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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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