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: Suitable Sextants - Mirrors
    From: Herbert Prinz
    Date: 2005 Oct 14, 15:01 -0400

    Yourname Here wrote:
    
    > here is a link to photographs of page 108, Figure 6-4 of Bauer's " The
    > Sextant Handbook". [...] For those interested, Bauer goes into great
    > detail in the methodology of swining an arc on pages 105-108 which
    > are the same as my own.
    >
    > http://www.villagephotos.com/pubbrowse.asp?folder_id=1485991
    >
    
    Joel,
    
    I hate to burst your bubble, but Bruce Bauer has it all wrong, and so do
    you. His figure 6-4 does not show clearly how he is swinging. But he
    tells us on p.106, which you chose not to reproduce in your post:
    
    "Swinging the arc is also called rocking the sextant and simply means
    rotating the instrument from side to side around the line of sight to
    the horizon."
    
    I think that there has long been a consensus on this list (at least
    since Frank Reed brought the attention to the subject a while ago) that
    this is the wrong way of doing it. Actually, it's an impossible way of
    doing it (see below).
    
    Instead of trusting a particular book, let's use common sense. The goal
    of swinging the arc is to find the exact point where the vertical line
    through the star intersects the horizon. If you pick a point on the
    horizon arbitrarily and then start swinging the sextant around the line
    of sight to this point, how do you ever find the correct point??
    
    That this wrong way does not work is further evidenced by Bauer's own
    claim that he cannot make it work for any other than medium altitudes.
    See p. 108, "One more wrinkle on swinging arcs [...]", full quote on the
    page you posted. Bauer says  "At high altitudes the arc becomes hard to
    manage because of its sharp curve." Not at all. The very problem with a
    star at high altitude is that it is difficult to guess the azimuth
    because the curve is so _flat_! Of course, if you decide on the wrong
    azimuth prematurely,  then face that direction and start swinging the
    arc on a horizontal axis, the star shoots out quickly on eiter side of
    the mirror. Neither will its arc ever touch the horizon - it may
    intersect. Even a Venetian wall mirror on your sextant will not help you
    solve this problem.
    
    Why Bauer says that swinging does not work for low altitudes, because of
    the arc flattening out, is beyond me. It makes me wonder whether he ever
    swung a sextant. For sufficiently low altitudes one can actually swing
    the object full circle (!) without loosing its view.
    
    Herbert Prinz
    
    
    

       
    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