NavList@fer3.com
Sent: Wed, 3 May 2006 11:43:31 -0400 (EDT)
Subject: [NavList 102] Re: first sextant sights
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From: Jim Hickey
Date: 2006 May 8, 13:14 -0500
I wanted to revisit the lunar distance <0.5' claim again. Since the moon was in such perfect postion late last week I took a number of lunar sights using the sun, mars, jupiter and spica. So, a good range of angles.
Out of 9 sights the largest error I had (using Frank's online utility) was 0.3'. I had 3 sights that reduced and showed 0' error (honest!).
Ignorance really is bliss. Prior to reading many of the posting on Navl or Nav list I would not have though much of this. My certificate for the sextant shows 0 error at all angles except a 10" correction at 20 to 30 degrees. Alamanc data assumed to be within 0.1 most of the time so I would expect measurements definitely <0.5 in great conditions. Clearly this is not everyone's experience.
One thing I did notice is that to get this kind of accuracy of sight, it is much much easier to take sights where the moon and object are in a configuration such that you can hold the sextant closely vertical (which is what I did this past week). Measuring large angles with higher power scopes holding the sextant at odd angles is a killer and getting that <0.5 accuracy is tough. I am going to see if I can mount my sextant on a tripod and see how much better I can do. Stay tuned.
As for the accuracy of Cook's equipment, what we need are some of his guys to use some of our equipment! I always had the impression that accuracy per se was good and rivaling modern sextants (though I have to believe that modern milling has to be an advantage). What strikes me about modern sextants versus the old are that the mirrors are larger and the optics better which has to expand the conditions for making reasonable sights.
In the end, I bet the great results from Cook's voyage are due to great observation skills by these early navigators. I have seen amazing results with incredibly cheap plastic sextants with great observers but rarely great results with a good sextant and a poor observer. Hard to separate and quantify the two factors.
Jim
-----Original Message-----
From: Alexandre E Eremenko <eremenko@math.purdue.edu>
To: NavList@fer3.com
Sent: Wed, 3 May 2006 11:43:31 -0400 (EDT)
Subject: [NavList 102] Re: first sextant sights
Jim, On Wed, 3 May 2006 jimattac@aol.com wrote: > I have posted some results I have taken > under excellent conditions Where did you post these results? > and found that the <0.5 claim not unreasonable and certainly > not if an artificial horizon is used. Well, of course, if the art horizon is used, one has to multiply the error by two, when comparing with ordinary shots. > Obviously you simply are not going to get the > same result with a single shot on the deck of > a pitching sailboat. Sure. These are the separate things, of different nature. So I am trying to analyse them SEPARATELY. a) small boat problems b) horizon problems c) reduction and almanac errors and d) sextant and observer acuracy in taking sites. In discussing d) as a separate issue from a) and b) and c), I am thinking of the observations of star-star and Lunar distances on land under ideal conditions only. One can add art horizon observations to the same category. > Maybe with ideal conditions, lots of work on IE, > averaging, graphing, good knowledge of instrument > error and close attention to almanac data > and reduction method you could but this would > really be taxing the limits. That's what I am trying to do. > It is not founded on any analysis but, > I do have to believe that top end modern > sextants are better that what Cook This is not evident to me. I've seen the results of Cook's astronomers, enough of them to judge about accuracy. But I have seen very few modern results with best sextants to compare. Maybe someone will post such results? (I mean distances and art horizon from land). Alex.
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To post to this group, send email to NavList@fer3.com
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