A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Sean C
Date: 2015 Aug 27, 13:03 -0700
Mark Coady's recent remarks about side error reminded me of something I meant to post a while back, but forgot. In my first post after receiving my Astra, I commented that I had used a couple of technical pen caps placed on the arc to check the perpendicularity error of the index mirror. I had previously seen a thread that mentioned the technique and decided to try this method as opposed to directly sighting the arc as I did with my Davis. Unfortunately, I think this was a mistake. I found it more difficult. Perhaps because of my choice of cylinder (the caps), or because of their orientation. I don't know. But as Frank pointed out several times after I posted some of my results, I wasn't getting as consistently good measurements as I should have with a metal sextant.
I had noticed that when I was taking sights, it seemed that I had to hold the sextant slightly tilted from what I felt was true vertical for the images of the bodies to align vertically as I brought them together. Something which struck me as odd, but I chalked it up to me not knowing what vertical "felt like". I should have listened to my gut. After more research, I discovered that the cylinder method was really only recommended for mirrors that had been replaced with the front surface variety, consequently moving the reflecting surface away from the pivot point of the index arm. So I re-checked the error using the method I was used to and...voila! Much more consistent results and the sextant "felt" more vertical when aligning the images. I even shot my first "perfect" sun sight and lunar! (Zero error from the calculated number.) I don't know if this is because of the error in the index mirror, or just that I feel more comfortable with the sextant now...or a little of both. But I guess it doesn't really matter in the end.
As another self taught enthusiast, I identified a lot with what Mark wrote. Particularly the bits about authors leaving steps out and the need to be patient with one's self. I, too, would get a little frustrated if I didn't understand what was presented as a simple concept. I still don't fully understand the trig, though. I only made it to geometry in school, and math was never my best subject. But I'm getting there. So, if you're reading this, Mark: thanks for sharing your thoughts. :)