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: Sun Slow Day
    From: Patrick Goold
    Date: 2011 Feb 10, 12:53 -0500
    Wiki has a good article on the equation of time. 

    The equation of time results mainly from two different superposed astronomical causes (explained below), each causing a different non-uniformity in the apparent daily motion of the Sun relative to the stars, and contributing a part of the effect:

    • the obliquity of the ecliptic (the plane of the Earth's annual orbital motion around the Sun), which is inclined by about 23.44 degrees relative to the plane of the Earth's equator; and
    As for Ptolemy:

    The irregular daily movement of the Sun was known by the Babylonians, and Ptolemy has a whole chapter in the Almagest devoted to its calculation (Book III, chapter 9). However he did not consider the effect relevant for most calculations as the correction was negligible for the slow-moving luminaries. His calculation did not correctly account for both of the two major annual variations as they are now known, and he only applied it for the fastest-moving luminary, the moon.

    Thus Wikipedia.

    My question is why, if the maximum variation is 14 min 13 seconds, has the astro website I have been using to tell me the time of meridian passage been telling me LAN is 12:19 here in Portsmouth?    Are they just wrong or am I misinterpreting what they are telling me?

    Best regards,

    On Thu, Feb 10, 2011 at 12:22 PM, Apache Runner <apacherunner@gmail.com> wrote:

      The asymmetry is I think the result of an elliptical orbit around the Sun (Kepler discovery ?).

     I believe Ptolemy caught this one first.   He had circular orbits with the center of the orbit displaced in order to get this in a geocentric model.   

    Dr. Patrick Goold
    Department of Philosophy
    Virginia Wesleyan College
    Norfolk, VA 23502
    757 455 3357

    Charles Olson: "Love the World -- and stay inside it."

    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