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    [Fwd: lunars hard to shoot?]
    From: R. Winchurch
    Date: 2000 Sep 11, 7:01 AM

    "R. Winchurch" wrote:
    
    > Excuse my ignorance but would someone give me a primer on lunars.
    > I understand that prior to the effective use of chronometers lunars were the
    > only method for determining latitude.  As I understand it one measures the
    > angle between the moon and selected planets (Jupiter and ?) and certain
    > stars.  How does this translate into latitude?
    >
    > Dick and his Navy Mk III
    >
    > Carl Herzog wrote:
    >
    > > Lunars are definitely a bigger challenge than shooting a horizon, but not
    > > because of the brightness of the bodies. As you surmised, Paul, shades
    > > take care of that pretty easily.
    > >
    > > I've shot a few lunars and I've found the biggest challenge is keeping
    > > the bodies lined up accurately while I shoot -- particularly in a rolling
    > > sea. The wider the angle, the more difficult it is. I shot a couple with
    > > angles of well more than 100 degrees, with the rig of the ship in
    > > between. Those are difficult.
    > >
    > > Assuming you have a sextant with shades on both the index mirror and
    > > horizon glass, it doesn't matter which body is viewed through the
    > > telescope and which is viewed through the index mirror. My decision
    > > usually depends on which choice requires less of a contortionist act from
    > > me. Lying on my back on deck is often the best way to get it. I try to
    > > shoot the moon's lower limb (that is, the one closest to the other body).
    > > Depending on the azimuth to the other body and the phase of the moon, you
    > > may need the further limb. When it's possible, the sun and the moon make
    > > a good combination, and it's probably the easiest to shoot.
    > >
    > > I use a modern whole horizon sextant with shade glasses for both the
    > > index mirror and horizon glass. I've never shot lunars with a traditional
    > > octant, but I hope to. I suspect the tools available in the mid-18th
    > > century made this a more difficult task, although I've seen ingenious
    > > solutions from the time that I would love to try out.
    > >
    > > I should also point out that all my experience doing this has been on
    > > large sail training ships -- schooners and square rigs of 100 ft. or
    > > more. I've never bothered to try it on small yachts, where I'm certain
    > > the impact of the seas would make it even more difficult.
    > >
    > > I've been pretty happy with some of my sights, but I confess that more
    > > than a couple have come out utter nonsense. I haven't done them regularly
    > > enough for the paperwork to become second nature, so some of my errors
    > > could have been there.
    > >
    > > -- Carl Herzog
    

       
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