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Doug,

The worst problem with the Davis horizon is that the roof sits on the pan.
Any little breeze can shake the roof and disturb the water. Lewis and Clark used
a horizon with the "house" sitting outside the pan, not touching it. Last
summer I made a horizon to fit over a pie pan without touching. It was fairly
quick to put together and take apart. It had a cloth skirt around the bottom to
prevent drafts getting in when it was set up on rough ground. It got a real
test on the plains of Montana. Had to pile rocks on sextant box, notebook, and
Almanac to keep them from blowing away. Not a flicker in the sun's image!

It did seem there was a slight rocking, perhaps from the gusts shoving the
cover, and thus the air in inside it. Just as likely it was me being rocked by
the wind.

Gave that Horizon away and came up with a better design. The glass is hinged
at the top with masking tape. When opened, strips of ribbon act as the
hypotenuse. It opens to 90? and is self supporting. The "gables" are glued together
layers from a cardboard box. Two strips of ribbon, the same length as the glass
is wide, join the gables to each other. When this is put in place the two
ribbons cross the glass on the side away from you. One crosses next the bottom,
the other next the peak. Then, on your side, you bring a single ribbon from the
lower corner of one gable across the glass and latch it with Velcro to the
lower corner of the other gable.

One layer of the cardboard gables fits just inside the glass roof, the other
goes outside and comes to the upper edge of the roof. In other words, there's
a step, or notch, that fits the glass and keeps out drafts. A lifting handle
makes it easy to pick up the "house" with one hand.

As to the lunar intercept method, I expect you'll get reasonable results on
land in lower latitudes, using a reflecting horizon. At sea, in a small boat,
I'd put no trust whatever in the altitude lunar.

Bruce

   

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