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In adjusting the octant or sextant for index error, there is nothing to stop 
you from adjusting or determining
it on land and then using that value at sea.  Being as land based as Chauvenet, 
all my star-star sights are on
a stable platform.

There is another dilemma concerning the BackSight Octant that I am attempting to 
resolve in my head.  The
octant has only one arc.  That is, the scale on the arc is singular.  There aren't 
two scales, one for foresights
and the other for backsights.

For a foresight, for a given angle, the nonius will  indicate that value.  But the 
backsight is
to be used to measure the same angle.  I believe that the index mirror does not 
move, because the nonius
itself cannot move, else it would indicate a different angular value.   What 
therefore, is the arrangement
of the backsight mirror relative to the foresight mirror?

The backsight mirror must be parallel to the foresight mirror, in order to obtain 
the same angular value.
The optical path from the backsight mirror to the index mirror must then somehow 
cross through the
foresight horizon mirror.  Is the backsight mirror slit further away from the plane of 
the arc, permitting
the optical path to go through the clear part of the horizon mirror?

If the paths are parallel but not co-linear, then the index mirror must be larger 
to support this offset.

Is there some graphical optical path explanation existent, one perhaps for a real 
octant we can examine online?

Best Regards
Brad




-----Original Message-----
From: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] On Behalf Of George Huxtable
Sent: Monday, March 22, 2010 4:20 AM
To: NavList@fer3.com
Subject: [NavList] Re: Back sights

Brad has been asking detailed questions about how to obtain or correct the
index error of an octant (=quadrant) at sea, when a backsight was being
taken for measurement of angles greater than 90º.

To which Frank responded, in 19 March,

"... this problem of getting the index correction for the back sight is
actually identical to the problem of finding the arc error at some odd spot
on the arc of a normal sextant. And therefore any of the solutions that
we've discussed would also work. So, for example, you could do a star-star
sight for any angle in range for the back sight."

===============

The kindest words I can find for that proposal is that it's a landsman's
notion.

Measuring star-star distances isn't easy, even on land, due to stars being
so numerous, and one star looking so much like another. It isn't easy to
locate the intended star-pair.

And measuring stat lunars isn't easy, either, especially at sea, but at
least there is one unmistakable object that can be kept in view; the Moon,
which the observer can swing his view-line about, to locate the other.

But we come, next, to the proposal for measuring star-star distances, 90
degrees apart, at sea, where nothing is stable. Has anyone ever done that?
Has Frank? I certainly haven't, and wouldn't even attempt it. Has anyone,
on this list? In the whole of navigational history, has anyone ever claimed
to measure a star-star angular distance, from the deck of a vessel at sea?

I don't claim that it's inconceivable, but it would call for such
extraordinarily calm conditions, that it isn't a practical navigator's
technique. If Brad proposes to try it out, I wish him well.

==================

I've pondered about this, when evaluating some of the Lewis and Clark
observations, made inland. Measurement of star-star distances would have
been quite beyond their powers, in my view. But on land, there were other
options open to them, depending on the details of the local topography, at
the time. A procedure, which would call for a rather open landscape, would
call for two distant landmarks such as identifiable trees, a mile or more
away from the observer to minimise parallax errors. These trees would need
to subtend around 90º at the observer, which he could adjust be approaching
or receding from them. Then a comparison of horizontal angles measured
between them, by foresight and by backsight, would provide the desired
information.

At sea, this difficulty of checking index error in back observations
provided a serious limitation to the use of backsights for measuring
large-angle lunars, which the Dollond modification was intended to bypass.
Navigators, who had attempted lunar distance observations using only their
octants, would find its limitations restrictive. That's an important
reason, in my view, for rapid acceptance of the sextant, for those
navigators who indulged in ocean travel,  calling  for knowledge of
longitude. For those voyaging shorter distances, an octant remained
perfectly good enough for altitude navigation.

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: "Frank Reed" 


Brad, you wrote:
"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. "

That would work, but it wouldn't be easy, and good luck getting a star in
the right place! If you haven't already said so in a post I've missed, this
problem of getting the index correction for the back sight is actually
identical to the problem of finding the arc error at some odd spot on the
arc of a normal sextant. And therefore any of the solutions that we've
discussed would also work. So, for example, you could do a star-star sight
for any angle in range for the back sight. Or, if you have three
"lighthouses" in a row (far enough out on the horizon so that they fall on
a great circle), let's call them A, B, and C, and you can measure the
angles less than 90 between A and B and then the angle between B and C.
Then of course the large angle between A and C is known. So you measure it
as a back sight and any error is the index correction.

-FER





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