NavList:

Frank, it is a pleasure to read posts like this, for its clarity and compendium.

Frank says:
"There are several ways to do this. First, you need to decide whether you will determine the sextant's index correction separately and measure the diameter always "on-arc", like any standard sextant observation, or instead use on-arc and off-arc observations to cancel out the I.C. Note that this is a special advantage of short angle sights, and this also can apply to short distance lunars (but only under special circumstances). It's convenient to cancel out the I.C., of course, but it's also easy to screw up the off-arc readings so the benefits may not be that great.

The math of this is simple enough. You take one or more on-arc observations where you bring the Sun and its opposite limb together. Each of this will read somewhere around 32' plus or minus a bit depending on the date and the IC of the instrument. You average those. Call that sum A. If you have previously determined the IC, then the reduction is relatively simple: the diameter is A+IC. If you also take off-arc observations, you average those and call that sum B. In this case, the diameter is (A+B)/2 and, though we don't need it, the IC is then (A-B)/2."

... things are a bit more messy when one lecture is on the arc and the other off the arc.

Linked some examples from an excellent book in spanish:
Manual de Navegación. Martínez Jiménez, Enrique, 1978. ISBN: 84-400-5327-4

regards,
--
Andrés Ruiz
Navigational Algorithms
https://sites.google.com/site/navigationalalgorithms/

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