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
    Longitudes greater than 180
    From: Paul Hirose
    Date: 2020 Mar 3, 14:07 -0800

    Anyone writing software which takes geodetic longitude as an input
    should think carefully about rejecting values greater than 180°. In
    astronomy, geodetic longitude is sometimes measured from zero to 360.
    For instance, the JPL Horizons online calculator accepts -180 to +180,
    but its output normalizes negative (west) values to an equivalent east
    longitude greater than 180.
    In this matter I follow an old rule of thumb for user interfaces: "Be
    liberal on input, conservative on output." For longitude I accept any
    real number, but in the output it's normalized to a value between -180
    and +180.
    Astronomical computation maven Jean Meeus insists on measuring longitude
    positive westward:
    "This convention has been followed by most astronomers during more than
    one century... We cannot understand why the International Astronomical
    Union, having first decided to measure all planetographic longitudes in
    the direction opposite to that of rotation, then alters the system for
    the Earth (1982). We shall not follow this IAU resolution..."
    [Astronomical Algorithms, 1991]
    He also puts the azimuth zero point in the south, an old convention not
    in step with the modern practice.

    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