NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Back sights.
From: Bill B
Date: 2010 Mar 18, 22:01 -0400
From: Bill B
Date: 2010 Mar 18, 22:01 -0400
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.