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
    Noon sight for longitude
    From: Gary LaPook
    Date: 2008 Jun 04, 23:05 -0400

    I just recieved this article from Ocean Navigator.



    May 2008
    Celestial Navigation
    Plotting the noon curve
    by David Berson
        This is the third article in a series concerning the use of the noon sight as a means of finding longitude. Although most often associated with the determination of latitude, the meridian passage of the sun — if it can be timed precisely — can be utilized for ascertaining longitude. The timing, however, is the tough part. As any navigator who has ever done a noon sight knows, at the moment of its maximum altitude, the sun appears to hang in the sky. This makes it very difficult to mark the exact time that meridian passage occurs. If, on the other hand, we could know the exact time of meridian passage then we can enter that time — in GMT — into the Nautical Almanac and find the GHA of the sun. Then we can find our longitude since the Local Hour Angle of the sun at Meridian Passage is 0°. In this case the GHA is actually the longitude of the observer. Of course, there are qualifications for this assertion, but for all intents and purposes, an accurate longitude can be found using a noon sight. The proviso being that we need to know the exact time of the meridian passage and can shoot it at that moment.

        This brings us to an idea known as the “noon curve.” What we do in this case is graphically plot the rise and fall of the sun beginning at about 20 minutes before calculated noon, and continuing for another 20 minutes after the meridian passage. The graph is constructed with the sextant altitude on the left side of the page and the time at the bottom. Perhaps 10 shots of the sun are taken as it rises and another 10 as it descends. At each shot time, a pencil mark is made on the page that corresponds to the altitude at that moment. When completed, and the points joined and faired, they should look like a sine curve. The top of the curve is the maximum altitude of the sun and should be marked with an exact time. Actually the top of the curve will be flattened a bit, this is where the sun hangs before losing altitude.

        The exact time and altitude of the sun can thus be found for meridian passage. It is true that this method requires a great deal of time, and a calm sea helps as well. In one sense it is almost a contradiction of the raison d’etre of the noon sight which was developed as a method that was quick and simple and required very little time or math. Still it is important to know how to wring every bit of information from the sight. There might come a time when you may need it.
    More Ocean Navigator Articles

    About the Writer
    Contributing Editor David Berson writes the Nav Problem page in every issue of Ocean Navigator. He is also the owner and operator of Glory, an electrically powered excursion boat, in Greenport, N.Y.
    Question for David? editors@oceannavigator.com
    About Ocean Navigator 
    Ocean Navigator provides stories and articles that will make you a complete voyager. From ocean voyaging experiences to in-depth articles on marine electronics, weather, electrical systems, communications and more, you'll find informative, useful editorial that will improve your skills and your voyages.
    Customer Service 
    Phone: (207) 772-2466     





    This message was sent from Ocean Navigator to glapook@pacbell.net. It was sent from: Navigator Publishing, P.O. Box 569 , Portland, ME 04110. You can modify/update your subscription via the link below. Email Marketing Software

    To be removed click here  
    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