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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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