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Fred Hebard wrote-

>Having been prodded by Frank Reed, I previously wrote that I would
>"poke around in Chauvenet to see whether I can find reference to the
>value of taking lunars in both directions."
>
>This is the subject of article 256 of the 5th Edition, in the section
>on determining longitude by lunar distances.  Doing so cancels out the
>the index error.  If the distances are similar it also cancels out
>errors due to eccentricity in the arc.
>
>Chauvenet is presuming here that either the horizon mirror or the index
>mirror will always be pointed toward the moon, usually the index
>mirror, so that the direction of the sextant will be reversed between
>the two classes of distances.
>
>He said regarding series of sun distances that it is best to average
>the leading from the trailing ones separately.

I no not differ at all from the main thrust of Fred's posting, just with
his third paragraph-

>Chauvenet is presuming here that either the horizon mirror or the index
>mirror will always be pointed toward the moon, usually the index
>mirror, so that the direction of the sextant will be reversed between
>the two classes of distances.

Chauvenet doesn't specifically say that, and why should he presume it? Does
it matter? Surely, reversing a sextant shouldn't affect its reading at all,
nor its index correction. Well, there may perhaps be some slight
gravity-distortion of the framing, I suppose, and it may be helpful to use
exactly the same combination of shades.

Isn't the aim to keep the angular errors exactly the same in amount, when
observing a lunar distance to the East of the Moon, and another to the
West, so that their net effect on the averaged longitude cancels?

George.

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