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: Death to the Intercept Method!
    From: Robert Pearson
    Date: 2016 Nov 5, 18:59 -0700

    Sumner's method for lop solved a "time sight" twice for longitude using 2 different latitudes that differed only by a small amount - Frank suggests 0.1 deg. Plotting the 2 positions and drawing a straight line between them gives us a chord of the circle of position (cop) corresponding to the measured altitude. Marc St Hilaire's intercept method for lop gives us a distance between 2 cops one thru the ap and one corresponding to the measured altitude. With St Hilaire's method we also calculate the azimuth from the ap to draw the tangent to the cop corresponding to the measured altitude at the point where it intersects the azimuth line. Both methods give a good approximation to the same cop.

    We also have modified Sumner methods that solve the time sight for longitude and also the azimuth from that position. The resulting lop plotted thru that tangent to the azimuth would also approximate the cop thru the one position found. In fact it would be the same lop found by the St Hilaire method if the ap was the position found by time sight. Lops found by these methods were all called Sumner lines.

    Sumner's method came 1st by many years (1837 vs 1875) and if memory serves me well both were used in the US Navy in preference to the time sight for longitude. In the US merchant marine officers preferred the time sight and noon sights to a much greater extent maybe up to WW I even.

    To recap: with Sumner we solve 2 spherical triangles for 2 position points but they use the same altitude and declination. With St Hiliare we solve 1 spherical triangle for altitude and azimuth. Both give accurate results for ordinary navigation. Manual calcs using tables/logs vs calculator methods might favor one method over the other or as in the past familiarity and personal preference might also play a role.

    I guess what goes around comes around - right Frank.

    Bob Pearson

    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