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: Do We Still Need to Use Sextants?
    From: Frank Reed
    Date: 2013 Apr 5, 20:15 -0700

    David, you asked:
    "Did I get you right, that in location with various structures that would
    allow the drawing of lines of perspective, as an artist would draw them,
    the "true horizon" for purposes of using a sextant can be derived that

    Well, yes. In principle. The accuracy is a matter for details. Isn't it always... :) But the KEY point is that this finds the "true" horizon. The converging lines of perspective from well-constructed buildings leads to that great circle that's exactly 90 degrees away from the zenith. There's no dip, of course, for the true horizon. In fact, if you can look at converging lines from buildings with a sea (or large lake) horizon in the distance, you can directly see the dip --the difference between the sea horizon and the true horizon.

    And you wondered:
    "Would the ghost of George Huxtable (God bless his soul) say
    anything contrary?"

    For that, I can give you a direct answer. The first time I brought up this idea of converging perspective lines was in a little essay called "Easy Lunars" which I posted just about nine years ago. It's hard to believe it's been that long. Here's a link that will take you to that message and follow-ups:

    George Huxtable replied on that specific point of 'converging lines' as follows:
    "I liked his clear explanations, and his simple approach to estimating a
    land-bound rough-altitude of the Sun. In a flat and gridded city such as
    Chicago, with many shoebox buildings, there will be no shortage of parallel
    horizontal lines to converge at a vanishing-point on the horizon. Perfectly
    good enough in accuracy for estimating Sun altitude, to allow for the small
    correction for Sun refraction."

    The 'good enough' in this case, for the altitudes in a lunar observation, would imply within +/-5 minutes of arc or so. Can one do better than that with converging lines? Yes, sure. But it would take some detailed work.


    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

    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