NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar trouble, need help
From: Frank Reed
Date: 2008 Jul 01, 16:16 -0400
From: Frank Reed
Date: 2008 Jul 01, 16:16 -0400
Kent, you wrote: "The factor 60,35� has been provided to me from the Observatory in Stockholm to be used with the true zenith altitudes for altitudes above approx. 10 degrees. The factor is taken from modern English litterature." What color light? It's not often mentioned explicitly that this factor applies only for one specific frequency in the visible spectrum. That's one reason why two people can quote factors differing by an arcsecond and still be technically correct. It's also why we shouldn't worry too much about the tenths and hundredths of an arcsecond. And it's a big reason why we shouldn't shoot lunars when the objects are below about ten degrees: the stars and the limbs of the Sun and Moon are stretched out into colorful bands at very low altitudes. Incidentally, I use 58" usually. And: "Yes, this is correct. For altitudes below 20 degrees I use the formula defined by Smart." Since you're modeling historical methods, you could just input the whole refraction table as printed. Then the software would interpolate between values much as a 19th century navigator would have done on paper. This approach bypasses all this chatter about various formulae for different altitudes. It's also a reasonable approach for a modern analysis. Sometimes people doing these calculations obsess over the "Bennett" formula. This is because we carry around "baggage" from the calculator era and the early years of programming small computers. There's no real advantage to a short formula today (except saving a few keystrokes). And you wrote: "So again, what I have tried to achieve is a re-construction of how the LD�s were measured and reduced in the old days (by Swedish navigators) and not necessarily what will be achieved with todays know-how." May I ask, what is your source material? Are you looking at old navigation manuals and textbooks or old logbooks (or similar)? The textbooks tend to focus on minor issues (like second-differences, and finding local time from star altitudes, and correcting for the oblateness of the Earth) which were almost entirely irrelevant in practical navigation. And: "The first way is what I outlined and this way can be used for any celestial body, incl. the sun and provides the local time (MT). The other method was to use the sun for generating the local time by finding the sun�s apparent time and correct it with the TE. This later method required (requires) less calculations and was therefore probably preferred by navigators." The general method (which you outlined previously) was also preferred by one other group: sadistic navigation instructors! Seriously, by the second half of the 19th century, very few navigators ever used lunars at sea, but many studied them in schools and instructors used the arcane aspects of lunars as a means of "spreading the curve" (as we would say today in education) or more informally "separating the men from the boys." So an instructor might teach this very involved method of getting local time and then put that on an exam, just to see who was paying attention. Meanwhile, everyone knew that local time was determined, almost without exception, by observations of the Sun. Yet another reason some instructors and theoreticians emphasized the possibility of getting local time from the stars is because it seemed "pure" in a mathematical sense. If you take a lunar distance observation with a star or planet, then surely you should get local time from that same body. That sounds good "in theory." In fact, lunars with stars and planets were nowhere near as common as Sun-Moon lunars. From my experience with logbooks, I roughly estimate that around 80% of lunars were Sun-Moon observations. So once again, the common method of finding local time was to measure the Sun's altitude. In effect, it turns the sextant into a sundial. It's a simple sight and a simple calculation. These sights were almost as common as Noon Sun for latitude throughout the period from the late 18th century and into the middle of the 20th century. And you wrote: "Also keeping the local time with a watch was used but after what I have gathered this was not a common method likely because watches were not good enough. But this method, when working, was certainly much easier for the navigator than the first method outlined by me." Watches were not the issue. Plenty of good watches were widely available in the period. The real problem with carrying local time on a watch is that you have to correct it for the change in longitude since the last time sight. In modern terms, it's a running fix problem. It's hard to guess how much time since the last sight was considered acceptable historically, but a few hours was certainly no problem and even twelve hours does not appear to have caused much concern. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---