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If I recall, I have read a mention of backsights being used aboard naval 
vessels when other vessels blocked the view they wanted.  I do not 
recall the specifics.

Bill B.
On 3/18/10 5:57 PM, George Huxtable wrote:
> The original purpose of the backsight, in which the horizon was viewed in 
the opposite direction (and was never, as far as I know,
> used with a telescope) was to allow a lunar, greater than 90º, to be 
measured with an octant. The mirror-view would then cover the
> arc from vertically above, right round to observing the horizon at the back 
of his neck. Another application for such an instrument
> was on land, in measuring the altitude of a body, when it was greater than 
45º, using a reflecting artificial horizon. Lewis and
> Clark had such an instrument, which they used with their usual lack of understanding and competence.
>
> Sometimes such an instrument could be used to measure an altitude of the Sun 
off a coast, not from the horizon beneath it, if that
> was blocked by nearby land, but from the horizon at an opposite azimuth. For 
example , Malaspina, in his circumnavigation, recorded
> a morning Sun observation, off the Andean coast of South America, with a similar instrument.
>
> Backsights were possible with the early wooden instruments, which could be 
up to 18 inches radius, because they were tall enough to
> allow the back view from the index mirror to clear the top of the observer's 
head. Indeed, early brass sextants were almost as big,
> and some carried backsights to extend their angular range.
>
> Cook's second circumnavigation, departing 1771, was provided with two 
15-inch brass sextants, one by Dollond and another by Ramsden,
> both fitted with backsights, as described on page xx of  Andrew David et al, 
 "The charts and coastal views of Captain Cook's
> Voyages", vol 2. Hakluyt Society, 1992.  It reports that Wales and Bayley , 
the astronomers of the expedition, measured Sun lunars
> of up to 155º!
>
> As instruments became smaller, the backsight view would no longer clear the 
observer's head. As lunar distance predictions were
> never quoted for angles greater than 120º, the limit of a standard sextant, 
there was no call to go further. When the Sun-Moon angle
> exceeded 120º, navigators had to switch to a star lunar instead, which 
presented no great difficulty; it was part of their training.
>
> The big problem with the backsight was this: With the familiar geometry of a 
foresight, the index error of the instrument was
> quickly obtained by aligning an object with itself. That was not possible 
with a backsight. It might be possible to check one end of
> its scale, by aligning the fore horizon with the aft horizon, if they could 
both be seen together, but that observation would
> include twice the dip, an unpredictable quantity. Some index mirrors had a 
special facet ground exactly 90º from the main surface,
> to aid such alignment, which could be as good as was the precision of that 
set angle. But otherwise, I imagine that mariners
> accepted any backsight index-error as it came, without checking. On land, 
given appropriate distant landmarks, I can imagine ways of
> doing the job with some difficuly. However, there's no sign that Lewis and 
Clark ever verified the error in their backsights. If any
> reader can suggest ways of dealing with such offset error, on land or sea, I am ready to learn.

   

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