Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Sun Moon Lunars to 155 degrees
    From: Peter Fogg
    Date: 2010 Apr 7, 15:55 +1000
    George opines:
    "I often find that it's instructive to push
    situations to the physical limit, to see what happens"
     
    I disagree, George.  You're just up to your old trick of finding some absurd and impractical extreme at which some general procedure may be shown - or not - (this varies, doesn't it?) to have shortcomings.  What you refuse to acknowledge is that this somewhat pointless nitpicking in no way invalidates the use of the particular procedure within its practical parameters.

    On Wed, Apr 7, 2010 at 9:21 AM, George Huxtable <george{at}hux.me.uk> wrote:
    I had written-

    "Here is a simple case, with lunar distance at 90 degrees. The Sun is at
    the zenith, the Moon is on the horizon. How sensitive is the cleared
    distance to the altitude of the Moon? And that of the Sun?"

    and Frank replied-

    George, first of all you've picked a case that would never come up in the
    real world of lunars. Today no one would shoot lunars with altitudes below
    three to five degrees, and historically anything below ten degrees was
    considered off-limits. Second, as I've said many times, the formulas for
    altitude accuracy which I've given apply when the refraction is
    small --above ten degrees in the altitudes. The behavior when either body
    is below ten degrees is qualitatively similar but there are moderate
    quantitative changes. When both bodies are above the thick air near the
    horizon, it is the parallax of the Moon that drives the whole
    problem --EVEN with respect to the required accuracy in the other body's
    altitude.

    ==========

    As I understand it, the only reason for not taking lunars at such low
    altitudes is because the refractions are unpredictable, on account of
    fluctuations in the atmosphere. There's nothing fundamental to disallow
    such lunars.

    So let's imagine that we happen to know that the atmosphere is in such a
    state that atmospheric refraction follows the tables given in the almanac,
    precisely. Under those imaginary conditions, am I then allowed to repeat my
    question? What goes wrong? I often find that it's instructive to push
    situations to the physical limit, to see what happens.

    George.

    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.





       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    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:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    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