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Brad has the right idea about backsights. I imagine that's just the way the 
index adjustment was made, at sea.

There are a few things to bear in mind.

It's a rather indirect procedure, in that it relies on first measuring the 
altitude of a near-zenith star. That depends on both the index-index error 
adjustment in the forward direction, then the 0º to 90º scale calibration 
being error-free, then a precise observation of the star in the forward 
direction, then another precise observation in the back direction, from the 
same part of the horizon.  But on the other hand, as there was never a 
telescope involved in  back observations, they would never be very precise, 
at the best of times.

The disc confined within 5º of the zenith only offers a tiny fraction of 
the total sky, which is unlikely to offer a bright star at a particular 
time, so it may be necessary to wait until one turns up.

Because one is shooting at a moving target, a star, careful timing, or 
better, careful interleaving, of forward and back observations would be 
called for. If a star at culmination is chosen, then its angle, from the 
North point of the horizon, would be unchanging, and it would be best to 
alternate fore and back observations from a due-North spot on the horizon, 
chosen with reference to another star near that spot, the observer facing 
alternately North and South. Then, to add a bit more data, another pair of 
altitudes could be measured with respect to the South point of the horizon.

A normal quadrant / octant, for forward observation,  uses some sort of 
telescope or at least a peep-tube to put the eye in the right place. In the 
back mode, there's no such guidance. So I wonder how sensitive the 
observation is to error, from the observer's eye being out of the plane?

But I have never ever handled such an instrument, or attempted a back 
sight, so I'm guessing, a bit.

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.
----- Original Message ----- 
From: "Brad Morris" 
To: 
Sent: Friday, March 19, 2010 1:24 PM
Subject: [NavList] Re: Back sights


| Hi George
|
| Thank you for the information about backsights.  That was precisely what 
I was after!
|
| You wrote:
| 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.
|
|
| I think I may have the answer to the index error for backsights. 
Consider NAV1268 at the National Maritime Museum in Greenwich.  It is a 
backsight Octant.  As is typical of Octants, it is a vernier type, with the 
arc from -2 degrees to +101 degrees.  Due to the length of the vernier 
itself, however, the octant can only measure to 95 degrees.  The 
measurement beyond 90 degrees is the key.  Using a FORESIGHT, measure the 
altitude of a star whose apparent altitude is greater than 85 degrees.  Why 
greater than 85 degrees?  There is a doubled region between 85 degrees and 
95 degrees, in which we can measure the altitude of a star with EITHER a 
foresight or a backsight observation.  Since it is possible with either 
method, we must perform the observation with BOTH methods.  In knowing what 
the altitude is with a foresight observation, we therefore know what the 
vernier must read for a backsight observation, given the same star.  Set 
the octant's vernier to the arc for a backsight observation and adjust the 
backsight horizon mirror until the altitude is correct.  Viola!
|
| Best Regards
| Brad
|
|
|
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