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: DIY long-term almanac for calculators
    From: Antoine Couëtte
    Date: 2018 Jan 24, 03:37 -0800

    Dear Paul,

    You wrote, "Curiously, his Sun GHA is as good as any method despite the greater error in GHAA."

    I had long observed this.

    Although I am far from any document to elaborate in further details, and to the best of my memories, the Sun Longitude is initially computed in Ecliptic 2000.0 . To get it into Ecliptic of date, you simply add "+ Delta Psi".

    Then you transform the Ecliptic Longitude into Right Ascension in the Equator of date.

    The precession effect in Equatorial coordinates has then become equal to "+ Delta Psi Cos (Obliquity)".

    The Calculation of GHA Aries also involves the very same term, i.e. "+ Delta Psi Cos (Obliquity)".

    When you compute GHA Body = GHA ARIES - Body RA, any error on the precession term then cancels out.

    Hope it may help,

    Antoine Couëtte

    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