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    Re: Lunar Exercise
    From: Frank Reed
    Date: 2008 Sep 24, 23:25 -0400

    JCA, you wrote:
    "The lunars were a deliberate exercise, using a wind-up watch that is known
    to run fast, and a longitude that is truly suspect since I did not know the
    datum of Washington DC that was used. The Fix of the stone comes from 1881."
    A longitude in the eastern US from 1881 is likely to be very accurate by the
    standards of sextant navigation. By then, telegraphic longitude
    determinations were standard.
    If you want to determine a longitude there by traditional lunar observation,
    bear in mind that you will need an observation for local apparent time.
    Frequently with historical Sun-Moon lunars, the Sun's altitude taken for
    clearing the lunar distance was pressed into service to give local time,
    too. A navigator might also take a time sight and set a common watch to
    local time whenever convenient and then record the apparent time of the
    lunar distance when that sight was taken. The apparent time has to be
    adjusted for any change in DR longitude between the setting of the watch and
    the LD observation.
    On the other hand, you could do a modern "lunar distance line of position"
    (take two distances for a fix) if you throw in known GMT.
    You wrote:
    "While I like the sextant, I now remember how much I dislike the 3.5x scopes
    that now come standard with most sextants.  On the ship I have a 4x, and I
    had previously used a 6x scope that is no longer available.  I have on order
    from Celestaire, a 7x scope, but since I haven't received it yet, I am stuck
    with the standard scope."
    Yes, I find that this makes a big difference. It's obvious enough when you
    think about it, but magnification is really important for lunars. A 7x scope
    gives results twice as good as a 3.5x scope. Geoffrey Kolbe even constructed
    a 25x scope for his sextant. It's hard to hold the images of the Moon and
    Sun in the field of view, but for land-based observations, this should yield
    essentially perfect lunars, limited only by the Moon's uneven, mountainous
    limb (this assumes that the sextant has been calibrated so that arc errors
    can be taken into account).
    Navigation List archive: www.fer3.com/arc
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