NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Index correction sun sights
From: Greg R_
Date: 2012 Feb 20, 12:47 -0800
From: Greg R_
Date: 2012 Feb 20, 12:47 -0800
> Convincing myself that the reflected Sun disk is indeed sitting on the > horizon took a few seconds, which is why UT is rounded to the nearest 5 or > 10 seconds One thing that I started doing (which also improved my shooting accuracy) was to position the sun so that it's just slightly above the horizon, and then wait for the Earth's rotation to bring it down to exactly on the horizon. Once it "kisses" the horizon (i.e. the gap between it and the horizon disappears) I mentally call "Mark", move the sextant away from my eye, and read the time on my Casio "atomic" watch (maybe subtracting a second or two for the time it takes to do that - though if I remember right you only need to be within 4 seconds of real time to get be accurate). That's a lot easier (at least for me) than trying to bring the sun down to exactly match a moving target (not to mention rocking the sextant back and forth on the way down to make sure it's perpendicular) - with the Earth doing the work for you there's never any guess as to when it's exactly on the horizon, and it's easy enough to rock the sextant while waiting for that to happen). -- GregR ----- Original Message ----- From: P H To: NavList@fer3.com Sent: Monday, February 20, 2012 11:31 AM Subject: [NavList] Re: Index correction sun sights Same here with my Mark 15. Last July I wrote: "The main difficulty I had was establishing the index error of the instrument. The index mirror held perpendicular to the frame very well but I spent a lot of time tinkering with the horizon mirror. I repeatedly measured and adjusted the index error by looking at the horizon, overlapping the two sun disks, and also using the "Solar IC procedure" from David Burch's book on plastic sextants. The IE tended to gravitate toward 8' to 9' off the arc but I would have to attach an "error bar" of a few arcminutes to that value from what I saw. Using this Solar IC method to recover the Sun's semidiameter also showed differences of the order of 1'. Between sights I turned my back to the Sun and shielded the sextant from direct sunlight." The full posting is here: http://fer3.com/arc/m2.aspx/Santa-Barbara-sights-PeterHakel-jul-2011-g16765