NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Using star-star distances
From: Frank Reed
Date: 2008 Sep 23, 00:15 -0400
From: Frank Reed
Date: 2008 Sep 23, 00:15 -0400
Bill, you wrote: "He was on, I kept coming up 1' off, despite other pairings being on. We swapped sextants, I hit the mark with his and he was 1' off with mine. That pretty much tells the tale. Not an IC problem, not personal error, so definitely a scale error. " Just to emphasize, this is not bad news. It's good news. Arc error is a 100% correctable error, every bit as much as index error, if it has been measured in some way. Because arc error has been a bit of a mystery and difficult to assess, many navigators get the feeling that a sextant with arc error is "bad". But this is not necessarily the case. Any error which is repeatable in multiple trials is no error at all. It's just like a clock that gains exactly one second every day. That's a good clock. Predictable, repeatable errors are not a problem. Arc error is only a true source of error when the calibration table is wrong or when the arc error varies randomly between sights. In some of my recent sextant calibration tests, I've had the pleasure of watching, in real time, the calibration of a plastic sextant glide up two minutes of arc postive for a couple of minutes (of time) then drift in the opposite direction until it was zero and then negative by a couple of minutes of arc. Something like that simply cannot be corrected in live navigation. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---