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
    Name or NavList Code:
    Email:
       
    Reply
    Re: Spur of the moment long by chron
    From: Robin Stuart
    Date: 2019 Jan 5, 11:35 -0800

    Frank,

    I’ve been pondering the implications of your comments earlier in this thread;

    “Navigators did not worry about taking sights on the prime vertical for local time except in relatively rare cases.”

    and

    “there is a standard practice of taking sights at some fixed time of day, typically 9am and/or 3pm”

    With regard to the first: I wonder to what extent the navigator got to choose the procedures that were followed. If I were a ship’s owner I think I’d probably want to some qualified party to take a look at the log and check that the navigator is following best practices to protect my valuable asset. If he’s not making his time sights at the prime vertical when the opportunity existed I might ask why. This would presumably only make sense in an age when the chronometer was reasonably reliable and the uncertainty in latitude became a significant factor. One might think that observations on the prime vertical would become more prevalent as chronometers improved but your statement that your survey spans the “19th century through the early 20th century” suggests not.

    With regard to your second: I always had in mind the image of the navigator stood to attention just after the crack of dawn ready to take time sights when the rest of the crew are still snoozing in their hammocks. Sights on the prime vertical cannot be made for more than half the year (when latitude and declination are contrary name) or if you are in the tropics at a latitude lower in the Sun’s declination. I’ve plotted (hopefully correctly) the local apparent time (noon at 12h) when the Sun is on the prime vertical and find that things are not as onerous as I was imagining. I’ve assumed an altitude cutoff of 10° why the  5° curve ends at around a latitude of 30°.

    So my question to you is how certain are you that at the “fixed time of day typically 9am and/or 3pm”, the Sun is not near the prime vertical. Is it something that was unerringly followed throughout the voyage through periods when prime vertical sights were and were not possible or did it drift? Unfortunately, unless there’s a shortcut I don’t know about, the azimuth can’t be immediately read off from the numbers in the time sight reduction but it requires a separate calculation to be made. It was therefore an eye opener when I carried this out on Worsley’s log.

    Regards,

    Robin

    File:


       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    NavList is a community devoted to the preservation and practice of celestial navigation and other methods of traditional position-finding. We're a group of navigators, navigation enthusiasts and hobbyists, mathematicians and physicists, and historians interested in all aspects of navigation but primarily those techniques which are non-electronic.

    To post a message, if you are already signed up as a NavList member, start a new discussion or reply to any posted message and use your posting code (this is a simple low-security password assigned when you join). You may also join by posting. Your first on-topic messsage automatically makes you a member, and a posting code will be assigned and emailed to you for future posts.

    Uniquely, the NavList message boards also permit full interaction entirely by email. You can optionally receive individual posts or daily digests by email, and any member can post messages by email (bypassing the web site) by sending to our posting address which is "NavList@NavList.net". This functionality is similar to a traditional Internet mailing list: post by email, read by email, reply by email. Most members will prefer the web interface here for posting and replying to messages.

    NavList is more than an online community... more about that another day.

    © Copyright notice: please note that the rights to all messages and posts in this discussion group are held by their respective authors. No messages or text or images extracted from messages may be reproduced without the explicit consent of the message author. Email me, Frank Reed, if you have any questions.

    Join / Get NavList ID Code

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    A NavList ID Code guarantees your identity in NavList posts and allows faster posting of messages.

    Retrieve a NavList ID Code

    Enter the email address associated with your NavList messages. Your NavList code will be emailed to you immediately.
    Email:

    Email Settings

    NavList ID 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