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    Re: Lunar Distance in Wikipedia
    From: George Huxtable
    Date: 2007 Aug 24, 00:03 +0100

    Renee wrote-
    
    | I have recreated the simple summary in METHOD, and put Jim's practical
    | details in a sub-section called "in practice".  Rather than describe
    | how two sights and a time can be used to find position, I link the
    | interested reader to the article on celestial navigation.  This, I
    | think, removes George's objection that the reader could be confused
    | about how time, longitude, and star sights were actually used in the
    | age of lunars, as opposed to how they might be used by lunarians
    | today.
    |
    | In Errors, I again simplified and reorganized the discussion, moving
    | the discussion of longitude prize into the History section.  I find it
    | easier to follow in its new form.  I did mention the wreck of
    | Shovell's squadron.  I noticed that, though the section says clearing
    | involves atmospheric corrections, it does not say where, when, or how
    | the corrections are made.  Are the corrected altitudes plugged into
    | the parallax formula?
    |
    | I moved the mention of the longitude prize to "history", and could
    | have moved the wreck of Shovell's squadron there as well, but I
    | thought the wreck of the squadron gave context to the subject of
    | Error.
    
    =======================
    
    Renee is giving this topic plenty of necessary attention, but in some
    respects it seems that the latest changes may be a step back, rather than an
    improvement. Also, perhaps some earlier statements could do with some
    revision. I will get out my nit-picking tool, and wield it about.
    
    But please, anyone, argue back if you disagree.
    
    
    
    Figure caption-
    
    "The altitudes of the two
    
    bodies are used to make corrections and determine the time."
    
    
    
    
    
    
    
    "In Celestial navigation, precise knowledge of the time at Greenwich and the
    
    positions of one or more celestial objects are combined with careful
    
    observations to calculate latitude and longitude."
    
    
    
    
    
    
    
    "But reliable marine
    
    chronometers were unavailable or unaffordable until well into the 19th
    century."
    
    
    
    What about inserting "to many mariners" after "unaffordable"?
    
    
    
    The navigator then consults a prepared table of lunar distances and the
    
    < insert "Greenwich"?> times at which they will occur
    
    
    
    Having measured the lunar distance and the heights of the two bodies, the
    
    navigator can find Greenwich time in three steps.
    
      Step One -- Lunar Semidiameter
    
      Almanac tables predict lunar distances between the centre of the Moon and
    the
    
      other body.[citation needed] However, the observer cannot accurately find
    the
    
      centre of the Moon. Instead, lunar distances are always measured to the
    
      sharply lit, outer edge ("limb") of the Moon. The first correction to the
    
      lunar distance is the distance between the limb of the Moon and its
    center.
    
      Since the Moon's apparant size varies with its varying distance from the
    
      Earth, almanacs give the Moon's semidiameter for each day.[citation
    needed]
    
    
    
    
    
    
    
    Because the Moon is so much closer to the Earth than the
    
      stars are, the position of the observer on the surface of the Earth shifts
    the
    
      relative position of the Moon by up to an entire degree[citation needed].
    
    
    
    
    
    
    
     The
    
      parallax correction is a complicated function of the observed lunar
    distance
    
      and the altitudes of the two bodies[citation needed].
    
    
    
    
    
    
    
    
    
      Step Three -- Finding the Time
    
      The navigator, having cleared the lunar distance, now consults a prepared
    
      table of lunar distances and the  times at which they will
    occur in order to
    
      determine the Greenwich time of the observation.[1][5]
    
    Having found the  time, and having the altitude of a star as
    well as the Moon,
    
    the navigator can now use the techniques of Celestial Navigation to find the
    
    vessel's position at the time of the observation.[1]
    
    
    
    
    
    
    
     Errors
    
      Effect of Lunar Distance  Errors on calculated Longitude
    
      A lunar distance changes with time at a rate of roughly half a degree, or
    30
    
      arc-minutes, in an hour[citation needed]. Therefore, an error of half an
    
      arc-minute will give rise to an error of about 1 minute in Greenwich Time,
    
      which (owing to the Earth rotating at 15 degrees per hour) is the same as
    one
    
      quarter degree in longitude (about 15 nautical miles at the equator).
    
    
    
    
    
      Almanac error
    
      In the early days of lunars, predictions of the Moon's position were good
    to
    
      approximately half an arc-minute[citation needed], a source of error of up
    to
    
      approximately 1 minute in Greenwich time, or one quarter degree of
    longitude.
    
    
    
    
    
    
    
     "Knowledge of latitude to within 30
    
      nautical miles would have prevented the wreck of Admiral Cloudesley
    Shovell's
    
      squadron in 1707.[citation needed]"
    
    
    
    < Here, what is referred to is knowledge of latitude, not of longitude. And
    it's quite true that the Shovell disaster was related to a poor acount of
    latitude being kept. Made worse by a 10-mile error in the positioning of the
    Scillies, 10 miles too far North on the chart. Even so, Shovell had no
    business to be so far North. Although Sobell makes much of the Shovell
    disater, it had little or nothing to do with longitude, and it's quite
    misleading to refer to it at all, in the context of longtitude. It could,
    and should, be scrubbed.>
    
    
    
    George.
    
    
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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