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: Lunar Distance in Wikipedia
    From: Renee Mattie
    Date: 2007 Jul 25, 17:45 -0700

    I would not object in the least if someone completely replaced the
    I have been tinkering with.  Or if someone tinkered with it further.
    That's exactly the way Wikipedia works.  It wouldn't be fair for "SV
    to get all the credit!
    Another convenient place for thrashing out a text is in the discussion
    page for the article --
    anyone at all can contribute.  You don't even need to log in.  As long
    as everyone behaves
    themselves, no-one will object.
    On Jul 25, 7:51 pm, "George Huxtable" 
    > Renee Mattie is applying a bit of discipline to the Wikipedia entry on Lunar
    > Distance (Navigation), and a good thing too.
    > But it's still not right, as I see it, and a bit more tinkering may be
    > called for.
    > It now reads-
    > "Theory
    > The local time at Greenwich is always one hour later than the local time 15
    > degrees West of Greenwich. Suppose one person observes the moon at Greenwich
    > at local apparent noon (when the sun crosses the observer's meridian). Then
    > suppose a second person observes the moon 15 degrees West of Greenwich, also
    > at local apparent noon. The two observations take place one hour apart. The
    > second observer will see the moon in a different position than the first
    > observer did because, during that hour, the moon would have moved
    > approximately its own diameter across the sky."
    > But in that hour, the Moon, just like any other object near the ecliptic,
    > moves through something like 15 degrees across the sky; far more than its
    > diameter.
    > What the observers have to measure is the position of the Moon with respect
    > to the star background, or the Sun, which is where the lunar distance comes
    > in. It's that angle that changes by a Moon diameter in an hour, roughly
    > speaking. And of course, there's no call for that mythical observer at
    > Greenwich anyway; what he would measure has been predicted in the almanac.
    > What I'm suggesting is that the whole paragraph could do with a wholesale
    > revision, rather than tinkering with one clause at a time. What's the best
    > way for that to be done, collectively, by this list? We're as clueful a body
    > as any to apply our wisdom to the job. It seems counterproductive for Renee
    > to make a change, then another of us to follow it up by another piecemeal
    > edit process, and so on. Wouldn't it be better for us, on this list, to
    > thrash out a text we all can agree on, if that's possible, and then Renee,
    > who has shown herself to be so adept, can act as our ambassador and make the
    > change on the list's behalf. Does that make sense? Would anyone like to set
    > the ball rolling?
    > And it's not just that paragraph. The introductory paragraph to the whole
    > article reads-
    > "In celestial navigation, lunar distance is the angle of the Moon's centre
    > from the Sun or a bright star as measured using a sextant. Given a lunar
    > distance and a nautical almanac, it is possible to calculate the difference
    > between local current time - obtained by observing the height of the moon
    > and the second celestial object - and current time at the meridian of the
    > nautical almanac - usually Greenwich - which gives the difference in
    > longitude between the two places."
    > Indeed, taking a lunar, it is necessary to observe "the height of the
    > Moon and the second celestial object", but that's mainly in order to get
    > information for "clearing" the lunar distance: correcting it for the effects
    > of parallax and refraction. It's quite true that under certain
    > circumstances, either the Moon altitude or the star altitude (no need for
    > both), might be used
    > to determine local timebut usually local time will be
    > found separately, from a different observation. The text gives quite a
    > misleading impression of why those altitudes are needed.
    > ===================
    > If we took a rigorous look at the whole piece, it's likely we would uncover
    > more deficiencies. All I have done, so far, is to quickly scan it over.
    > George.
    > contact George Huxtable at geo...---.u-net.com
    > or at +44 1865 820222 (from UK, 01865 820222)
    > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to 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