Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Real life accuracy...and thoughts on learning
    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. :)

    Regards,

    Sean C.

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site