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: Lunar Distance in Wikipedia
    From: George Huxtable
    Date: 2007 Jul 31, 00:04 +0100

    Paul Hirose has suggested alterations to the Wikipedia entry proposals, all
    of which are in the right direction; and to Renee has provided words of
    encouragement.
    
    Getting the rest of the entry right still requires a bit more pondering, in
    my view. Trouble is, I'm knee-deep in a little project which has a serious
    time-limit. This is a contribution that I'm committed to write, about taking
    a lunar distance, for the RIN's "Navigation News". That's their more
    populist newsletter, much less serious than their authoritative "Journal of
    Navigation". It was researching for that article that led me into Wikipedia.
    
    As putting the Wikipedia thing right doesn't have any time constraints, it
    will be on my own back-burner for a while, but I hope it won't lose momentum
    for that reason. I would like to encourage others to have their say, and
    implement their own changes to Wikipedia as they see fit.
    
    ======================
    
    Paul asked whether planets were used for lunar distance, and the answer is-
    No, not initially, but planet lunar distance predictions were added to the
    almanac later, in the early 1800s. I'm not certain exactly when. The
    "navigational" planets have two advantages; they tend to be bright, and
    easily recognisable, and they are never far from the Moon's path, near to
    the ecliptic, around the sky.
    
    On the other hand, precise calculations planet distances, for the almanac
    were rather more complex and demanding than for the Sun and stars. Also, for
    Venus and Mars, there was an added complication that when they came nearest
    the Earth they were close enough to show a significant parallax, which
    needed correcting for. There were also phase effects on the effective centre
    of their light-crescent, when that was large.
    
    Paul ended- A "obituary" paragraph at the end would give a nice sense of
    completeness to the article. E.g., "Lunars went out of fashion when..."
    
    I concur, and perhaps this should be a part of a short reference to the
    historical context, particularly the contributions of LaCaille, Mayer, and
    Maskelyne.
    
    George.
    
    ====================
    This is what Paul wrote-
    
    | George Huxtable wrote:
    | > Trouble, is, the various sections on that page all interrelate, so it's
    hard
    | > to tinker with one without altering others. Nor do I think the way the
    topic
    | > is divided under different headings is entirely sensible.
    |
    | I think the article has too many headings. One for each paragraph is
    | excessive. The main part of the article (if you omit the Theory
    | paragraph) actually reads more smoothly if you pretend the headings
    | aren't there.
    |
    |
    | > I wonder if there are specific rules about the length of such
    | > contributions.
    |
    | There is a Wikipedia style guide, and even an article about article size:
    | http://en.wikipedia.org/wiki/Wikipedia:Manual_of_Style
    | http://en.wikipedia.org/wiki/Wikipedia:Article_size
    |
    | The amount of stylistic guidance is somewhat intimidating. It's a good
    | thing you can't get fired for breaking a rule.
    |
    |
    | > "In celestial navigation, lunar distance is the angle between the Moon's
    | > centre and the Sun or a bright star, slanting across the sky, as
    measured
    | > using a sextant. Such an observation, usually abbreviated to just "a
    lunar",
    | > can be made by a mariner, anywhere in the World, if the Moon is visible,
    | > together with the Sun or a special star. Without needing a chronometer,
    it
    | > allowed him to calculate what the time was at some reference longitude
    | > (usually Greenwich) at the moment of that observation, using data which
    used
    | > to be published in a nautical almanac. That was an important step in
    finding
    | > his own longitude, from Greenwich."
    |
    | All of that is correct as far as I can tell. However, I believe less
    | detail and precision are appropriate for a lead paragraph. E.g., "In
    | celestial navigation, lunar distance is the angle, observed with a
    | sextant, between the Moon and the Sun or a star." (Were planets used too?)
    |
    | It should be obvious that both bodies must be visible, so to continue,
    | I'd just say, "From such an observation, usually called 'a lunar', a
    | navigator can obtain Greenwich time. That enables the determination of
    | longitude without a chronometer."
    |
    |
    | > In Celestial navigation, precise knowledge of the time at a reference
    point
    | > and the positions of several celestial objects are combined with careful
    | > observations to calculate latitude and longitude. But reliable marine
    | > chronometers were not invented until 1761, and were not generally
    available
    | > for many decades afterwards. For nearly one hundred years (from about
    1767
    | > until 1850), the method of lunar distances was used to determine
    Greenwich
    | > time, in order to deduce the longitude at the time of the lunar
    observation.
    | > Such time information could also be used to check chronometer error.
    |
    | In the first sentence, rather than "a reference point", I would use
    | "Greenwich". While the former term is more strictly correct, it's also
    | more abstract. I think most readers will be best served by simply using
    | Greenwich as the basis for the almanac, time, and longitude.
    |
    | The paragraph puts the era of lunars *after* the invention of the
    | chronometer. I don't question the historical accuracy, but it reads
    | oddly. Perhaps the second sentence could simply say, "But accurate
    | Greenwich time from chronometers was not generally available at sea
    | until well into the 19th century." I believe that would make a smoother
    | transition between the first and third sentences.
    |
    |
    | > "This method relies on the relatively quick movement of the Moon across
    the
    | > background of the stars. Although the Moon, with every other body,
    appears
    | > to circle round the sky in about 1 day, with respect to the star
    background
    | > it completes a circuit in 27.3 days, and with respect to the Sun in 29.5
    | > days. This implies that with respect to the Sun and to stars that lie
    near
    | > its path in the sky, it is moving by approximately its own diameter,
    about
    | > half a degree, every hour. So lunar distances to those bodies are
    generally
    | > changing at about that rate, some increasing, others decreasing. That
    motion
    | > of the Moon is by far the fastest such change that can be seen in the
    sky,
    | > and because it is predictable in advance, it can be used as a measure of
    | > time. Wherever on Earth the Moon is seen from, at that moment, observers
    | > will agree about that time. If the predictions they use are based on
    | > Greenwich, that time will be Greenwich Time.
    |
    | That's better, though I believe the apparent daily rotation of the
    | heavens is familiar enough to the general reader that it need not be
    | mentioned. So for the second sentence I'd say, "It completes a circuit
    | of 360 degrees in about 30 days, equivalent to 12 degrees per day. This
    | implies..." This combines less mathematical precision with a bit more
    | filling-in on how the figure for angular rate is derived.
    |
    | Is this a good place to mention the almanac? Perhaps the last two
    | sentences could be replaced with something like, "If lunar distances to
    | selected stars are tabulated at intervals of Greenwich time in an
    | almanac..."
    |
    |
    | > As for the bit that caused the trouble to start with; the section headed
    | > "Theory", I think it's awful, in so many ways, that just tinkering with
    it
    | > will not do. For example, why invoke two observers, when in reality
    there's
    | > only one? It needs a complete rewrite, and I will think about that.
    |
    | Amen. Maybe delete the current Theory paragraph and use the one under
    | the Method heading instead.
    |
    | A "obituary" paragraph at the end would give a nice sense of
    | completeness to the article. E.g., "Lunars went out of fashion when..."
    
    =======================
    
    contact George Huxtable at george---.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
    -~----------~----~----~----~------~----~------~--~---
    
    

       
    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