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: Azimith forumula for Great Circle sailings- having problems
    From: Gary LaPook
    Date: 2014 Dec 24, 15:56 -0800
    By definition, LHA is measured only towards the west, from 0 to 360 degrees, so for any LHA between 0.0001 degrees and 179.99999 degrees the destination must have a longitude further to the west than the starting point. So your example has the destination about 10 minutes of longitude west of the starting position (about 7.5 NM at a 41 degree latitude) so the course must be to the west of straight north. In fact it is about 4 degrees to the west of north. The standard formula provides a -4 degrees that you add to 360 to find the initial course of about 356 degrees true.

    In the olden days and using old formulas and tables, what we now call LHA today was known as "angle t" or "hour angle" and was measured both ways, both to the west and to the east, from 0 to only 180 degrees so perhaps you are having a problem with definitions, perhaps you are looking at one of those old usages. 

    To clear this up, give us the actual longitudes of the starting and finishing positions.


    From: Samuel L <NoReply_SamuelL@fer3.com>
    To: garylapook---.net
    Sent: Wednesday, December 24, 2014 12:11 PM
    Subject: [NavList] Re: Azimith forumula for Great Circle sailings- having problems

    Unless I'm really, really, really wrong the Azimith is 3.917400689 and not -3.917400689. Forget about the places to the right of the decimal for the moment.
    The coordinates of both positions were plotted on a UPS and gave the same result of +3.917d
    Several online great circle sailings calculators gave the same result.
    Peter Hakel's Sailings.xls spreadsheet gave the same result of +3.917d
    I think the error occurs when the LHA (Difference in Longitude) isn't examined for a greater or less than 180d value. If a negative sign is applied to the LHA when using the formula then the answer comes out correctly.
    I've read your lengthy comments but learn only by trail and error. Thus, if the GPS says such and such a figure and Gary says something contrary....for the moment I'm going to work with the GPS figures until study and practice shows otherwise. It's like the "rounding up or down" situation I had a few months ago.
    Would you please show me how in the world you arrived at a negative Azimith?
    Sam L.


    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