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Thanks George,

My question is theoretical, since I have only tried a backsight with a
sextant (not octant) once, for fun, and then did not really understand what
to do with the observation.  Also I wanted to include more detail in my
website at
http://jimthompson.net/boating/CelestialNav/CelestNotes/SightReduction.htm#B
ackSights , and a correspondent asked me via email to expand on the process.
I was unable.

I am interested in using a sextant for situation number 4: "At sea,He wishes
to measure the altitude of a body above a horizon which is obscured by
nearby land or by mist [or ships], though the horizon in the opposite
direction is clear."

Jim Thompson
jim2@jimthompson.net
www.jimthompson.net
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-----------------------------------------

> -----Original Message-----
> From: George Huxtable

> A further question now arises: what does Jim Thompson wish to use
> an octant
> backsight FOR? I can think of four applications.
>
> 4. At sea,He wishes to measure the altitude of a body above a
> horizon which
> is obscured by nearby land or by mist, though the horizon in the opposite
> direction is clear.
>
> In cases 1, 2, and 3, dip doesn't enter the question at all,
> because angles
> aren't being measured from the horizon.
>
> Whatever purpose he wishes to use the back-sight quadrant for, Jim has to
> decide just how he is going to determine its index correction, on land or
> at sea. Only then can we say how the observation should be reduced.
>
> Over to you, Jim.

   

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