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: Use of Sun Sights for Local time, and Lunars for Longitude
    From: Bruce Stark
    Date: 2002 Oct 12, 13:34 EDT

    Good posting from Arthur and George. Anyone interested in the navigation of
    that era should go over that posting several times to absorb not only what
    they've said, but the implications.
    
    The procedure Arthur suggested will work today for someone who's lost
    Greenwich time and is uncertain of the dead reckoning. I'll lay that out in
    more detail:
    
    Take and work a time sight in the morning. You won't get accurate time from
    it, not unless the sun was due east, or the DR latitude correct, but you'll
    get it close enough to know, within a few minutes, when to start monitoring
    the sun's altitude for the noon latitude. Anyone who's tried to get noon
    latitude with only vague idea of when the sun will "dip" will appreciate the
    value of this first, approximate, working of the time sight.
    
    Once you've got the correct latitude, work the time sight again to find
    exactly how fast or slow the watch is on local apparent time. Be sure to
    write that down because, until you get the next time sight, it will be the
    basis of your calculations.
    
    That's all the regular "sight reduction" you'll have to do, and the only
    thing you took from the Almanac was the sun's declination. At the very most,
    that never changes more than 1' per hour. Your estimated GMT would have to be
    a long way from the truth to get you in trouble.
    
    Now let's say you get a set of distances of the sun from the moon. If you are
    able to take the altitudes before and after the set of distances you can go
    ahead and clear it, find, from the Almanac, the GMT that fits, and apply the
    equation of time to convert GMT to GAT.
    
    To the average watch time of the lunar observation apply the correction you
    found with the time sight: so many hours minutes and seconds to be added or
    subtracted to convert watch time to local apparent time. The difference
    between the LAT and GAT of the lunar is the longitude of the place where you
    took the time sight.
    
    Keep that in mind. The time you're using is specific to the meridian where
    you took the time sight, so the longitude you find is specific to that
    meridian also.
    
    You've found latitude and longitude, and the lack of accurate GMT was no
    hindrance whatever in working the observations. Besides the noon latitude and
    lunar, which took no more work than if you'd had accurate GMT, you've worked
    a time sight twice, using different latitudes. That's exactly what was
    required in order to plot one Sumner line, using an accurate chronometer.
    
    Modern navigators find this hard to swallow. In the system they've been
    taught, everything is founded on, and must begin with, accurate GMT. They've
    come to accept, as a bedrock truth, that to work observations successfully
    you have to have accurate GMT. If you don't have it the only hope, in their
    view, is to flounder toward it by iteration.
    
    So much for now.
    
    In case a list member is wondering what to do when he can't measure altitudes
    for the lunar, I recently stumbled on a way of calculating them that fits
    present procedures, but is no more dependent on accurate GMT than the method
    posted under "It Works." Given time, I'll explain it soon.
    
    Bruce
    
    
    

       
    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