NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: How did Sumner navigate in 1837?
From: Herbert Prinz
Date: 2003 May 14, 11:37 +0000
From: Herbert Prinz
Date: 2003 May 14, 11:37 +0000
Jim Thompson wrote: > 2. Was the celestial LOP completely unknown in 1837, or were academicians > aware of the concept, but nobody had been able to operationalize it at sea? Globes had been used since the 16th century for the graphical solution of astronomical problems. They had to be rather large to be useful and were impractical at sea. > 3. Exactly how would he [Sumner] have determined his longitude in 1837? By chronometer and time sight, using one of the methods given in a contemporary Bowditch (preferring No.3 over No.1, passing over No.2). > 4. How aware were navigators in his day that their longitude calculation was > dependant on latitude? They must have been. It must have been part of the > sight reduction? Not every longitude calculation is dependent on latitude. For example, the "method by equal altitude" isn't - well, at least not to an extend where an error of 1 deg of latitude would matter. But with most commonly practised methods there was indeed such a dependency and navigators were well aware of it. The problem was by no means new or unique to celestial navigation. Already before the chronometer, dead reckoning was based on Traverse Tables that kept track of changes in latitude as well as meridian departures and thus, longitude. If possible, the Sun was observed at noon. This observation overruled the DR latitude. Now the the Traverse Table had to be adjusted so as to reflect the new latitude, thereby yielding a new DR longitude. J. H. Moore, for instance, (Practical Navigator, 1800) has a whole chapter on this. Although he claimed to present to this end "the most rational methods", I can't help the feeling that this was an area where navigation had more to do with art (or magic) then with science. Some of the worked examples in Moore are charmingly naive and quite amusing to read. > Putting together some information on 18th century celestial navigation from > www.lunardistances.com, I assume that he [Sumner] probably had done this: > > 1. Determined latitude by DR from his last fix, 900 NM to the west, at 21 > deg W longitude (he was now at 6 deg W). Is there no way he could have > determined latitude from the sextant altitude of the sun and his chronometer > time? Only if he would have had his longitude in addition to Greenwich Time, or local apparent time instead of it. Neither is a realistic assumption. As Sumner himself stated convincingly, from one observation you get exactly one position line, no more, no less. If this position line happens to be perpendicular to your meridian (it will be, if, and only if the celestial body is on it), then you happen to have got your latitude. > > 2. Determined longtitude by using these known variables: DR latitude, his > chronometer GMT time, the altitude of the sun from his 1000 shot, and tables > showing declination of the sun. From those he could determine local time, > from which he could determine the difference in time between his local > meridian and Greenwich. I still don't understand the steps he used, but I > think that's the basic process he would have used. Am I right? Yes, you are. > The critical point is that his longitude estimate was dependant on his > latitude. He seemed to understand that, which is why he reworked the 10 AM > sight with two presumed latitudes. How common was that practice in 1837? How common was it to do a time sight at all? And would not the first person to plot three lat/lon pairs have made Sumner's discovery? > If that old DR latitude was way off, then his longtitude was too -- which > was one of the points that navigators in those days might not have > appreciated, because they did not commonly understand the concept of a > celestial LOP.latitude. As I said before, I do think that navigators were aware that wrong input lead to wrong results. But before Sumner, an observation was either good or bad, the result right or wrong. By contrast, Sumner adopts a quantitative approach to the problem. He asks how much error one may expect under certain given conditions. In the treatise where he presents his new "Method by projection on Mercator's chart", he spends over 13 pages out of 90 on error analysis. He even has it in the title: "[...] First, The True Bearing of the Land; secondly, The Errors of Longitude by Chronometer, Consequent to Any Error in the Latitude; thirdly, The Suns True Azimuth.[...]" I believe he is the first one to undertake any serious attempt at error analysis in a work of navigation and is perhaps not sufficiently appreciated for this by historians. Later in the century this would become a big topic, specially in France. Herbert Prinz