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"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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