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: Early sextants & Lunars...
    From: Brad Morris
    Date: 2014 Mar 26, 15:40 -0400

    My apologies Bill for misquoting you.  I meant no harm.  That's my memory modifying your statement.

    As to reading the vernier and deciding which is the appropriate reading, I like to do the following.
    1) There is a modest parallax between the vernier lines and the arc lines.  It is therefore important to view the final alignment when those lines are centered in the field of view of the microscope.  That microscope is on an additional axis of rotation on the index arm, permitting centered view.  I take advantage of that.  If you don't, its easy to be off by 10 or (in terrible cases as you point out) 20".
    2) Once I decide which pair is best aligned, I center the FOV over the pairs on either side of the central pair.  The vernier lines for these pairs should appear slightly offset towards the center pair (on my Heath).  This is an important criteria for making the final call.
    3) Since the vernier reading is an offset to the nonius reading, I like to record each individually and do the quick sum on paper.  Its easy to make an arithmetic error when doing this in your head.  {Note: the same applies when doing the autocollimator reading.  The AC has independent minutes and seconds, with independent signs.  +1' -30" is the same as 0' +30" although the presentation is different at the graticule}

    That said, reading a vernier takes practice and care.  The resultant minor loss in accuracy due to lost motion and imperfect worm gears (by your measurements) is outweighed for many by the ease of reading the micrometer.  Its a tradeoff that history has decided for us.

    There is one artifact that has troubled me for some time.  Nearly all of the non-electronic means of sight reduction for LOP based navigation is based upon tenths of a minute (6").  Yet the vernier sextant yields 10".  This mismatch, when doing reductions, forces a round off to the detriment of the vernier reading.

    10" ~ 0'.2
    20" ~ 0'.3
    30" = 0'.5
    40" ~ 0'.7
    50" ~ 0'.8
    60" = 1'.0

    What happened to .1, .4, .6 and .9?  They don't line up to the modern tables. 

    I believe these vernier instruments more appropriately align to the Latitude at Noon // Time Sight in the AM PM method, and the micrometer sextants align more appropriately to the LOP era.  I wish to treat this as a supposition and not a statement of fact to be trolled.


    On Mar 26, 2014 3:07 PM, "Bill Morris" <engineer{at}clear.net.nz> wrote:

    <<Bill Morris has called the Heath Hezzanith as *the finest sextant ever* made.>>

    Not quite, Brad. I described "Heath and Company's best vernier
    sextant" here: http://sextantbook.com/2011/12/ ,but didn't go as far as to call it the finest ever made. That honour probably has to go to the Soviet era SNO-T. There is probably nothing to choose between the turn-of-the-century vernier sextants made by Heath and Co, Hughes and Son and C Plath.

    Most sextants of the time are divided to read to 10 seconds, but it is very difficult to decide which pair or even 3 or 4 pairs of lines coincide when reading the verniers. I look either side to see which two pairs just do _not_ coincide and choose the middle value between them.

    Bill Morris
    New Zealand
    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

    View and reply to this message: http://fer3.com/arc/m2.aspx?i=127383

    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