Welcome to the NavList Message Boards.


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

Compose Your Message

Add Images & Files
    Re: I.C. and sunspot
    From: Frank Reed
    Date: 2019 May 12, 12:24 -0700

    Yes. I saw the same spot yesterday while shooting some lunars! We are currently near sunspot minimum so spots are small and few in number. I experimented with the process you're suggesting some years ago when there were larger sunspots. There's no real benefit. Yes, you can do it, but I find that it's just as easy to do this using limb overlap, and the limb is much easier to see and define clearly. Further, if you're in circumstances where you can use this method, then you might as well use the alternating solar limb method: you place the reflected sun with its lower limb just touching the upper limb of the direct sun, read the angle, then reverse and read again, and half of the difference between the two readings gives your I.C. That's generally more accurate than any method involving a side-by-side comparison. 

    General advice: don't measure IC. Instead zero out the index error. With plastic sextants, get in the habit of doing this before each and every sight. Don't test and record index correction --just zero out the index error. With metal sextants, you can check it once a season and typically find no changes for months (barring sharp temperature changes and shocks to the instrument). And since you're only doing this once a season with a metal sextant, set the sextant on a table with a view of some distant line (horizon, radio tower, power line, etc.) remove the standard scope and place a 20-40x power spotting scope inline with the normal optical path (or a camera with a big lens). With that magnification, you can just about guarantee that your index error has been reduced to less than a tenth of a minute of arc. 

    See the images below for a novel index error test that you can use inland when you don't have a sea horizon. I suspect the images are self-explanatory here among NavList readers. A key feature here: contrails are typically six to seven miles away.

    Frank Reed



    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    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 Settings

    Posting Code:

    Custom Index

    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