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: What is a "Class A" sextant?
    From: Brad Morris
    Date: 2014 Jan 20, 17:49 -0500

    Thanks for that Frank.  It slipped by me in the Star-Star discussion, as it was just on passing, and not the focus of the conversation.

    Today, concentricity (eccentricity or centering error) is denoted in linear units, not angular.  I'm interested how that translates.

    I've approached Richard Dunn, who has promised to try and dig out the information.  We may hear from him in a few weeks on this topic.

    One reference I found seemed to indicate that 'Class A' sextants needed to resolve to at least 10 seconds.  That may be true as lesser resolutions were produced. 

    That said, I think that there is more to the Class A rating than resolution and centering error.  What about accuracy of scale, etc?  You've got perfect centering and 10 second resolution, but the arc could be divided by a drunken monkey. 

    This story is not complete.


    On Jan 20, 2014 12:12 AM, "Frank Reed" <FrankReed@historicalatlas.com> wrote:

    Hi Brad,

    My understanding of the Kew "Class A" rating was that it was an overall rating. It was the certification required for sextants given to Royal Navy cadets. It combined several factors, and the instrument had to meet various standards on several tests.

    You may remember a NavList discussion a few years back about tables of "star distances" published in about 1905 for use with Lord Ellenborough's method of testing sextant arc error at sea (*). In the introduction, the authors say that a "Class A" certification implies among "other things" that the centering error (or "arc error" as we would call it today) amounted to less than 1' of arc maximum. Classes B and C would presumably permit progressively greater arc error, and this same source says that the sextant would be "rejected" (in other words, worse than class C) if the arc error was greater than 3'.

    *that discussion was in March 2010, and here's my first message on thee subject, specifically addressed to you personally, in fact. :)


    NavList message boards and member settings: www.fer3.com/NavList
    Members may optionally receive posts by email.
    To cancel email delivery, send a message to NoMail[at]fer3.com

    Attached File:

    (class-A-sextant.jpg: Open and save)

    View and reply to this message: http://fer3.com/arc/m2.aspx?i=126560

    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