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: Time sights
    From: Paul Hirose
    Date: 2005 Jun 1, 20:22 -0700

    Fred Hebard wrote:
    > There are ways of transmitting signals both ways
    > to account for the various delays.  Paul Hirose was kind enough to tell
    > us how this was done a few years ago; unfortunately, I didn't
    > understand the mechanism well enough to reproduce it here.
    
    I probably explained it badly. To illustrate the principle, suppose
    the clocks at either end of the line are in perfect synchronism. If I
    tap my telegraph key precisely at the top of a minute (according to my
    clock), your telegraph sounder clicks, say, 0.1 s later. From your
    viewpoint, your clock is 0.1 s *ahead* of mine.
    
    But if we exchange roles, I hear your ticks coming late by
    0.1 s. From my viewpoint, your clock is 0.1 s *behind* mine. By taking
    the mean of these results, the delay is cancelled and
    the true time offset determined. As a byproduct, the discrepancy between
    the two measurements, divided by two, equals the propagation delay.
    
    In practice, break-circuit clocks automatically generated the seconds
    pulses, which were recorded as helical ink traces on paper by rotating-drum
    chronographs. The equipment and techniques were developed with amazing
    rapidity. By the time of this report in 1858, telegraphic longitudes
    were routine:
    
    http://www.history.noaa.gov/stories_tales/cs1858_6.html
    
    The great length of the trans-Atantic cable did create technical
    challenges, but they were were quickly overcome:
    
    http://www.lib.noaa.gov/edocs/BACHE2.htm#CARDINAL
    ("A Cardinal Point for Longitude")
    
    According to footnote 26 at the bottom of that page, "The seconds in
    time of longitude of the Cambridge Observatory [i.e., the seconds
    portion of the Harvard College observatory time offset with respect to
    Greenwich] as determined with the 1927 North American Datum was 30.928
    and as determined in the 1983 North American Datum was 30.802. The
    original telegraphic determination of longitude fell approximately
    half-way between these values demonstrating beyond a doubt the accuracy
    of the 'American Method' of longitude determination."
    
    I recommend science writer Trudy Bell's article on telegraphic
    longitude, "The Victorian Global Positioning System" (about 840 k PDF):
    http://www.tbp.org/pages/publications/BENTFeatures/BellSp02.pdf
    
    
    

       
    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