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On Jan 12, 2004, at 5:46 AM, George Huxtable wrote:

> 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 is correct that the direction in which the sextant is pointed
would not affect index or calibration errors. However errors due to the
shades would not be canceled unless the same combination were used for
sights both east and west of the Moon, which usually would require
reversing the sextant orientation.  Shade errors can be significant
with lunars; with decent shades, these are due to the shades not being
exactly perpendicular to the axis of illumination.

Chauvenet does not say in this article, #256, to always point the index
glass to the moon, but in an earlier one in the lunars section, he says
to do this to reduce the brightness of the moon, due to losses in
brightness upon reflection in the mirrors.  There he additionally
mentions sliding the telescope out to reduce the moon's brightness.
This was the source of my statement that "Chauvenet is presuming...."

Fred

   

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