NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Photo sextant sights
From: Frank Reed
Date: 2008 Aug 02, 23:43 -0400
From: Frank Reed
Date: 2008 Aug 02, 23:43 -0400
George H, you wrote: "This may perhaps suggest a possible enhancement to Frank Reed's lunar distance website. Instead of just multiplying the discrepancy in lunar distance by a constant factor of 30, to obtain longitude error, I wonder if it would be possible (and relatively easy) to actually compute that error, allowing for such geometric misalignments (and perhaps also, differences in the angular speed of the Moon). That would then at least draw the attention of a user to the fact that he was measuring a lunar distance in a silly direction; such a warning would have been useful here." I've thought about this issue a number of times over the past four years since the calculator first came online. It depends on what people are "really doing" when they're measuring lunar distances today. MOST people, that I have spoken with, are just trying it out to see what it's like and to experience a famous historical method of navigation. For them, the meaningful error in terms of longitude is the equivalent error in a typical lunar observation --and that's why I designed it that way. They're not necessarily worried about the exact longitude error in a specific case, though they do want the exact error in terms of the observation itself; they want to know, "how close was I? did I do well?". Then SOME people are using lunars to calibrate their sextants. For them, they want to know the exact error in the observation, and the 30x value is useful simply because it reveals an extra digit of precision in the observational error. And SOME people (like me) are using the calculator to analyze historical lunar observations, and in those cases, the longitude is adjusted via the DR longitude, to whatever accuracy is desired, and the "longitude error" in the analysis page is simply an indication of how close the DR is to the correct position given the observed lunar distance (under the assumption of no error in the observation). Finally, some people may, in the future, want to know the error in the corresponding "lunar LOP" and in that case a different measure of error is required. But I am still thinking about it, and perhaps the best option would be a click-through to a rather long explanation of the various possibilities INCLUDING the true error in longitude, calculated along the lines you've described, resulting from the error in observation. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---