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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.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. 

   

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