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: Polaris SHA
    From: Bill B
    Date: 2008 Sep 19, 03:28 -0400

    I asked:
    > "Why does the SHA of Polaris shift so dramatically through the month/year
    > (as opposed to other stars?)"
    Frank responded
    > Fundamentally, this is a "coordinate singularity" effect. Polaris isn't
    > moving faster. It just happens to be moving fast very close to the North
    > Celestial Pole. It's a lot like someone walking past one of the geographic
    > poles on Earth at a distance of fifty feet. You can walk through a dozen
    > time zones in a few minutes.
    > The largest change in the coordinates of the stars (and therefore the
    > coordinates of Polaris) is due to precession....
    A lot of your answer I had absorbed (in my own vague manner) from your past
    posts.  What you say makes a lot of sense, I just had trouble picturing it.
    I had briefly seen the magnificent star ball Alex owns (Alex too long absent
    from the list).  How it accommodates some of the changes in star
    relationships you mention is beyond me.
    Given the coordinates of the celestial sphere etc. are semi-arbitrary
    man-made constructs IMHO, sometimes I have trouble syncing up with them
    lacking the benefit of a formal education in those areas.
    Regarded precession, I had expected the declination of Polaris to get
    smaller month-by-month and year-by-year (in my lifetime) with a *extremely*
    small wobble factor. I would expect the pole star, from an earthbound
    perspective, to move in an ellipse through the stars on an approx.
    26,000-year cycle.
    Doing a little reading, apparently nutation affects precession so the change
    is not as constant as I had naively expected.  More like a very dull
    handsaw's teeth applied to a smooth representation of an ellipse.
    As a side bar, what is the current projection of when Polaris will be
    closest to the axis of Earth's rotation?  Susan P. Howell stated 2101 if I
    I did not expect SHA to vary so much and move up and down from year
    beginning to year end.  I regarded SHA as an angular distance from the first
    point of Aires.  While the reference point changes with precession, the
    relationship to the reference point remains relatively constant noting the
    change of relationships due to proper motion and shifts (relativity) from
    the Earth moving at nominally 70,000 mph around the Sun and their
    relationships to the celestial sphere you have mentioned in the past.
    If I read you correctly, any small changes in the relationships between the
    stars (for many reasons) that might seem minor on a star ball or a
    conceptual celestial sphere are magnified by the coordinate
    Bill B.
    Navigation List archive: www.fer3.com/arc
    To post, email NavList@fer3.com
    To unsubscribe, email NavList-unsubscribe@fer3.com

    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