NavList:

G'day from New Zealand, Bruce.

The sextant can of course be used with its frame horizontal or, for that matter, at any other inclination, including upside down. You could use your theodolite to measure the angle in the horizontal plane between two or more distant objects to check your sextant's calibration. That way, you don't have to worry about refraction or curvature of the earth. Purist may object to this method and say that an instrument should be calibrated in the position in which it is used, but sextants are normally calibrated lying on their sides, so any error introduced must be tiny indeed.

You can use your level or theodolite to adjust a bubble sextant to zero, by adjusting the levelled instrument to focus on a distant object, illuminating the eyepiece and directing your bubble instrument to look through the objective at the cross wires, which will appear to be at infinity. It's a way of importing a horizontal plane into your shed. The cross wires are very fine and quite difficult to see, but if you have a spare diaphragm you could perhaps fit it with a coarser horizontal wire, e.g. some cat or baby hair glued to the diaphragm, and you will need to tape a bit of white paper over the eyepiece to act as a diffuser.

As I wrote the last paragraph, it occurred to me that what I have used frequently in the workshop to zero bubble sextants might also be used by the land-locked navigator out of the workshop with a nautical sextant, and indeed the sun's image can be brought down into coincidence with the crosswires' image (or at least, where they would be if I could see them). I used the star or Galilean telescope,as each half of its objective "sees" its own half of the visual field. It's much harder with the inverting or Keplerian telescope and I gave up on it.

I leave the field open for others to develop the "Morris method" ;-)> ...

Bill Morris
Pukenui
New Zealand
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