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
    Lunars with and without altitudes
    From: George Huxtable
    Date: 2006 Nov 12, 00:03 -0000

    Henry Halboth's interesting posting [NavList 1668] Latitude by lunar
    distance, has rather changed the direction of this discussion, so I
    have tried renaming the thread, to correspond.
    Henry must have as many sea-observations under his belt as the rest of
    us put together, and I am pleased to agree with much, though not all,
    of what he has said.
    I fully concur, when he writes- "I am obliged to say that backyard
    Lunars, taken under antiseptic conditions from a known position,
    although they may be a fair exercise in the determination of sextant
    accuracy/capability, do not replicate conditions of observation at
    sea; if the altitudes are obtained by use of the artificial horizon,
    they do replicate conditions probably faced by land explorers. To the
    best of my knowledge, gleaned through both experience and research,
    the best average accuracy attainable at sea by the use of Lunars
    was/is in the range of fifteen arc minutes of Longitude."
    I would quibble a bit with the following-
    "For example: to clear a Lunar Distance normally requires (at sea at
    least) observations of the respective altitudes which, with multiple
    observers, does not present much of an obstacle, but with only one
    observer requires additional calculation;..."
    That was the case, I'm sure, in the days of lunars in the Royal Navy,
    overstaffed with officers and middies under training, and making a
    ceremony out of it.. However, one skilled man could do the whole job:
    any additional calculation, if the job was done systematically,
    involved no more than an extra bit of time-averaging. The most logical
    sequence was to take a star (or Sun) altitude, then a Moon altitude,
    then some lunar distances, then a Moon altitude, then a star altitude,
    in a regular time-sequence like clockwork, so that the averaged times
    for all three sets of measurements were the same. That was how a
    lightly-officered merchant vessel would proceed, and was how Kieran
    Kelly's postings described Gregory land-navigation techniques in the
    Australian bush in the mid 19th century.
    Henry continued-
    "... yes, I know it's possible to calculate the altitudes - however,
    does not that require a position/time of reasonable accuracy - some
    authorities stating these calculated altitudes should be within four
    arc minutes of the truth,..."
    Asking for the calculated altitude to be within four arc-minutes of
    the truth may be a bit over the top, but it's not entirely
    unreasonable. The biggest effect would be on the parallax correction
    for a high Moon at about the same azimuth as the relevant star. With
    such an error, the resulting error in the parallax correction could be
    no more than about 4 arc-sec.
    and he goes on to ask-
    " ...and does not such accuracy require a position/time accuracy
    better than is ultimately determinable by the Lunar Distance. "
    Here, my answer is "No, it doesn't. It's possible to proceed by a
    process of iteration. Knowing latitude and local apparent time, you
    can assume a trial-value for your longitude, and work out what the
    apparent altitude of the Moon and star would be on that basis. It's
    the Moon's altitude, and the resulting parallax, that has by far the
    biggest effect on the corrections for clearing the lunar distance.
    Then proceed by clearing the observed distance on that basis, and then
    from the almanac tables you get Greenwich time, and from the known
    local apparent time, a new value for longitude. If that is far from
    your trial value, go round that loop once again, with the new value
    for longitude as a trial-value. Each time you go round such a loop,
    the error reduces by a factor of 30 or so, and you very quickly home
    in to the correct value. Once or twice around will normally suffice.
    Done by hand, this involves a lot of tedious calculation, but of
    course it's just the sort of tedious calculation that a calculator can
    do in a twinkling.
    Nevil Maskelyne, bless his soul, included, in his "British Mariner's
    Guide" of 1763, four years before the first Nautical Almanac appeared,
    "A rule to compute the apparent altitude of the Moon or a star,
    supposing they were not observed". However, I would not claim to
    defend the details of that section.
    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.
    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