NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunar Occulation in Practice at Sea
From: Frank Reed
Date: 2009 Feb 4, 21:40 -0800
From: Frank Reed
Date: 2009 Feb 4, 21:40 -0800
Brad M., you wrote: "Here is a clear case of occultations in use at sea. While his use of the small telescope may bring the claim that the occulatation was not at sea, the short answer is that the Endurance was trapped in the ice during this time. The position within the Weddell Sea, albeit it frozen in the pack ice, still qualifies as "at sea"!" Ha! Yeah, that's funny. :-) I don't think too many mariners stuck in place in pack ice would call themselves 'at sea' but technically they are. Some of those 19th century whalers might have found themselves in the same circumstances, but I haven't seen any cases of occultations observed on ordinary commercial vessels. Explorers and scientific expeditions, incidentally, are exactly the sort of people who would have used occultations since they are likely to carry good telescopes, likely to include people with astronomical observation and calculation skills on their teams, and likely to be able to set up good observing sites on solid ground (or solid ice!). If you're interested in this particular case, you could probably find out the reduction method that was used either by digging around in the original documents or you might be able to make a very good educated guess by researching the background of Shackleton's astronomer. Occultations could also be used by a modern "navigator" (I use the term in quotes simply because most people would be doing this for the fun of it or perhaps to win points in a sailing race rather than for navigation in the literal sense). You could use a few occultations to check a chronometer, as they did historically. Or... you could also use a couple of occultations at known GMT to get a real position fix. This is fundamentally the same problem as the "position fix by lunar distances" that Dave and Andres and I (and a few others --sorry if I've left anyone out who is enthusiastic about this method) have been talking about for a couple of years. In fact, for a land-based observer, it could yield a very accurate fix, comparable to a standard celestial fix. Since decent models of the lunar limb are available, and better ones should become available very soon, the reduction can be much more accurate than it was historically. How good would your position fix be if you could time and reduce occultations with an error on the order of half a second or better? A big advantage of this method is that you don't need a visible horizon or a sextant. An ordinary spotting scope would probably suffice. On the other hand, unless you're prepared for some really laborious paper work, it also calls for a computing device of some sort. This method would be largely useless "at sea" unless you have some sort of stabilization system or perhaps a digital camera recording continuously. And needless to say, it would only work now and then, when the Moon is well-placed for observing and when there happen to be stars conveniently placed for occultation observations. -FER www.HistoricalAtlas.com/lunars --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---